Male Heterogeneity and Female Choice in human Mating:
Maximising Women’s Fertility in Offsetting Stress, Age and Unwanted Attention, While Facilitating Extra-Pair conception
Moxon SP (2021) New Male Studies 10(1) & 10(2) Preprint.
— the paper is to appear in two parts, in the successive NMS issues of 2021
Steve Moxon, Deepcar, Sheffield, UK. firstname.lastname@example.org
Women’s mate choice, given profoundly differential male genetic quality (specifically genomic integrity), is heavily skewed towards topmost-ranked males, producing polygyny with residual monogamy and bachelordom. Polygyny is ancestral, as in gorilla harems (apparently homologous with human female cliques): originally predation-avoidance grouping, male-interposed to obviate female-female stress depressing fertility to sub-replacement (Dunbar). Pair-bonding ensures successive highest-possible-quality offspring while offsetting age-related fertility decline, and dissuading low-mate-value social-sexual approach, thereby actually facilitating access by (or to) high-mate-value males for extra-pair conception. It’s a female fertility platform and springboard for its enhancement. Failure properly to incorporate male heterogeneity and female discernment explains a longstanding theoretical impasse, with infanticide prevention a default mistaken hypothesis attempting to account for monogamy’s chimerical opportunity costs.
Keywords: male heterogeneity, female choice, monogamy, polygyny, pair-bond, genomic integrity
The centrality of differential male mate value
That the human mating system cannot be understood without making central male heterogeneity and female choice was the conclusion in a key review (Human Origins and the Transition from Promiscuity to Pair-Bonding) by Gavrilets (2012), as reiterated by Rooker & Gavrilets (2016). This should have come as no surprise, and was no new insight. It’s a commonplace if not universal perception that men (as for males generally across species) are seen to vary considerably, indeed enormously as mate prospects. Complementing this, women (as for females generally across species) are seen to be very particular in their choosing. With variation, if more limited, also among women, then males and females pair up according to their equivalent albeit very differently based mate values — the perception by those of the opposite sex of an individual’s overall worth as a sexual partner (the sum of traits likely to confer fitness in terms of reproductive output). Men and women mate assortatively, that is, according to their similar level of attractiveness. Profound differential male mate value, keen female discernment and consequent assortative mating would be expected to be at the core of any and every hypothesis of the basis and evolution of the human mating system (and of pair-bonding — evolved neuro-hormonally cemented enduring attachment — or simply pair-living, across primates, mammals, and species in general). Yet this has not been considered properly, if at all. One if not the only exception is the 2013 paper by the present author, which here receives major update, development and different presentation.
With Gavrilets’ corrective unheeded, there is an absence of competing cogent hypotheses to test, and trying to decide between existing ones unsurprisingly has not been fruitful. There continues to be a widely acknowledged impasse in understanding of pair-living/-bonded (monogamous) species generically, not just regarding humans specifically. Reviewers agree on the current impossibility of coming to robust conclusions, though often cite low data quality and inappropriate comparisons (perhaps scapegoating methodological for unrecognised root conceptual problems). Most recently, Fernandez-Duque, Huck, Van Belle & Di Fiore (2020) in examining comparison studies of the evolution of monogamy have “serious doubts” about all of them, given data so poor as to be “distressing”. Lambert, Sabol & Solomon (2018) conclude there is no best model, and Kvarnemo (2018) finds overall monogamy is “still surprisingly poorly understood”, with no single overriding explanation, instead being more taxon-specific (pertaining to a particular group of individuals; for example, populations of the same species in different ecological niches, sub-species, a genus or family of species, etc). Klug (2018) sees no way forward but pluralism plus different explanations of origin as distinct from maintenance (an impasse familiar from the study of the origin of sex and the sexes, indicating a likely similar failure to see the wood for the trees).
Without considering male heterogeneity, female choice and the resulting assortative mating, paired-off men and women may be seen and studied simply as interchangeable binary units entirely independent of each other and of others within the local population not paired up, despite all individuals being part-and-parcel of what is a mating system: a whole-group phenomenon. The resulting misconstrued hypotheses generate equivocal data, prompting their over-complex elaboration, and the risk that what seemingly key factors emerge may be artefactual. However, the failure here is likely not merely an unfortunate constraint of scientific inquiry through a need for parsimony and circumspection. A driver, at least in part, appears to be the denial of the reality of hierarchy in a fundamental misrepresentation of the nature of male heterogeneity. Instead of comprehending hierarchy as the foundational facet of human male sociality it is — as it is for animal species generically — there is the notion that hierarchy is an historical, social construct. Inasmuch as this isn’t simply inappropriate ideological intrusion into science, it both stems from and creates the false notion that forager / hunter-gatherer societies are egalitarian, in that males do not display markedly differential possession of or access to resources. But this is because there is little or no facility to do so in such societies, so rank hardly could be distinguished on that basis (and hierarchy usually would not be expected to be overt if it is settled and thereby much less salient). Resource-holding is merely indicative of rank in male hierarchy, which functions not to allocate resources but sexual access (Moxon, 2009). Accordingly, trying to use resource-competition scenarios to assess the ranking of individuals fails: the rank order appears resource- and situation-specific (Lanctot & Best, 2000). Resource-holding cannot constitute rank, otherwise males in species where there is no resource-holding to indicate rank could not display the clear and stable ranking they do. For ranking to be meaningful — stable and transitive (such that alpha being dominant to beta, and beta dominant to gamma, then gamma is sub-dominant to alpha) — it has to be resource-neutral, as it were. Either the summation of a male’s intrinsic qualities or some major aspect of them (which would be embodied and revealed in ramification across the whole genome through genic capture, the mutual influence of most if not all genes, such that each and every one has huge ramification in expression across the whole genome). Belief in an ancestral non-hierarchical sociality precludes accepting that any sort of inherent distinctions in quality have profound social salience; not least that anything concerning hierarchy could be invoked in mating system aetiology.
Gavrilets’ 2012 paper remains the key recent one in the field in that not only is it a rare actual test of several rival models of the evolution of the human mating system, but there is the addition, across all of them, of the parameters of male heterogeneity and female choice. His results lead Gavrilets to conclude that no hypothesis is feasible without them, as they “point to the crucial importance of female choice and emphasize the need for incorporating between-individual variation”. Whether this means existing hypotheses nevertheless identify factors that are necessary but not sufficient, or that male heterogeneity plus female choice in themselves, in their ramification, may be the only main factors required, Gavrilets does not venture. The task herein is to develop this as an hypothesis, along with any necessary qualifications.
The problem with the whole body of research, reiterated in 2016 (Rooker & Gavrilets), is echoed (looking across primates generally) by Huck, Di Fiore & Fernandez-Duque (2020), who urge incorporating into models “competition and variance in reproductive success across a population” — that is, taking into consideration all individuals in the reproductive group, including non-reproducers. Kvarnemo (2018), in discovering, contrary to what is commonly expected, substantial sexual selection under long-term mutual monogamy, stresses that “mate quality is obviously more important than mate numbers, which in turn affects the need for pre‐mating mate choice”. Unfortunately, rather than by modelling, hypotheses continue to be examined usually only by phylogenetic analysis, an examination of the evolutionary history of the diversification of species. Though usefully delineating the change in possibly relevant factors, often this does not allow either their separation or to establish causal direction. It tends to miss discovering the need for missing parameters, and hence why theorising has managed to continue without being obliged to incorporate male heterogeneity and female choice.
Surprisingly, having outlined what at the very least is a necessary major corrective, Gavrilets himself fails to apply it properly to his own comparative modelling, restricting interpretation of male heterogeneity to fighting ability (determining the minority of males who could, supposedly, monopolise multiple females) and of female choice to only the services males may provide, and not in terms of intrinsic quality of males. Without such self-hobbling, Gavrilets’ conclusions likely would be doubly underscored. Albeit along the right lines, the outline of a cogent aetiology is obstructed by largely ignoring foundational biological principle.
