Sexual Selection in Primates – New and comparative perspectives (II)

You can read my first post about the book here. Let’s talk some more about what we can learn from this publication…

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“In a variety of mammals and a few birds, newly immigrated or newly dominant males are known to attack and kill dependent infants […]. Hrdy (1974) was the first to suggest that this bizarre behaviour was the product of sexual selection: by killing infants they did not sire, these males advanced the timing of the mother’s next oestrus and, owing to their new social position, would have a reasonable probability of siring this female’s next infant. […] Although this interpretation, and indeed the phenomenon itself, has been hotly debated for decades […], on balance, this hypothesis provides a far better fit with the observations on primates than any of the alternatives […] several large-scale studies have estimated that the time gained by the infanticidal male amounts to [25-32] per cent of the mean interbirth interval […] Because males rarely, if ever, suffer injuries during infanticidal attacks, and because there is no evidence that committing infanticide leads to reduced tenure length, one can safely conclude that, on average, infanticide is an adaptive male strategy. […] Infanticide often happens when the former dominant male, the most likely sire of most infants even in multi-male groups […], is eliminated or incapacitated. […] dominant males are effective protectors of infants as long as they are not ousted or incapacitated.”

“Conceptually, we can distinguish two kinds of mating by females that may reduce the risk of infanticide. First, by mating polyandrously in potentially fertile periods, females can reduce the concentration of paternity in the dominant male, and spread some of it to other males, so that long-term average paternity probabilities will be somewhat below 1 for the dominant male and somewhat above 0 for the subordinates. Second, by mating during periods of non-fertility […], a female may be able to manipulate the assessment by the various males of their paternity chances, although she obviously cannot change the actual paternity values allocated to the various males. […] The basic prediction is that females that are vulnerable to infanticide by males should be actively polyandrous whenever potentially infanticidal males are present in the mating pool (i.e. the sexually mature males in the social unit or nearby with which the female can mate, in principle). There is ample evidence that primate females in vulnerable species actively pursue polyandrous matings and that they often engage in matings when fertilisation is unlikely or impossible […]. Indeed, females often target low-ranking or peripheral males reluctant to mate in the presence of the dominant males, especially during pregnancy. […] In species vulnerable to infanticide, females often respond to changes in the male cohort of a group with immediate proceptivity, and effectively solicit matings with the new (or newly dominant) male […] It is in the female’s interest to keep individual males guessing as to the extent to which other males have also mated with her […] Hence, females should be likely to mate discreetly, especially with subordinate males. […] We [expect] that matings between females and subordinate males tend to take place out of sight of the dominant male, e.g. at the periphery and away from the group […] it has been noted for several species that matings between females and subordinate males [do] tend to occur rather surreptuously”

“Even though most primates have concealed ovulations, there is evidence that they use various pre-copulatory mechanisms, such as friendships […] or increased proximity […] with favoured males, copulation calls that are likely to attract particular males […], active solicitation of copulations around the likely conception date […], as well as changes in chemical signals […]; unique vocalizations […]; sexual swellings […] and increased frequencies of particular behaviour patterns during the peri-ovulatory phase […] to signal impending ovulation and/or to increase the chances of fertilization by favoured males.” [Recall from the previous post also in this context that which males are actually ‘favoured’ changes significantly during the cycle].

“Thornhill (1983) suggested that females might exhibit what he called ‘cryptic female choice’ – the differential utilisation of sperm from different males. The term ‘cryptic’ referred to the fact that this choice took place out of sight, inside the female reproductive tract. […] Cryptic female choice is difficult to demonstrate [as] one has to control for all male effects, such as sperm numbers or differential fertilising ability […] Cryptic female choice in primates is poorly documented, even though there are theoretical reasons to expect it to be common. […] The strongest indirect evidence for a mechanism of cryptic female choice in primates is provided by the observation that females of several species of anthropoids (mostly macaques, baboons and chimpanzees) exhibit orgasm […] Physiological measures during artificially induced orgasms [have] demonstrated the occurence of the same vaginal and uterine contractions that also characterise human orgasm […] and are thought to accelerate and facilitate sperm transport towards the cervix and ovaries […] female orgasm was observed more often in macaque pairs including high-ranking males (Troisi & Carosi, 1998). A comparable effect of male social status on female orgasm rates has also been reported for humans […]. Orgasm therefore has the potential to be used selectively by females to facilitate fertilisation of their eggs by particular males […] This hypothesis is indirectly supported by the observation that female orgasm apparently does not occur among prosimians […], but rather among Old World primates, where the potential for coercive matings by multiple males is highest […]. Seen this way, female primate orgasm may therefore represent an evolutionary response to male sexual coercion that provided females with an edge in the dynamic competition over the control of fertilisation” [Miller’s account/explanation was quite different. I think both explanations are rather speculative at this point. Speculative, but interesting.]

“It has long been an established fact in ethology that interactions with social partners influence an individual’s motivational state and vice versa, and, through interactions, its physiological development and condition. For example, the suppression of reproductive processes by the presence of a same-sex conspecific has been documented for many species, including primates. […] The existence of a conditional [male mating] strategy with different tactics has been demonstrated in several species of mammals. To mention but one clear example: in savannah baboons, a male may decide what tactic to follow in its relationships with females after assessing what others do. Smuts (1985) has shown that dominant males follow a sexual tactic in which they monopolise access to fertile females by contest competition. A subordinate male may use another tactic. He may persuade a female to choose him for mating by rendering services to the female (e.g. protecting her in between-female competition) and thus forming a ‘friendship’ with the female. Similar variation in tactics has been found in other primates (e.g. in rhesus macaques, Berard et al., 1994).”

