Sexual Selection in Primates – New and comparative perspectives (I)
Okay, here’s the short version: This book is awesome – I gave it five stars and added it to my list of favourites on goodreads.
It’s the second primatology text I read this year – the first one was Aureli et al.; my coverage of that book can be found here, here and here. I’ve also recently read a few other texts as well which have touched upon arguably semi-related themes; books such as Herrera et al., Gurney and Nisbet, Whitmore and Whitmore, Okasha, Miller, and Bobbi Low. Some of the stuff covered in Holmes et al. turned out to be relevant as well. I mention these books because this book is aimed at graduates in the field (“Sexual Selection in Primates is aimed at graduates and researchers in primatology, animal behaviour, evolutionary biology and comparative psychology“), and although my background is different I have as indicated read some stuff about these kinds of things before – if you know nothing about this stuff, it may be a bit more work for you to read the book than it was for me. I still think you should read it though, as this is the sort of book everybody should read; if they did, people’s opinions about extra-marital sex might change, their understanding of the behavioural strategies people employ when they go about being unfaithful might increase, single moms would find it easier to understand why their dating value is lower than that of their competitors without children, and new dimensions of friendship dynamics – both those involving same-sex individuals and those involving individuals of both sexes – might enter people’s mental model and provide additional angles which might be used by them to help explain why they, or other people, behave the way they do. To take a few examples.
Most humans are probably aware that many males in primate species quite closely related to us habitually engage in activities like baby-killing or rape, and that they do this because such behavioural strategies lead to them being more successful in the fitness context. However they may not be aware that females of those species have implemented behavioural strategies in order to counteract these behaviours; for example females may furtively sleep around with different males in order to confuse the males about who’s the real father of their offspring (you don’t want to kill your own baby), or they may band up with other females, and/or perhaps a strong male, in order to obtain protection from the potential rapists. I mention this in part because a related observation is that it should be clear from observing humans in their natural habitat that most human males are not baby-killers or rapists, and such an observation might easily lead people who have some passing familiarity with the field to think that a lot of the stuff included in a book like this one is irrelevant to human behaviour; a single mom is unlikely to hook up with a guy who kills her infant, so this kind of stuff is probably irrelevant to humans – we are different. I think this is the wrong conclusion to draw. What’s particularly important to note in this context is that counterstrategies are reasonably effective in many primate species, meaning for example that although infanticide does take place in wild primate species, it doesn’t happen that often; we’ve in some respects come a bit further than other species in terms of limiting such behaviours, but in more than a few areas of social behaviour humans actually seem to act in a rather similar manner to those baby-killing rapists and their victims. It’s also really important to observe that sexual conflict is but one of several types of conflicts which organisms such as mammals face, and that the dynamics of such conflicts and aspects like how they are resolved have many cross-species similarities – see Aureli et al. for an overview. It’s difficult and expensive to observe primates in the wild, but when you do it it’s not actually that hard to spot many precursors of- or animal equivalents of various behaviours that humans engage in as well. Some animals are more like us than people like to think, and the common idea that humans are really special and unique on account of our large brains may to some extent be the result of a lack of knowledge about how animals actually behave. Yep, we are different, but perhaps not quite as different as people like to think. Some of the behaviours we like to think of as somehow ‘irreducible’ probably aren’t.
Observations included in a book like this one may well change how you think about many things humans do, at least a little. Humans who are not sexually active have the same evolutionary past as those that are, which means that their behaviours are likely to be and have been shaped by similar mechanisms – an important point being that if even someone like me, who at the moment consider it a likely outcome that I’ll never have sex during my lifetime, is capable of finding stuff covered in a book such as this one to be relevant and useful, there are probably very few people who wouldn’t find some of the stuff in there relevant and useful to some extent. Modern humans face different decision variables and constraints than did our ancestors, but the brains we’re walking around with are to a significant extent best thought of as the brains of our ancestors – they really haven’t changed that much in, say, the last 100.000 years, and some parts of the ‘code’ we walk around with are literally millions of years old. You need to remember to account for stuff like birth control, ‘culture’ and institutions when you’re dealing with human sexual behaviours today, but a lot of other stuff should be included as well, and books like this one will give you another piece of the puzzle. An important piece, I think.
Although there’s a limited amount of mathematics in this book (mostly limited to an infanticide model in chapter 8), as you can imagine given the target group the book is really quite dense. There’s way too much good stuff in this book for me to cover all of it here, and I don’t know at this point how detailed my coverage of the book will end up being. A lot of details will be left out, regardless of how many posts I decide to give this book – more than a few chapters are of such high quality that I could easily devote an entire post to each of them. If the stuff I include in my posts sparks your interest, you’ll probably want to read the rest of the book as well.