Mating deals with the core problem of maintaining genomic integrity
The foundational biological principle that is the missing underpinning of hypothesis is this: the core problem faced by all biological entities of the relentless accumulation of gene replication error, necessitating repeated purging, the elimination of defective genes by natural and/or sexual selection, either through reducing the reproductive output of individual carriers, or consigning them to reproductive or actual oblivion. Given that the female as the gestating sex is always the limiting factor of reproduction, in effect purging has to be quarantined on the male half of the lineage, leaving the female to reproduce unhindered. Hence the male functions as the genetic filter, to use Atmar’s (1991) label for the process, or mutational cleanser, to use West-Eberhard’s (2005). As Martinossi-Allibert, Rueffler, Arnqvist & Berger (2019) put it: “sexual selection in males has the potential to purge deleterious alleles from the population while leaving females, who experience weaker selection, spared of the demographic cost of adaptation”. Consequently, selection acts far more on the male than on the female, as is now extremely well evidenced and modelled (for citations and a mini review see Moxon, 2019, pp. 42-43).
Several mechanisms have evolved to express pointedly the male genome and to expose it to natural and especially sexual selection, resulting in males to some degree failing to reproduce if to reproduce at all, in proportion to the extent to which they carry deleterious genetic material (for an outline, see Moxon, 2012; 2016, pp. 16-25). It is not just that the male is obliged to take on this function, but that this functional distinction between the sexes appears to be their origin (Moxon, 2019), with sex itself having evolved to deal with mutational load (not to produce variation, as has long been mistakenly thought), as also concluded most recently by MacPherson, Scot & Gras (2021) and Zadorin & Rivoire (2021).
Male heterogeneity here is genetic quality in the special sense of genomic integrity, the maintenance of good inter-functionality of the whole of an individual’s complement of genetic material (both coding and non-coding, regulatory genes), rather than the default notion in biology of the quality distinguishing males being good genes, the possession of the genes or genetic variants most obviously conferring incremental competitive advantage; for example, height. A good genes understanding is notwithstanding equivocal meta-analytic findings (Prokopet al., 2012) suggesting males instead may be attractive to females in ways unrelated to fitness (the sexy sons notion of benefit to females in greater reproductive output of their own in turn attractive male offspring), plus methodological issues and questions about interpretation (Achorn & Rosenthal, 2020). The evidence and modelling fall short in looking for fitness gains when instead it’s an absence of fitness decline that should be sought, in keeping with simply managing to stand still in what is a red queen scenario — thus dubbed from the Alice in Wonderland character obliged to keep moving simply to stand still. Rather than good genes per se, “female mating biases align with the avoidance of bad genes” (Dugand, Kennington & Tomkins, 2018). See also Velando, Torres & Alonso-Alvarez (2008).
The principal mechanism here is the assortment of males to rank according to their relative lack of deleterious genetic material. The resulting dominance (and/or prestige) hierarchy, as already pointed out, is shown to function to allocate sexual access (not resources per se) (Moxon, 2009). The principal mode of allocation is the intrinsic part females play in correspondingly keying into the male hierarchy as the mate-choosers. Given how crucial a function is purging deleterious genetic material, together with the low overall reproductive output of the female making it important that each conception counts, then it would be expected that all females vie for the male with the very topmost genomic integrity — that is, the most-highly-purged male genome. Thus, male heterogeneity, given the imperative of purging, is greatly amplified in being very heavily skewed in its translation into male mate value.
Note that if it were quality per se, then it would be additive, and all that would be needed of a male would be for him to be good enough. All individuals above a certain threshold thereby might be considered interchangeable. But distinctions between purged male genomes are not in absolute but relative terms. It’s because constantly emerging genetic defects are potentially lethal, even as a single mutation, and likely to be synergistic across the whole genome, leading to depressed performance across-the-board, that the fight against them is a red queen phenomenon. Having to run just to avoid slipping backwards, as it were: chasing and closing, though never quite catching. Winning an endless series of battles in a war that never can be won requires constant greatest effort, utilising whichever is the currently top performing individual as the template for all. The chosen male can never be too good. He needs to be as good as it’s possible to be, with no female settling for less than the highest-possible genetic quality (genomic integrity) male obtainable in the circumstances. Hence an evolved enormous skew in the perception of male mate value towards the apex of an ordinal series. Male hierarchy potentially is a winner takes all scenario in terms of reproduction, assuming it were practically possible for the alpha to sire all offspring in the local group. This reproductive skew would be reinforced by the differential reproductive suppression of males that appears to be inextricable with ranking (Moxon, 2009), increasing in degree with descent down the male hierarchy, compounding falling male mate value in terms of genomic integrity. The impact of lower status on male stress physiology, in turn depressing male reproductive physiology and motivation (and the converse re higher status), has been well studied, and can be either direct behavioural reproductive suppression by high-rankers or its anticipation by low-rankers in order that it’s obviated in evolved self-suppression mechanism.
With the female functioning as the conduit to the next generation of the filtered / mutationally cleansed male genome, then males as well as females have a basis of mate choice through opposite-sex heterogeneity. Female differential ability to act as the conduit is summed up in fertility. However, this is a quite particular and at the same time common quality, unlike the general but pre-eminent one looked for in the male, and concerns a narrow set of restricted parameters manifest in correspondingly narrow and restricted indicators, so here quality is according to a threshold model. There is no imperative to be as good as it’s possible to be; just good enough. There is, then, far less scope for variation in mate value among females than among males. In any case, the great bulk of the variation among human females is simply age (see below): females not very long after their fertility-peak more or less cease to be under consideration as part of the mating pool. Most tellingly, females are subject to far less stringent mate-choosing than are males, given the minimal cost and great reproductive benefit to males of mating (compared to the huge costs to females): males, given the opportunity, focus on gaining sexual partners in number more than — or at the same time as — just one of notable high fertility. It is easy to see, then, why the female is considered the choosing sex.
The different basis of mate choice according to sex is illustrated by findings regarding MHC (the Major Histocompatibility Complex), the key factor in immune response. Hanne, Simmons & Rhodes (2009) find that a woman seeks a man with a strong MHC indicating immuno-competence, whereas a man seeks a woman who possesses MHC that merely contrasts with his own. The genetic complement under selection here clearly is that of the male, with the female’s mate choice being in terms of a particularly well-functioning important aspect of the male requiring, as far as is possible, a perfectly non-compromised genetic basis. By contrast, the male’s mate choice is in terms just of adding some hybrid vigour to assist in transmitting his filtered genome through to the next generation.
The need for pair-bonding and same-partnered successive conception
With the prodigious sexual capacity of the male, promiscuity at first glance would seem potentially to suffice as a mating system. The most-highly-purged male could be selected by all the females in the local reproductive group or wider mating pool. However, this would entail low regularity of sex (if not also compatibility issues, both physical and/or physiological), on average substantially delaying time taken to conceive if to conceive at all, including even with the most fertile females. Output/efficiency presumably would be greatly improved by the most-highly-purged male focusing on a number of very-high-fertility females over time, as in pair-living/-bonding, thereby ensuring iterated sex on a regular basis, and, therefore, conception and in minimal time. This presumably is the proximal function of pair-bonding. The number of these females would need to be restricted — likewise the extent of sex outside pair-bonding — so as not to reintroduce the problem of the male’s mating effort being spread too thinly.
The principal benefit of pair-bonding, however, would be if it is long-term. Conception then would be repeated, leading to another benefit stemming from male heterogeneity. Offspring not only would continue to be assuredly produced and at minimal intervals, but all would be the most highly purged possible, as all would be sired by the same, most-highly-purged male. The alternative of only short-term pair-bonding, merely sufficient to ensure conception (or, as has been suggested, long enough to support the female during lactation), would be no different to promiscuity. It would risk the quality of offspring progressively decreasing with iterated assortative mating.