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…And there you probably have at least part of the explanation for why millions of romantically frustrated (…’pathetic’?) human males waste significant parts of their (reproductive) lives catering to the needs of women who already have a sexual partner and are not sexually interested in them – they might not even have been born were it not for the successful application of this type of sit-and-wait strategy on part of some of their ancestors in the past.

The chapter in question has a lot of stuff about male orangutans, and although it’s quite interesting I won’t go much into the details here. I should note however that I think most females will probably prefer the above-mentioned ‘sneaky’ male tactic (I should perhaps note here that in terms of the ‘sneakiness’ of mating strategies, females do pretty well for themselves as well. Indeed in the specific setting it’s not unlikely that it’s actually the females who initiate in a substantial number of cases – see above..) to the mating tactic of unflanged orangutans, which basically amounts to walking around looking for a female unprotected by a flanged male and then raping her when he comes across one. In one sample included in the book of orangutan matings taking place in Tanjung Puting national park (Indonesia), of roughly 20 matings by unflanged males recorded only 1 or 2 (it’s a bar graph) did not involve a female resisting. These guys are great, and apparently really sexy to the opposite gender… The ratio of resisting/not resisting females in the case of the matings involving flanged males was pretty much the reverse; a couple of rapes and ~18-19 unforced mating events. It should be noted that the number of matings achieved by the flanged and unflanged males is roughly similar, so judging from these data approximately half of all matings these female orangutans experience during their lives are forced.

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“Especially in long-lived organisms such as primates, a male’s success in competing for mates and protecting his offspring should be affected by the nature of major social decisions, such as whether and when to transfer to other groups or to challenge dominants. Several studies indicate dependence of male decisions about transfer and acquisition of rank on age and local demography […]. Likewise, our work on male long-tailed macaques […] indicated a remarkably tight fit between the behavioural decisions of males and expectations based on known determinants of success […], suggesting that natural selection has endowed males with rules that, on average, produce optimal life-history trajectories (or careers) for a given set of conditions. […] Most non-human primates live in groups with continuous male-female association [“Only a minority of about 10 per cent of primate species live in pairs” – from a previous chapter], in which group membership of reproductively active (usually non-natal) males can last many years. For a male living in such a mixed-sex group, dominance rank reflects his relative power in excluding others from resources. However, the impact of dominance on mating success is variable […] Although rank acquisition is usually considered separately from transfer behaviour and mating success, the hypothesis examined here is that they are interdependent […]. We predict that the degree of paternity concentration in the dominant male, determined by his ability to exclude other males from mating, determines the relative benefits of various modes of acquisition of top rank […], and that these together determine patterns of male transfer”

“the cost of inbreeding may cause females to avoid mating with male relatives […]. This tendency has been invoked to explain an apparent female preference for novel (recently immigrated) males”

“a male can attain top rank in a mixed-sex group in three different ways. First, he can defeat the current dominant male during an aggressive challenge […] Second, he can attain top rank during the formation of a new group[…] A third way to achieve top rank is by default, or through ‘succession’, after the departure or death of the previous top-ranking male, not preceded by challenges from other males”

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The chapter which included the above quotes is quite interesting, but in a way also difficult to quote from given the way it is written. They talk about multiple variables which may affect how likely a male is to leave the group in which he was born (for example if there are fewer females in the group, all else equal he’s more likely to leave); which mechanism he’s likely to employ in order to try to achieve top rank in his group, if that’s indeed an option (in small groups they always fight for the top spot and the dominant male will have a very dim view of other mature males trying to encroach upon his territory, whereas in large groups the dominant male is more tolerant of competitors and they’re much less likely to settle things by fighting with each other – the reason why fighting is less common is probably because the male in the latter group is in general unable to monopolize access to the females because of the size of the group, so you to some extent ‘gain less’ by achieving alpha male status), and when he’s likely to act (a young male is stronger than an old male and he can also expect to maintain his tenure as the top male for a longer period of time – so males who try to achieve top rank by fighting for it are likely to be young, whereas males who achieve top rank by other means tend to be older). Whether or not females reproduce in a seasonal pattern also matters. It’s obvious from the data that it’s far from random how and at which point during their lives males make their transfer decisions, and how they settle conflicts about who should get the top spot. The approach in that chapter reminded me a bit of optimal foraging theory stuff, but they didn’t talk about that kind of stuff at all in the chapter. Here’s what they concluded from the data they presented in the chapter:

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“We found not only variation between species but also remarkable variation within species, or even populations, in the effect of group size on paternity concentration and thus transfer decisions, as well as mode of rank acquisition and likelihood of natal transfer. This variability suggests that a primate male’s behaviour is guided by a set of conditional rules that allow him to respond to a variety of local situations. […] Primate males appear to have a set of conditional rules that allow them to respond flexibly to variation in the potential for paternity concentration. Before mounting a challenge, they assess the situation in their current group, and before making their transfer decisions they monitor the situation in multiple potential-target groups, where this is possible.”