“In this review I have emphasised five points that modern students of sexual selection ought to keep in mind. First, the list of mechanisms of sexual selection is longer than just the two most famous examples of male-male combat and female choice. Male mate choice and female-female competition are two frequently noted possibilities. Other between-sex social interactions that can result in sexual selection include male coercion of females […] and female resistance to male coercion or manipulation […] sexual selection among females should be as important as male sexual selection to dynamical interactions between the sexes. Sexual selection among females will favour resistance to male attempts to manipulate and control them […] Second, even when a mechanism of intersexual selection depends on interactions between members of opposite sexes, the important thing for selection is the variance in reproductive success among members of one sex. Think about female mate choice for a moment. Whenever choosers discriminate, mate choice may cause variation among the chosen in mating and reproductive success […] Thus, mate choice is a mechanism of sexual selection because it theoretically results in variance among individuals of the chosen sex in mating success and perhaps other components of fitness. […] Third, sexual selection can result in individual tradeoffs among the components of fitness […] Fourth, for a trait to be under selection, there must be variation in the trait. For sexual selection to operate the trait variation must be among individuals of the same sex. […] To argue that an opportunity for sexual selection exists, variation among same-sex individuals in reproductive success must exist. Fifth, between-sex variances in reproductive success alone are […] an insufficient basis for the conclusion that sexual selection operates […], as within-sex variances may arise because of random, non-heritable factors”
“In summary, sex roles fixed by past selection from anisogamy or from parental investment patterns so that females are choosy and males indiscriminate are currently questionable for many species. The factors that determine whether individuals are choosy or indiscriminate seem relatively under-investigated.” (One factor which does seem to be important is the encounter frequency with potentially mating opposite-sex individuals; this variable (how often do you meet a potential partner?) has been shown to affect the sexual behaviours of individuals in species as diverse as fruit flies, fish and butterflies).
“Because most primates live in stable, long-lasting social groups, pressures for direct sexually selected communication cues may be less than in species with ephemeral mating groups or frequent pairings. Primates are likely to accumulate information about competitors and mates from many sources over a longer time frame. […] Although there do appear to be some communication signals that may be sexually selected, it may be best to consider these signals as biasing factors rather than the determinants of mate choice. For primates, human and non-human, as well as for Japanese quails, gerbils, rats and blue guramis, there is more to successful reproduction than simply responding to a sexually selected cue. Although I might be initially attracted to a woman with the ‘correct’ breast-to-waist-to-hip ratios, a symmetric face and all of the other hypothesised sexually selected cues, I will quickly learn if she is intelligent or not, if she is emotionally stable, and many other things that should be more important in my reproductive decisions than mere appearance. It is important to keep this in mind in any discussion of sexual selection. […] The strongest evidence, so far, for intersexual selection of traits is observed in female primates, suggesting that male mate choice and female competition may be as important as male competition and female mate choice. […] The data suggest that intersexual selection is as strong if not stronger on female primates than on males.” [As should be very clear at this point, male primates do have standards, despite what the third cartoon at the beginning of this post would have you believe…]
“One form of polyandry that has received much attention is extra-pair copulation (EPC) – sex that a female with a social mate has with a male who is not the social mate. […] Because an evolved adaption is a product of past direct selection for a function, the question of whether EPC by women is currently adaptive or currently advances women’s reproductive success (RS) is a distinct one. An evolved adaption may be currently non-adaptive and even maladaptive because the current ecological setting in which it occurs is different from the evolutionary historical setting that was the selection favouring it […] Female EPC is not a rare occurence in humans. […] Female EPC may be a relatively common occurence now. But was it sufficiently common in small ancestral populations of humans or pre-hominid primates to be an effective selective force of evolution? Evidence suggests yes, and perhaps the best evidence comes from design features of men rather than women. Men, but not women, can be duped about parentage as a result of EPC, leading to the unknowing investment in another man’s offspring. Men show a rich diversity of mate guarding and anti-cuckoldry tactics ranging from sexual jealousy, vigilance, monopolising a mate’s time, pampering a mate, threatening a mate with harm if she shows interest in other men, and adjusting ejaculate size to defend against the mate’s insemination by a competitor […] Some mate guarding tactics appear to be conditional, such that men guard mates of high fertility status (young or not pregnant) more intensely than ones of low-fertility status (older or pregnant) […] and hence appear not to be caused by general male-male competitive strivings but rather concern for fidelity of a primary social mate […] We […] asked women in [a] study to report their primary mate’s mate-retention tactics. Our questionnaire measures two major dimensions, ‘proprietariness’ and ‘attentiveness’. Women reported their partners to be higher on both when fertile [i.e., mid-cycle].”