The quality (genomic integrity) of males subsequently secured in serial short-term pair-bonding would correspond to the age-related declining fertility of the female herself. Not only is there deterioration of stored ova (leading to quickly rising rates of implantation failure), but also progressive shortening of the potential reproductive lifespan (Conroy-Beam & Buss, 2019). A sharp early diminution in a female’s sexual attractiveness occurs with a first pregnancy, and then the impact of childbirth, as indicated by low waist-to-hip ratio and BMI (Lassek & Gaulin, 2018), or, more accurately, shallow waist depth and small circumference (Rilling et al., 2009). It is not fertility that is indicated but being nulligravid (that is, not currently and never having been pregnant). The female signals not only her current immediate availability for reproduction, but that she has her full reproductive life ahead of her (that is, none of its being already in the past); assuredly starting at her fertile peak. This enables a female to secure for a pair-bond partner a male with the greatest genomic integrity consonant with her other fertility indicators. Pair-bonding in effect projects forward in time female peak fertility, if at or circa the female age of peak fertility (and, therefore, attractiveness to males) the female looks for the male with the greatest genomic integrity she can find who is willing to pair-bond with her. There will never be a more auspicious time to do this, and the pair-bond effectively time-capsules her mate choice, which she could not do in reproducing promiscuously, as the genomic integrity of each successive father for her children is likely progressively to diminish in line with her own rapidly declining mate value. In other words, with pair-bonding the female in effect can cheat time.
Pair-bonding in the human case does appear to be of sufficient duration to effect repeat conception, albeit its estimation is extremely difficult, as discussed in Moxon (2013), with extremely inconsistent and very poorly comparable measures. Not least, data on marriage and even cohabitation is missing any period of initial informal pair-bonding, including the portion of this within those pair-bonds that later become marriages or cohabitations. The human pair-bond typically is about five years, according to Sefcek, Brumbach, Vasquez & Miller (2006), which is greater than inter-birth intervals in hunter-gatherer societies (those presumed closest to ancestral populations) of three to four years (Kaplan, Lancaster & Hurtado, 2000) or 3.65 years (Robson, van Schaik & Hawkes, 2006). Note that pair-bond duration would not have to be decisively longer than the inter-birth interval but merely comparable for pair-bonding to be adaptive in respect of repeat conception, as generally in the evolutionary creation of an adaptation even merely statistical biasing of behaviour usually suffices. Furthermore, pair-bonding would fulfil the putative function by merely providing a platform for other, usual modes of affiliation and attachment to augment and perhaps then supersede it. Such affiliation and attachment could have served to reduce in the first place the requirement for a lengthy period of pair-bonding per se.
Most often cited or used as a basis for estimation is Fisher’s four years (Fisher, 1989, 1994), which by other measure is a substantial under-estimate, being the modal average of the time from marriage to divorce. First, it’s only a subset of all marriages, and a severely systematically biased one at that, as all those that did not end in divorce are excluded. Second, ignored is all of the period of pair-bonding prior to its formalisation even as cohabitation. Third, all of the very high proportion of marriages that are very lengthy are missed out in not using the median or mean as the measure. Note, though, the inter-birth interval data is also lacking, as it does not include the period from the inception of pair-bonding to conception. Taking the median rather than the mode, even for divorcing couples the duration of the average marriage almost doubles to seven years, and far longer than that for the whole set. Fisher’s rationale for taking the modal peak is that in being what is typical rather than merely average we may detect the underlying natural extent of pair-bonding. Yet divorce itself in not being typical of marriage may represent less an absence of other affiliative and attachment processes piggy-backing the pair-bond than extrinsic factors precipitating pair-bond termination. If the latter pertain only to a contemporary and not an ancestral context, they would not be part of the milieu in which pair-bonding became adaptive. Consequently, pair-bonding that was highly adaptive ancestrally may be weaker in contemporary scenarios, reducing average duration to below what it may have been when originally evolved, providing other than a full picture today, clouding its function.
A key aspect and revealing feature of pair-bond duration is that it’s U-shaped: typically, either long (if not very long) or very short. Fisher’s data misses both these poles, neither of which can be considered atypical. Of those pair-bonds that dissolve, there is a very heavy skew toward this being very early in the relationship, as routinely commented on in the relationships-advice literature and journalism. Data for both developed-world and hunter-gatherer societies bear this out. Furthermore, specifically marriage data greatly understates relationship breakdown overall. In the USA, of unmarried couples, 70% break up within their first year (Rosenfeld, Thomas & Falcon, 2018). In the case of marriages, given the hurdle of the formalisation of the relationship causing lead times for dissolution, it’s necessary to measure failure within the first year other than just by the divorce rate. Then it is found that over a fifth — amounting to half or more of all those marriages that end in failure — if not already dissolved were well on the way to that outcome (Leonard & Roberts, 1998): 1% to 2% of couples had already completed divorce, 5% had permanently separated, and a further 14% had separated for a period at some point (heralding their demise). Additionally, much of the divorce rate in the second year should be attributed to the first, given that’s when proceedings began or when the instability in the relationship had already made one or both parties minded to initiate them. Regarding hunter-gatherers, Blurton-Jones, Marlowe, Hawkes & O’Connel (2000), in studying across four quite different populations, note the great number of marriages breaking up before children are born is owing to their failure in fecundability, the probability of a female conceiving in a given period of time. Partners wait only so long to test the concrete fertility of the pairing before baling out. Similar is found with pastoralists. Du & Mace (2019) conclude: “The production of offspring, regardless of their survivorship, also had a positive effect on marriage duration, as did trial marriage, a time period before formal marriage”. So, it’s not producing children per se but confirmation of the potential to do so that is at issue. Trial marriage is commonplace. Pair-bonding generally appears indeed to be on trial until reproductive output is assured by conception, so it may be that in an important sense this is when bonding properly begins or fully completes. So, Fisher could well be right to exclude both extremes of duration: long because it’s not pair-bonding per se sustaining the relationship; short because in important senses it’s pre-bonding. Matching or exceeding the inter-birth interval might, then, be a good measure of pair-bond duration to evidence the putative function of ensuring at least one successive conception. The data clearly supports this.
The logic of the most rudimentary polygamy — polygyny, that is (one male multiply paired with females) — with the male of the greatest genomic integrity being the only male females choose for pair-bonding, affords much less opportunity for most females to mate with him than would be the case promiscuously, driving fierce female competition. Females would do better to vie for a parallel pair-bond with not just one but a number (albeit a very limited number) of top-ranking males (the very topmost plus a tiny few of the next highest-ranking males). Hence a number of polygynous males. A trade-off of a degree of diminished (compared to the alpha) male genomic integrity against reliability of long-term regular sex in pair-bonding, then can lead to assortative mating to encompass a larger portion of the male hierarchy. Males ranking high-to-middling additionally could also pair off polygynously, though each may attract fewer females with decreasing rank. Given the very strong skew in mate value, this would quickly fall from a multiple to just one. Polygyny becomes monogyny (just one female per male; in other words, monogamy), leaving a surfeit of mateless males — zerogamy, as now popularly labelled. However fine-scaled is this assortment, as it proceeds the relative genomic integrity of males would decrease at least roughly in step with the corresponding lesser ability of the female partner to transmit the male’s genes into the next generation. In this way, whatever the degree of male genomic integrity, it is not compromised by any less inter-generational transmissibility than it warrants, and vice-versa, overall producing high (if not the highest possible) reproductive output (quantity x quality) and efficiency of the local reproductive group or mating pool.
Note that the females are never selecting some interchangeable also-ran male. Those females mating with males who don’t have the mate value to warrant multiple pairings, and therefore mate monogamously, still will have chosen a male with a degree of genomic integrity greater than that possessed by still lower-ranked males (and certainly more than that of the lowest-rankers). The supply of females will have run out before not just many but (ancestrally, at least, as evidenced below) most males get any opportunity to assort with them, owing to so many females having paired off multiply with just the one male — or even eschewing monogamy with well-below-average-ranked males, preferring instead intermittent consortship or promiscuity with males of higher rank. This is why it is so mistaken to view a basis of assortment as merely varied, idiosyncratic or arbitrary, as is implied by and the hidden assumption of usual hypotheses. The real possibility of complete matelessness would drive males to settle for partnering merely monogamously. Indeed, partner scarcity is proposed as the basis of monogamy (Schacht & Bell, 2016).
It is held that monogamy is difficult to explain because the male sustains something of the order of a fivefold opportunity cost of not having multiple pair-bond partners as in polygyny (for example, Dunbar, 2018), but although this can be argued in the case of obligate monogamy, it is not applicable to monogamy as a residue of polygyny. Polygyny is not a potential alternative for males who struggle to gain even a single partner. Far from monogamy entailing opportunity cost, it is a clear benefit to non-polygynous males because otherwise not only will they be zerogamous but they won’t be able to acquire a sexual partner outside of a pair-bond either. They will not be able to reproduce at all, because a male finding himself in the lower realms of assortative mating — in monogamy clearing, as it were — will have even less (much less) chance of mating promiscuously. Unlike men, women require substantially more attractive partners for extra-pair (no-strings) sex than for pair-bonding (Szepsenwol, Mikulincer & Birnbaum, 2013), just as do females in other species (eg, Cochaset al., 2006; Kempenaerset al.,1992). Again, male heterogeneity enters as crucial to reveal what is going on.
A very different picture emerges here to that envisaged in modelling by Gavrilets, who recognises only the very top-ranking males as pairing up through any intrinsic quality — and only their fighting ability, which may recruit only a subset of the full range of genetic quality demonstrating genomic integrity — and this by imposition, not female choice. All other males are considered to possess little if any, or effectively zero mate value, and therefore in effect are interchangeable. Not until some of them realise they can adopt an alternative mate acquisition mode of offering to provide services beyond mating itself, such as provisioning. Only then and not before, in Gavrilets’ modelling, does female choice emerge as a factor in mating systems. Clearly, this is a highly restricted model not reflecting biological reality.
The polygyny-monogyny-zerogyny mix arising from the simple logic of male-female assortment according to differential intrinsic mate value is an accurate characterisation of the human mating system, which is not monogamy. Monogamous species are obligately so. Polygyny and monogamy (and bachelordom) are inextricably linked, with polygyny central and monogamy (and bachelordom) in its shadow, notwithstanding that not only is there monogamy in all societies, but polygyny never accounts for more than a minority of pair-bonds. The very severe skew in male mate value produced by the imperative of purging means it can only be a small minority of males who are polygynous. So it is that in half of the societies thus categorized, less than five percent of men take on more than one wife (Labuda, Lefebvre, Nadeau & Roy-Gagnon, 2010). A large preponderance of societies (85%) in the anthropological record allowed polygyny (White et al., 1988). As well as polygyny being formally sanctioned in most societies, it occurs overtly in all societies in the guise of serial monogamy (Schacht & Kramer, 2019), as well as covertly in the de facto polygyny of mistress-keeping. Suda (2007) in a paper on contemporary and traditional African practice, titled Formal monogamy and informal polygyny in parallel, outlines that “although much of the ethnographic literature indicates that heterosexual monogamy remains the statistical marriage norm, polygyny was nevertheless empirically widespread in traditional Africa and is increasingly being reinvented, often clandestinely …” (p. 56). That polygyny is the human norm is the conclusion, in a book-length review of the topic, by Baresh (2016), who sees “the underlying prevalence of polygamy as the default setting for human intimacy” (p. 2).
A particular form of polygyny in the anthropological literature is concubinage (here with a more restrictive meaning than non-marital co-habitation), also known as polycoity, which might be considered a hybrid of monogamy and polygyny in that there is a primary pair-bond plus one or more secondary, substantially lesser ones. Actually, this is the usual nature of polygyny. Chika & Nneka (2014), for example, point out: “The husband in most cases showers much love on the most favoured wife” (p. 23). Almost always there is a preferred wife over all others, whether an always-favoured or the newest (youngest), with the latter case being usual (eg,Gwirayi, 2016), which in effect is serial monogamy. Essien puts it emphatically: “The hierarchy of wives is indisputable and the inferiority of the secondary wives is beyond any argument”. This distinction underlines that polygyny is not some collective bonding with one male but parallel monogamy. Distinguishing between polygyny and monogamy gets harder the more they are investigated.
The female as the chooser
The full extent of the heavy reproductive skew towards males with the most genomic integrity would be revealed by adding to the within-pair reproductive output of polygynous males that which is through extra-pair conception. This is the reproductive output from clandestine polygyny plus any sex outside of any pair-bonding (casual sex, as it were). The latter would be very largely if not exclusively with polygynous males, in that (as afore-mentioned) human females require partners of substantially higher mate value for extra-pair sex than for pair-bonding (Szepsenwol, Mikulincer & Birnbaum, 2013).
Extra-pair paternity is now shown to be extraordinarily high in populations most closely resembling ancestral ones: foragers or pastoralists. Among African pastoralists, Scelza et al. (2020) find extra-pair paternity to be 48%. A natural base rate of extra-pair paternity (non-paternity) had thus far been hidden. Macintyre & Sooman (1991) discuss the impossibility of verifying the usually cited 10% — medical students are told it’s 10-15%, which is in line with the worldwide median of 10% arrived at by Baker & Bellis (1995) — because known or suspected cases would self-select out of sampling to avoid discovery by the presumed father, leaving all measures inherently gross under-estimates. Nevertheless, a series of unpublished studies of English samples put the rate at 30% (20-30% according to a likewise unpublished Liverpool survey). Research hitherto anyway has been restricted to European or European-descent populations, and therefore to (long-)developed societies, that profoundly contrast with the traditional agrarian, let alone those that might more resemble ancestral populations. It might be expected that the former feature factors depressing non-paternity and/or its discovery that were not present until recently.
The new data from pastoralists on extra-pair paternity indicates monogamy may function in part or mainly as back-up to supply an already high level of male genomic integrity, for if there has not occurred extra-pair conception with a male of genomic integrity substantially greater still. The latter is not unlikely to occur given the heightened sexual motivation of females at ovulation, which, in temporarily reducing, relatively, the salience of anything but sex, reduces concern for pair-bond maintenance, allowing the risk-taking required in extra-pair sex. It has been thought that during their brief fertile phase around ovulation women switch from being attracted to their pair-bond partner to extra-pair males (for example, Larson, Pillsworth, & Haselton, 2012). However, regular failure to replicate has been reported, most recently by Thomas, Armstrong, Stewart-Williams & Jones (2021), Stern, Kordsmeyer & Penke (2021), and van Stein, Strauß & Brenk-Franz (2019) — who nevertheless confirm heightened sexual motivation — or that only a modest or slight effect is evident (Marcinkowska et al., 2020). A new review (Jones, Hahn & DeBruine, 2019) finds largely null results even in old studies. All indicate a faulty interpretation has been at play. Rather than a preference for extra-pair males over the pair-bond partner, it’s surely conditional on differential mate value. In the absence of an extra-pair male of substantially greater genomic integrity than the pair-bond partner, the female simply will have heightened sexual interest in her pair-bond partner. A general attraction to potential extra-pair males would not make sense. What has been dubbed the dual mating strategy or (good genes) ovulatory shift hypothesis is an artefact again of failing to incorporate male heterogeneity, instead to assume that the female, rather than looking for genetic quality (genomic integrity) in any and every male sexual partner, seeks different or additional qualities in a long-term partner: investment of some kind, usually assumed to be provisioning of offspring. But this requires evidence, which is absent, indeed countered (see below). Women instead seek genomic integrity in all male partners, but substantially more so in specifically extra-pair partners because of the risk of pair-bond dissolution, and thereby the loss of a genome with high genomic integrity by which to conceive a subsequent series of offspring — which she won’t be able to replace with anything like an equivalent male given her own steeply falling mate value. The extra-pair partner is available only for one conception, unless he too becomes a pair-bond partner of the female.