“Women’s preferences shift across the [menstrual] cycle in a number of ways. They particularly prefer the scent and faces of more symmetrical men when fertile. The face they find most attractive when fertile is more masculine than the face they most prefer when not fertile. They prefer more assertive, intrasexually competitive displays when fertile than when not. [An example: “The behaviours of men being interviewed by women for a lunch date were coded for a host of verbal and non-verbal qualities [by Gangestad et al.]. Through principal components analysis of these codes, two major dimensions along which men’s performance varied were identified; ‘social presence’, marked by a man’s composure, his direct eye contact and lack of downward gaze, as well as a lack of self-deprecation, and emphasis that he’s a ‘nice guy’; and ‘direct intrasexual competitiveness’, marked by a man’s explicit derogation of his competitor and statements to the effect that he is the better choice, as well as not being obviously agreeable.”] Furthermore, evidence indicates that their preferences when evaluating men as sex partners (i.e. their sexiness) is particularly affected; evidence shows that their evaluations of men as long-term partners shift little, if at all. […] symmetrical men appear to invest less time in and are less faithful to their primary relationship partners […] [The] pattern of findings suggests that it is not simply the case that all traits preferred by females are particularly preferred mid-cycle; that fertility status simply enhances existing preferences. Rather, it appears that only specific preferences are enhanced – perhaps those for features that ancestrally were indicators of genetic benefits. Preferences for features particularly important in long-term investing mates may actually be more prominent outside the fertile period.”
“STDs typically have been viewed as a curious group of parasites rather than established entities with important selective effects on their hosts […]. In recent decades, this view has changed, primarily through our increased understanding of HIV […] [There are] at least three major costs of STDs: (1) A large proportion of STDs increase the risk of sterility in males and females. (2) STDs commonly exhibit vertical transmission, with severe consequences for offspring health [see also this – Holmes et al. covers this stuff in some detail and actually the authors refer to an older version of that book in this context]. (3) Relative to infectious disease transmitted by non-sexual contact, STDs commonly exhibit long infectious periods with low host recovery, failure to clear infectious organisms following recovery, or limited immunity to reinfection. […] Many negative consequences of STD infection probably provide benefits to the parasites themselves, increasing the likelihood of invasion, transmission and persistence […] In mammals, for example, host infertility is likely to result in repeated cycling by females and may consequently increase their number of sexual contacts. [Mind blown! I’d never even thought about this.] Primates offer an important opportunity to test this hypothesis, because the frequency of infertile females within wild groups may exceed 10 per cent […]. Similarly, STDs that increase host mortality or possess short infectious periods are less likely to survive until the next breeding season, when contact is established with new, uninfected hosts […] Thus, in addition to long infectious periods, STDs tend to produce less disease-induced mortality relative to other infectious diseases”
“Because sexual reproduction offers an important mechanism for disease spread and may even be influenced by infection status, it is pertinent to ask whether animals can identify infected individuals and avoid mating with them. Symptoms such as visible lesions, sores, discharge around the genitalia or olfactory cues may provide evidence of infection. […] many human STDs are […] characterized by limited symptoms or, in the case of viruses, asymptomatic shedding […] reproductive success of an STD is correlated with partner exchange and successful matings of infected hosts. Therefore, virulent parasites that produce outward signs of infection will experience decreased transmission because they provide conspicuous cues for choosy members of the opposite sex to avoid infected mates. […] A parasite faces two main barriers, or defences, imposed by the host: behavioural counter-strategies to avoid exposure, and physical or immune defences […]. The order of events can vary, but behavioural mechanisms commonly are viewed as the first line of defence. An important point we wish to emphasise is that host behaviour to avoid exposure prior to mating is likely to have other reproductive costs, and these costs may outweigh their benefits. […] male and female behaviour indicates that STD risk is of secondary importance relative to other selective pressures operating on mating success. Females mate polyandrously to reduce infanticide risk […] and, for similar reasons, they prefer novel males, though risking infection with STDs acquired from other social groups. Males prefer females of intermediate age that have already produced offspring, as these females have high reproductive value […]. Both sets of decisions by males and females are expected to increase exposure to STDs by increasing the number of partners and mating events.”
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