Furthermore, women have an aversion to sex during the infertile (luteal) phase, presumably because sex poses a substantial risk of implantation failure should an ovum be fertilised (Steiner, Pritchard, Young & Herring, 2014). Hence, it would seem, an evolved implantation-failure avoidance mechanism in pre-menstrual syndrome (Moxon, 2020). Added together, these phenomena are likely severely to depress the frequency of sex, which is the age-old principal complaint men have about marriage. Albeit minimal sex usually suffices to maintain a pair-bond (given the very well-known phenomenon in psychology experiment of reinforcement regimes that although infrequent nonetheless are effective, especially if they are unpredictable), this female overall strategy risks partner defection. Should a woman detect lesser investment in the pair-bond by her partner than by herself — in other words, a strong possibility of her pair-bond male defecting — she increases the regularity of sex with him, though only in the luteal (non-fertile) phase of her cycle (Grebe, Gangestad, Garver-Apgar & Thornhill, 2013), thereby continuing to facilitate (albeit not to preference) extra-pair sex during the fertile phase. It’s a measure of the imperative to further boost fertility overall that even when the pair-bond is under threat the heightened sexual motivation at ovulation is not directed specifically towards the pair-bond partner.
All entails a large degree of female partner-choice, whether proceptive (taking the sexual initiative) or through selective acquiescence. Given the various difficulties attending engaging in extra-pair sex and potential costly implications, it is inconceivable that it could occur on this scale without females being not only fully complicit but actively the choosing sex. With so much at stake in extra-pair sex, women surely are the arbiters, whether or not they are the main initiators.
The centrality of female choice to add to that of male heterogeneity is shown in polygyny not being some male imposition as often supposed: “Many women in Africa choose polygynous over monogamous marriages even in the absence of pressure from relatives” (Anderson, 2000). There are even benefits in terms of childcare. Anderson finds other adult females acting as alloparents, taking on parental duties despite not being parents of the offspring themselves. Female kin and co-wives provide the resources that contribute to the reproductive success of women married polygynously. There are benefits from polygyny not just for first but also for subsequent wives (Uggla, Gurmu & Gibson, 2018), contradicting assumptions that women in polygynous marriages are just making the best of a bad job. There is not only no evidence that polygynous marriage is a harmful cultural practice, claim Lawson et al. (2015), but any costs of sharing a husband are offset by greater wealth of polygynous households, consistent (the author himself notes) with models of polygyny based on female choice. So even on an inappropriate economic analysis ignoring the centrality of intrinsic male heterogeneity, polygyny is not disadvantageous but beneficial to women.
That women are the choosers, not men, is very clear whether in polygyny or monogamy. Corresponding to male advertisement of quality so as to prompt female choice, the female choosing the particular male is basic in evolutionary theory and long ago confirmed in the case of humans. Cross-cultural verbal interaction research by Stephens (1963) showed that notwithstanding men being the sexual initiators, women are the choosers; or, as Moore (1985) put it, the controllers of mate choice (including initiation). This extends to arranged marriage (the dominant form of match-making across Africa, the Middle East and Asia, and all but ubiquitous across the world prior to industrialisation), which was and is invariably orchestrated by women: either family members (aunt, elder sister, sister-in-law, or possibly an older matriarch) and/or an outsider female matchmaker. It’s the cross-cultural norm today and throughout history (for a brief review, see Moxon, 2017, p146).
Our mating system’s gorilla-like ancestry
Polygyny is known to have been central to human mating ancestrally. DNA analysis reveals only 40% of men were ever fathers (Wilder, Mobasher & Hammer, 2004), fitting with data of worldwide Y-chromosome diversity indicating only a few men may have contributed a large fraction of the Y-chromosome pool at every generation until only recently (Dupanloup et al, 2003). Some evidence points to this change being in the era of early modern humans, circa 150,000 years ago (Walker, Flinn & Ellsworth, 2011). Note this would occur simply from a large increase in local populations, given no reason to suppose the sharp reproductive skew towards an apex of only a very few males would change. The tendency to winner takes all suggests that the residue of monogamy and bachelordom has grown without a commensurate proliferation of polygyny.
Also indicating that polygyny is human-ancestral are the oft-cited long-known results from standard comparison of sex dimorphism in body size and other parameters across the family tree of hominid species. From these it has been proposed that the human mating system was never chimpanzee- but gorilla-like: long-term pair-bonds of one male with multiple females (Nakahashi & Horiuchi, 2012; Geary & Bailey, 2011). An evolutionary transition from this to the human mating system is discounted by Chapais (2008) in his assertion that separate harems would be unlikely to fuse to form the human sociality of multiple males and multiple females all together, and that instead this is more directly our ancestral state — like the hamadryas baboon today. But this is to ignore a lot of research, notably in the 1990s, reporting large gorilla groupings of multiple one-male-+-multi-female sub-groups (see the book covering three decades of this research, by Robbins, Sicotte & Stewart, 2001). Gorillas evidently exhibit group fission-fusion of modules of a single male and his harem. This is analogous, likely homologous with human family grouping, and confirmed in gorilla-human comparative work (Morrison, 2019) demonstrating “common underlying multi-level social structure and the considerable similarities in inter-group territorial dynamics”; findings that “strongly emphasise the importance of gorillas as a model system for human social evolution”. Chapais therefore seems right that a multi-male-+-multi-female sociality is ancestral: it’s simply that he didn’t realise that this is what we have in the gorilla.
Reinforcing this is another window on the extinct ape common ancestor provided by the orangutan, in that this species is now shown to be gorilla-like and more human-like than is the chimpanzee. The orangutan in comparison to all other apes has evolved only very slowly (Locke et al, 2011), whereas human evolution comparatively in major respects has been effectively reversed by neoteny (Franchini & Pollard, 2017). Thus is explained the uncanny morphological and other human-orangutan similarity, whereas there is notable human-chimpanzee dissimilarity (Grehan & Schwartz, 2009). The molecular evidence has been held not to support this, but is not relevant in being in respect only of coding genes, when “most genomic changes that distinguish humans from their primate relatives are in non-coding sequences with regulatory functions” (Ligrone, 2019), especially in the so-called Human Accelerated Region (HAR), and not least those regulating neoteny. Tang et al (2020) demonstrate that some human features indeed may be explained by neoteny, notably in the brain, with infant human brain growth being a mere third the rate of that of chimpanzees. It’s likely, then, that the orangutan’s mating system much more closely resembles those of species in the human phylogeny than does the chimpanzee’s, which appears to be a profound divergence. The upshot is that there is no utility and instead only hindrance and false trails in looking to the chimpanzee for our ancestral mating system. The orangutan, despite its semi-solitary living, is, like the gorilla, polygynous. It’s dubbed short-term polygynous, though this is rather inaccurate given that the sexually exclusive consortships of up to seven months reported by Utami et al (2002) are a continuation of long-term affiliations formed in adolescence, which also revive between child-rearing (Grehan, 2006). Apparently, there is long-term pair-living/bonding in the orangutan, or there would be but for periodic separation owing to necessary dispersal to ensure sufficient food resources. Any dissimilarity to the gorilla pattern is through the orangutan’s relatively recent evolution to largely solitary living; the result of a major environmental change producing food scarcity, according to Harrison & Chivers (2007): “the orang-utan’s present-day mating system most likely evolved from a gorilla-like base, with one dominant male guarding a harem of females”.
The homologous human female clique
A gorilla-human seeming homology pertaining to mating system is the gorilla harem and the human female clique. The latter typically numbers four to six, or an average size of 5.11 members (Kwon & Lease, 2007); the former, a maximum of five individuals — the limit, across primate species, of the number of females per polygynous male (Pawlowski, Lowen & Dunbar, 1998). The workings and nature of the clique as the key form of girl, adolescent, and indeed adult human female sociality has been entirely neglected in the scientific literature in favour of consideration only of supposed unisexual sociality, despite it being abundantly clear that male sociality is entirely different, with male (but not female) sociality essentially group-based as opposed to dyadic (David-Barrett et al, 2015), and the female (but not the male) social dynamic being exclusion (Benenson et al, 2013). Presumably, this neglect is through fear of usual censure and career derailment of being considered in some way to negatively portray women. Insights instead come from parenting literature — most notably by Wiseman (2016), but also several other authors — finding its way into scholarship as teacher resources (Harley & MacNeil, 2017) and via Psychology Today (Powell-Lunder, 2013).
As Powell-Lunder stresses, girls in general belong to a queen bee-led small coterie; that is, a clique. Not just those who are the most popular (who are merely the most conspicuous and suitable as protagonists in famous films such as Mean Girls). Furthermore, Powell-Lunder elaborates, the cliques to which most girls belong are still fiercer environments than those of the populars, because of the fear that exclusion from this less exclusive social milieu may leave nowhere else to go. Clique members adopt the various well-documented (Wiseman, 2016) unranked roles, in an ethos of fierce avoidance of one-upmanship, with just one of them at any one time a potential replacement in being the deputy of the queen bee, who appears, therefore, not an alpha at the apex of a hierarchy but first among equals. The hallmark of clique sociality is of course the extremely well-known intense policing of membership, under the fierce control of the queen bee (unless and until she is deposed by her deputy). Also, the firm exclusion of all others, the difficulty in gaining membership, and the constant prospect of losing it. The imperative appears to be an all-encompassing desire to achieve and maintain clique membership by serving the queen bee and taking great care not to be seen to challenge her (on pain of expulsion), yet inhabiting or creating a springboard position from which to be the usurper should there be an opportunity (with sufficiently good prospects of success).
From a comparative, evolutionary perspective, the human female clique looks like a proto- (though possibly only facultative, or even vestigial) co-operative breeding unit, with a (potential) sole breeding female controlling a group of fellow females she reproductively suppresses so they can be utilised (through channelling their reproductive motivation) as alloparents. It may be that in the deputy there is a reserve breeding female or a subsidiary as a result of only partial reproductive suppression. Perhaps the rest of the females, though reproductively suppressed more than is the deputy, nevertheless are not fully so, and may then breed to a limited degree or in some circumstances. If not obligate, this could be facultative co-operative breeding triggered by local severe ecological stress, as an adaptation to head off the possibility of local extinction, by restricting reproduction, possibly to the extent of a 100% skew to the one most fertile female in the sub-group. In her being paired with one of the males with the least deleterious genetic complement, and the use of locally very limited resources (including the parenting efforts of other reproductive-age females) being reserved exclusively for their use, then there would be a good chance that the one most reproductively fit couple of each sub-group could manage to produce a next generation. Such a best shot likely would outdo the alternative of too many pairs attempting to breed, as this would place them in mutual competition over temporarily impossibly limited resources. It may be that in extremis only one pre-eminent sub-group — therefore, just one couple — gets to reproduce from out of the entire group (collection of sub-groups). This would account for the character of the one popular clique of the most highly fertile females.
The stresses attending human females together in a breeding unit are evident in the experiences of co-wives in polygyny. According to Essien (2018), “co-wife conflict especially in the early years of marriage is pervasive, and often marked by outbursts of verbal or physical violence”. “Ongoing and contentious rivalry” and “a recurrent motif of strident co-wife hostility” is how Jankowiak, Sudakov & Wilreker (2005) put it: “… we found women’ s sexual desire and reproductive interests paramount factors in promoting co-wife conflict (that is a) deep-seated resentment”. Oppong, Monebenimp & Nzefa (2019) discuss the “emotional suppression” felt by co-wives. The chronic stress experienced by co-wives is so severe that a principal theme of the findings by Tabi, Doster & Cheney (2010) is infertility, with it prompting co-wives to welcome additional wives so as to avoid divorce. The co-wives’ predicament in their contradictory social dynamic is neatly summed up in the title of Madhavan’s (2002) paper, Best of friends and worst of enemies: competition and collaboration in polygyny. Female clique dynamics in a nutshell.
The male bodyguard to fend off female aggression
The human-clique-homologous gorilla pattern of small female sub-groups interpolated by males is now understood (Dunbar, 2020) originally to be an adaptation to reduce stress on females from females in other sub-groups, which could not be addressed by group fission given that large overall grouping itself is an adaptation — to avoid predation. Failing to deal with female intra-sexual stress would directly reduce female fertility to sub-replacement, leading to local extinction. Dunbar writes:
“One possible mechanism would be the impact that social stress has on the mammalian female menstrual system (and hence fecundity), the endocrinology of which is now well understood (Abbott 1984; Abbott et al. 1984; Gordon et al. 1992; McNeilly et al. 1994; McNeilly 2001a,b; von Borrel et al. 2007; Chatterjee & Chatterjee 2009; Son et al. 2012; Iwasa et al. 2017; see also Wasser & Barash 1983; Huchard & Cowlishaw 2011)”.
Human females are no different in stress causing reproductive suppression (Wasser & Isenberg, 1986). Interpolating males to reduce female-female aggression would be a pre-adaptation to subsequent pair-bonding. Likewise appears to be concealed ovulation, which also functions to reduce female-female aggression, according to Krems et al (2021), who, in testing a male investment model of concealed ovulation against one of female rivalry, find clear support for the latter and none for the former. The evidence here is that females evolved concealed ovulation because cues to ovulation trigger aggression from other females trying to boost their own mating opportunities by reducing those of others.
It’s long been known that low level agonistic interactions between female gorillas increase dramatically in frequency with the number of females, and not with group size (males and females both) per se (Watts, 1985). A strong parallel is evident in the closely-related primates, baboons. Within-group female coalitions attack and thereby reproductively suppress their female victims, who experience more cycles before conception and longer inter-birth intervals (Wasser & Starling, 1988), with female group size causing increased female-female competition that correspondingly elevates stress levels (Hill, Lycett & Dunbar, 2000). “Conflicts between neighbouring harems are usually initiated by one, occasionally two, females; if the conflict escalates, more females will become involved, until eventually the harem males are drawn into the dispute and force the separation of the two groups of females (Dunbar 1983b, 2018)” (Dunbar, 2020). The parallel appears to be very general indeed, and recognised some time ago by Gowaty (1996): “In fact, female-female aggression may be the most parsimonious explanation for socially monogamous males in many species” (p 28).
Both sexes experience crowding stress from same-sex others, but whereas for male mammals this is from spatial proximity, for females it’s caused simply by their total number (Brown & Grunberg, 1995). It’s social dynamics per se. Splitting the group of course would merely reintroduce the very predation threat that increasing group size evolved to counter. A separation of females is required that is more complete than is the merely spatial. Interceding males, one per each female sub-group, would be an adaptive solution if this socially buffered the female sub-groups from each other, at the same time facilitating sufficient cohesion to create overall one coordinated large group to dissuade or deal with predators. As noted by Dunbar (2012): “It seems that in the large groups adult males act as bridges, or weak links, that help maintain the cohesion between clusters of females”. This is strongly echoed in the afore-mentioned new work (Morrison, 2019) showing unexpected ape contiguity with human multi-level social structure and inter-group dynamics.
This interceding male Dunbar dubs the bodyguard, which is to borrow a term coined by Mesnick (1997) but which was on the understanding that deterrence was of male aggression. Dunbar himself formerly had considered the bodyguard to dissuade unwanted social-sexual advances from males, which had been proposed by Lumkin (1983), and as a possible basis of primate pair-bonding by Norscia & Borgognini-Tarli (2008). It was also the view of the present author in Moxon (2013), but there qualified to be specifically in respect of social-sexual advances from low-mate-value males, thereby actually facilitating the female pair-bond partner’s accessibility to and by males of high mate value — that is, of a mate value substantially higher than that of the female’s pair-bond partner. In other, colloquial words: clearing away the riff-raff leaves the deck clear for hob-nobbing. Indeed, this may be the principal benefit of bodyguarding, on top of and in the wake of the original function to reduce female-female stress. Low-mate-value males are unlikely anyway to present a threat to the female in terms of unwanted conception, given rank-appropriate behavioural self-inhibition by males, backed up by community sanction as well as that by the pair-bond male. Being of greater genomic integrity (in this scenario), the pair-bond male is likely to be physically and in other ways superior (more confident, belligerent, part of an effective coalition, etc), so would be anticipated to win any fight. In this way, bodyguarding appears to be routinely facultative.
Its workings in this respect are tapped into by Hoplock, Stinson & Joordens (2019) in finding that “highly-attractive men engage in mate-poaching behavior (proximity seeking) when a romantically-involved woman is accompanied by a less-attractive boyfriend, but not when she is alone or accompanied by a boyfriend who matches her attractiveness”. Here, the redundancy and abandonment of bodyguarding in the scenario where it is not required by the female appears as if formalised, with high-mate-value males actually preferring the presence of the erstwhile bodyguard during extra-pair courting of women. Indeed, the tactic appears to be itself the very mode of courtship in attempts to poach pair-bonded females. High-mate-value males overtly usurp low-mate-value males with women partners they perceive may be in the market for extra-pair sex owing to substantial mate-value disparities. This is plainly what Moran, Kuhle, Wade & Seid (2017) discovered. And in this scenario, Fugère, Cousins & MacLaren (2015) find there is reciprocal behaviour from the female: more flirting, feeling less committed to the pair-bond and unconstrained by (what the authors term) mate-guarding.
Bodyguarding has been misconstrued as mate-guarding
Fugère, Cousins & MacLaren appear to misconstrue bodyguarding as instead mate-guarding, which is understandable in its being a far more familiar conceptualisation. A supposed defence of the female not in the female’s but the male guarder’s own interests: and of the female from all social-sexual approach (supposedly from high-mate-value as well as from low-mate-value male potential mate-poachers). It is a thus conceptualised defence that the aforementioned Lumkin (1983) suggested was usurped in the guarded female’s interests to become defence specifically against unwanted male approaches: what is herein termed bodyguarding. Mate-guarding is a notion based on a chain of assumptions. The assumed male need to be assured of paternity, as a consequence of assumed male investment in offspring. Underlying all is the default assumption of male proprietorial control of the female, which appears to be inappropriate incursion of contemporary ideology — that only male interests ever are served. This notion has no scientific basis and is flatly contradicted by a diametrically opposite insight from biology: the fact that the female is the limiting factor in reproduction dictates that the female interest must be preferenced, as indeed it is.
The human mating system did not evolve from a need for male investment in offspring. Not only is evidence for paternal investment equivocal (for a very brief review, see Moxon, 2013, p27), but as is now well evidenced, modelled and very widely agreed, any paternal investment that may have evolved did so in the wake of the evolution of the human mating system, so cannot be the basis of it. This was the conclusion of an across-species (including human) key investigation by Brotherton & Komers (2003), and specifically regarding humans by Chapais (2008). Chapais (2011) concludes that human pair-bonding originated as “a pre-adaptation for the evolution of parental cooperation in the provisioning of progressively altricial (helpless) children”. Similarly outlining a primate phylogeny of pair-bonding, Geary & Bailey (2011) conclude that the emergence of a specifically human mating pattern merely coincided with increasing male provisioning. Opie et al (2013) applied phylogenetic comparative methods widely to 230 different species, confirming an antecedent evolutionary pathway. Lukas & Clutton-Block (2013) confirm this.
With the evidence overwhelming that male provisioning was not the basis of the human mating system, then the notion that its foundation is the need for the male to be assured of his paternity (in order to protect his investment) evaporates. There is no investment to protect. As it is the male’s investment that is taken to be the reason why males supposedly persist in trying to prevent the female partner from mating with other males, then such attempts would be pointless. The evidence indeed is that mate-guarding does not work — that is, what is presumed to be mate-guarding actually does not thus function. It fails so badly that by some measures extra-pair paternity is actually greater with mate-guarding, according to Kempenaers, Verheyen & Dhondt (1995), who conclude: “despite mate guarding, paternity seems to be largely under female control and unattractive males guarding their mate are making the best of a bad situation”. Little if any relationship exists between the strength of mate-guarding and paternity in avian species (Johnson & Burley, 1998), despite their seeming strict monogamy. Kokko & Morrell (2005) ask “if females regularly escape mate-guarding attempts, we face an enigma: why does mate–guarding evolve if it is so inefficient?“ The answer is because the male is not guarding his own interests but the female’s, as in bodyguarding. What has been taken to be mate-guarding of the male’s interests would appear in fact — as outlined above — to deter social-sexual advance only by males of lower mate-value (lesser genomic integrity) — that is, lower than that of her pair-bonded partner — who would not serve the female’s interests to accept, and surely she would not want. In marked contrast, higher mate-value (greater genomic integrity) suitors — that is, higher than that of her pair-bonded partner — would be in the female’s interests to accept, if the mate-value difference were significant enough to outweigh the risk to the endurance of the pair-bond. Again to reiterate: even if the male wished to try to prevent such mating, ultimately he would not be able to do so, as the adverse difference in ranking would be reflected in the suitor’s greater fighting ability or coalitional strength. Not that an agonistic encounter likely would occur: it would be obviated by the psychological and physiological mechanisms that have evolved in the service of hierarchy.
Infanticide prevention is a red herring
Not considered bodyguarding, though which might be thought a putative special form of it, is infanticide prevention: supposed male defence of a female to keep at bay other males who might kill unweaned offspring so as to prompt the female to end lactation and thereby resume cycling and fecundability. As a possible basis of the evolution of pair-living / -bonding it’s in major doubt even in principle. Supposedly to cut her losses in anticipation of infanticide in a male takeover scenario, a female may spontaneously abort a foetus (what is dubbed the Bruce effect), but again there is here a failure to take account of male heterogeneity. With the new male necessarily of a higher genomic integrity sufficient to have been able to effect takeover, the female gains in fertility, which quickly manifests in improved reproductive output in quality-x-quantity terms, given an immediate return to cycling, and then not just a replacement offspring in short order, but subsequent offspring of the same high genomic integrity. The imperative to maximise purging manifests again. Not only has the female not sustained any net cost, but has benefited. It is anyway frequently acknowledged how extraordinarily difficult it is to find evidence of the occurrence of infanticide, let alone of its prevention, in any species.
Infanticide prevention as the foundation of monogamy in humans and primates generally is championed most recently by Opie et al (2019), but their phylogenetic analysis considers only a highly restricted set of other alternative key factors as hypotheses (just parental care and a distribution of females too thin for males practically to have harems, with neither of these incorporating male heterogeneity & female choice), so the analysis is set up as a forced choice where only infanticide prevention is likely to be supported. Fernandez-Duque, Huck, Van Belle & Di Fiore (2020) in reviewing competing hypotheses take particular issue with the infanticide prevention hypothesis, citing a phylogenetic analysis (Lukas & Clutton-Brock, 2013) completely at odds with Opie’s earlier outline (Opie, Atkinson, Dunbar & Shultz, 2013) — which last has been the subject of rebuttals and counters to counter-rebuttals — going into considerable detail about poor methodology. Fernandez-Duque et al explain that support for the infanticide prevention hypothesis (though here not alone among evolutionary explanations in sharing the same fault) is a case of the fallacy known as affirming the consequent. Making a poor case for its significance are unsatisfactory proxies of infanticide risk, that supposedly is addressable by the adaptation of pair-living/ -bonding. But the existence of the adaptation is itself the basis of inferring infanticide prevention is the problem the adaptation evolved to solve.
If infanticide were a significant problem for such as the gorilla, Dunbar (2020) points out, then fertility decline of females would be in line with increasing numbers of males, not exclusively through the increase in the number of females (as is found). Instead, infanticide in the wake of takeovers always remains a risk. If a harem’s male is deposed, there is no other male to defend against the usurping male if he chooses to be infanticidal. In the case of the harem male still being in place, there is no logical rationale for infanticide in the first place. There would be no benefit for another male to attempt to bring back into cycling a lactating harem female belonging to a harem still with its male, because the long delay between ending lactation and resuming cycling means the opportunity to capitalise on the initiative would be in the future, when the harem male would have to be challenged anew for the sexual access. Unless the male challenger has the wherewithal to take over the harem, then he’s unlikely to achieve sexual access, and the female won’t be co-operative in any case. This is to return to the situation in bodyguarding, where males of lower mate value than the pair-bonded (or harem) male are easily dissuaded from making socio-sexual advances, and indeed, would internalise this as self-inhibition. A male of substantially higher mate value than the pair-bond (or harem) male is a different proposition. His takeover would be part-and-parcel of the species’ reproductive system, serving to increase reproductive output and efficiency, and the only individual who loses out is the deposed lower-mate-value male. A female harem member may suffer the immediate loss of a newborn through infanticide, but this is not a loss in even slightly wider perspective — just as in the spontaneous abortion scenario (above) — as it’s a clear gain in the greater genomic integrity provided by the new harem male, which will be embodied in an immediate replacement offspring if one were to be lost to infanticide, and, more importantly, several subsequent offspring. Even if the harem male is replaced simply through his aging, this too is of significant benefit to the harem females in that the likelihood of gene replication error in the male’s gametes will have increased dramatically. It’s hard to see how the female would benefit in being protected from the threat of infanticide even if it were real.
Whether as social-sexual advance or attempting infanticide, the concern that lower-mate-value males may gain the system, as it were, is misplaced, as these are the very males not in a position to do so. By contrast, higher-mate-value males, who are of actual benefit to females, are not gaining the system, being very much a part of how the mating system works, which is to the benefit of females generally and only for a minority of males. The issue of infanticide-prevention and the scope for a male service to the female in this regard is a theoretical problem only — if that, in that the theory appears awry.
Another apparent misconception regarding infanticide prevention is that concealed ovulation serves this function. As already outlined, modelling now shows instead that it reduces female-female aggression (Krems et al, 2021). It has long been claimed that the most parsimonious conclusion about concealed ovulation is that it is not an adaptation at all: simply loss of function through the absence of selection pressure (Burt, 1992; Pawlowski, 1999). It is an untested presumption that concealed ovulation causes paternity uncertainty, sufficient that as a consequence a potentially infanticidal male sees a significant risk of unintentionally killing his own offspring.
Furthermore, strangely there has not been considered that concealed ovulation is of most utility to females in reducing the benefits and increasing the costs to lower-mate-value males in their social-sexual approaches in attempt to usurp pair-bonding. Together with what anyway are the difficulties for low-mate-value males in approaching a bodyguarded non-receptive female, the strategy of sneak mating attempts would not be worth the risks if it were made impossible to time them to when the female is fertile. In thus augmenting bodyguarding in keeping lower-mate-value males at bay, then again, as with the bodyguard, thereby is allowed mutual social-sexual approach of higher-mate-value males and pair-bond females. For a higher-mate-value extra-pair male, concealment of ovulation is not an issue. On the contrary, ovulation prompts female proceptive and receptive behaviour towards them, notwithstanding its not being specifically so: nevertheless it would be in this context — of extra-pair sex with a substantially higher-mate-value male being in the offing. There’s another advantage. With the female allowing, indeed encouraging him to be close, he is in a position to detect subtle indications of impending ovulation (eg, Lobmaier et al, 2018) and sexual arousal (Wiseman & Shrira, 2020), that a lower-mate-value male, who, in not being allowed close and instead positively distanced, could not. He will never be in a position to increase the probability of conception to sufficiently offset the likely costs, to render a bid for a sneak mating worth the risk. Further disadvantaging lower-mate-value suitors may explain why human coitus itself is concealed. With the timing of sex a good indicator of likely heightened female sexual receptivity prompted by ovulation, its discovery by a lower-mate-value male would encourage his social-sexual approach in the immediate future so as still to be within the female’s fertile window. Not concealing coitus would be provocative and potentially very destabilising, in line with the interpretation of new data by Yitzchak (2020) that hidden coitus serves to maintain cooperation with those within the group who are prevented from mating.
Placing infanticide prevention at the heart of the aetiology of the human mating system appears to be unwarranted inference, evoked as a default hypothesis through a false understanding that essentially humans are monogamous and that (as aforesaid) this has a seeming fivefold opportunity cost compared to polygyny, as might thus be considered obligate monogamy. In the human case, at least, with monogamy not being obligate, and simply a reduced form of polygyny for those males not able to secure multiple females, there is no opportunity cost to sustain. The misconception at root is through the failure to appreciate the centrality of male heterogeneity and female choice. Infanticide prevention is proffered for want of an alternative in the paucity of hypothesis generation that stems from mis-framing what is here at issue.
With pair-bonding to ensure repeat offspring of the greatest-possible genomic integrity (least mutationally compromised), featuring bodyguarding to dissuade low-mate-value male social-sexual approach, thereby actually facilitating approach by/to high-mate-value males for extra-pair sex; all while buffering against female age-related fertility decline, and built on a pre-adaptation of interposed males to reduce female-female stress … the overall picture of the basis of the human mating system is one of service by the male to the female. Yet neither provisioning (the erstwhile assumed mode of such service) nor infanticide-prevention (the still current oft-assumed mode) is its basis. Instead, it’s the set of ramifications from keen female choice and profound heterogeneity of males, in turn from the imperative to purge relentlessly accumulating gene replication error.
Service by the male is investigated in a new review of the evolution of monogamy in a primate model by Dolotovskaya, Walker & Heymann (2020) by pitching against each other three categories of hypotheses: male-services, resource-defence and mate-defence — this last being mate-guarding. The authors conclude that “Our data is most consistent with the ‘male-services’ hypothesis for pair-bond maintenance, where a female contributes more to the proximity and affiliation maintenance while a male provides beneficial services”. Note the authors are cautious in making their claims in respect of maintenance rather than origin per se, because they include care of infants, which they concede evolved in the wake of the evolution of pair-bonding. The principal male services they ascribe to protection from predation and territorial defence, though these are not from pair-bonding but are the platform on which it was possible. What remains may be simply the way male heterogeneity and female choice play out.
Male services, with the female the party keeping the pair together, chimes with the data on mate-retention tactics showing that most are women’s or mostly used by women (for a brief outline and citations, see Moxon 2020, p10), and the research concerning the various other ways women in comparison to men reveal a far greater concern for and interest in their pair-bond (for a referenced account, see Moxon 2016, pp73-75). Of romantic attachment itself, males but not females are dismissive — in a near universal sex difference (Schmitt, 2003). All lines of evidence converge on a conclusion that human pair-living/-bonding is not at all the presumed proprietorial control by men of women. Indeed, it appears more the converse. Objectively, the human mating system is just that; a system, in which all individuals are engaged to play a part in what is a group-level phenomenon, with the goal of maximising overall reproductive output/efficiency. This is rather too obvious understanding to have been lacking — though still there are those hidebound by the outdated group selection debate, unaware of the now multiple complementary theoretical perspectives on how/why mutualism works. Appreciating the purging imperative driving the profound skew amplifying male heterogeneity, and even accepting that profound male heterogeneity exists, may be a different matter. Yet (the pre-adaptation of reducing female-female stress aside) a highly parsimonious hypothesis unfolds from the one premise of purging.
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