Gauri R. Pradhan

Sexual Selection in Primates: Mating conflict in primates: infanticide, sexual harassment and female sexuality

INTRODUCTION In a variety of mammals and a few birds, newly immigrated or newly dominant males are known to attack and kill dependent infants (Hausfater & Hrdy, 1984; Parmigiani & vom Saal, 1994; van Schaik & Janson, 2000). 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 mothers next oestrus and, owing to their new social position, would have a reasonable probability of siring this females next infant. Infanticide would therefore be one of the most dramatic expressions of sexual conflict (Smuts & Smuts, 1993; Gowaty, 1997, this volume). Although this interpretation, and indeed the phenomenon itself, has been hotly debated for decades (e.g. Dolhinow, 1977; Boggess, 1984; Bartlett et al ., 1993; Sussman et al ., 1995), on balance, this hypothesis provides a far better fit with the observations on primates than any of the alternatives (cf. van Schaik, 2000a). First, several detailed studies showed that the males never attacked or killed their own offspring (Borries et al ., 1999; Soltis et al ., 2000), in accordance with the more anecdotal information compiled from all directly observed cases of infanticide in the wild (van Schaik, 2000a). Second, several large-scale studies have estimated that the time gained by the infanticidal male amounts to 25 per cent, 26 per cent and 32 per cent of the mean interbirth interval (Crockett & Sekulic, 1984; Sommer, 1994; Borries, 1997).

Social organization and the evolution of cumulative technology in apes and hominins

Culturally supported accumulation (or ratcheting) of technological complexity is widely seen as characterizing hominin technology relative to that of the extant great apes, and thus as representing a threshold in cultural evolution. To explain this divide, we modeled the process of cultural accumulation of technology, which we defined as adding new actions to existing ones to create new functional combinations, based on a model for great ape tool use. The model shows that intraspecific and interspecific variation in the presence of simple and cumulative technology among extant orangutans and chimpanzees is largely due to variation in sociability, and hence opportunities for social learning. The model also suggests that the adoption of extensive allomaternal care (cooperative breeding) in early Pleistocene Homo, which led to an increase in sociability and to teaching, and hence increased efficiency of social learning, was enough to facilitate technological ratcheting. Hence, socioecological changes, rather than advances in cognitive abilities, can account for the cumulative cultural changes seen until the origin of the Acheulean. The consequent increase in the reliance on technology could have served as the pacemaker for increased cognitive abilities. Our results also suggest that a more important watershed in cultural evolution was the rise of donated culture (technology or concepts), in which technology or concepts was transferred to naïve individuals, allowing them to skip many learning steps, and specialization arose, which allowed individuals to learn only a subset of the populations skills.

Infanticide-driven intersexual conflict over matings in primates and its effects on social organization

In many species, newly immigrated or newly dominant males are known to attack and kill dependent infants they have not sired. One of the female counter-strategies against this adaptive male infanticide in group-living taxa is polyandrous mating. A mathematical model demonstrates the existence of a basic conflict of interest between the dominant male and the female. We model the intensity of this conflict, and the pressure to evolve counter-strategies, in relation to (i) the risk of takeover of top dominance by males from inside the group rather than outside and (ii) variation in the relative strength of the dominant male and, thus, expected length of future tenure. The model predicts that dominant males prefer single-male groups, or failing that, prefer multi-male groups with takeovers by outside males. Females, in contrast, generally prefer to live in multi-male groups with takeovers by inside males, but prefer single-male groups if dominant males are extremely powerful. Empirical data suggest that females can control adult group composition, but cannot control either the source of takeovers or relative male strength. The main conclusion is that intersexual conflict in the form of infanticide may over time affect the social system in which a species lives.

Infanticide and reproductive restraint in a polygynous social mammal

Alpha male chacma baboons experience uncontested access to individual estrus females. Consequently, alpha male paternity certainty is high and underpins significant levels of infanticide by immigrant males that, in turn, has selected for male defense of infants. There is also, however, a high probability that alpha males will be absent during the period when their own offspring are vulnerable, suggesting selection for additional countermeasures. We use data from a long-term study to test the prediction that alpha male chacma baboons cede reproductive opportunities to subordinate males and that this leads to the presence of other fathers that can serve as a buffer against infanticidal attack. We found that subordinate males obtained significantly more conceptive opportunities than predicted by priority of access alone, and that this occurred because alpha males did not consort all receptive periods. There was no evidence that this was due to energetic constraint, large male cohorts, alpha male inexperience, or the competitive strength of queuing subordinates. The number of males who benefited from concession and the length of time that they were resident relative to those who did not benefit in this way greatly reduced the probability that infants of alpha males would face immigrant males without a surrogate father whose own offspring were vulnerable. The absence of such males was associated with observed infanticide as well as, unexpectedly, an increased likelihood of takeover when alpha males with vulnerable infants were present.

Why do chimpanzee males attack the females of neighboring communities

Our closest nonhuman primate relatives, chimpanzees, engage in potentially lethal between-group conflict; this collective aggressive behavior shows parallels with human warfare. In some communities, chimpanzee males also severely attack and even kill females of the neighboring groups. This is surprising given their system of resource defense polygyny, where males are expected to acquire potential mates. We develop a simple mathematical model based on reproductive skew among primate males to solve this puzzle. The model predicts that it is advantageous for high-ranking males but not for low-ranking males to attack females. It also predicts that more males gain a benefit from attacking females as the communitys reproductive skew decreases, i.e., as mating success is more evenly distributed. Thus, fatal attacks on females should be concentrated in communities with low reproductive skew. These attacks should also concur with between-community infanticide. A review of the chimpanzee literature provides enough preliminary support for this prediction to warrant more detailed testing.

The Conditions Favoring Between-Community Raiding in Chimpanzees, Bonobos, and Human Foragers

Chimpanzees, bonobos, and human foragers share a fission-fusion social system and a mating system of joint male resource defense polygyny. Within-community skew in male strength varies among and within species. In this study, we extend a mathematical model of within-group male coalition formation among primates to derive the conditions for between-community conflicts in the form of raids. We show that the main factor affecting the presence of successful raiding is the likelihood of major discrepancies in party strength, which are set by party size distributions (and thus community size) and the skew in strength. This study confirms the functional similarities between the raiding of chimpanzees and human foragers, and it supports the “imbalance of power” hypothesis for raiding. However, it also proposes two amendments to this model. First, the absence of raiding in bonobos may be attributable more to potential female involvement in defense against raids, which increases the size of defensive coalitions. Second, the model attributes some of the raiding in humans to major contrasts in instantaneous fighting ability created by surprise raids on unarmed victims; it also draws attention to the distinction between minor raids and major raids that involve multiple bands of the same community.

A model for the evolution of developmental arrest in male orangutans

Male Sumatran orangutans (Pongo abelii) may delay for many years the acquisition of the full array of secondary sexual traits, including their characteristic cheek flanges. Such flexible developmental arrest is unique among male primates. Among male Bornean orangutans (Pongo pygmaeus) such long delays appear less common. Here, we develop a simple model to identify the conditions under which developmental arrest can be adaptive. We show that the baseline strategy (i.e., males are not susceptible to arrest) cannot be invaded by the flexible strategy (i.e., males can arrest their development when the conditions are unfavorable) when the potential for high-ranking unflanged or flanged males to monopolize sexual access to females is low. In contrast, at high monopolization potential, the flexible strategy is the evolutionarily stable strategy. We also derive the proportion of flanged males in the population for each combination of monopolization values. This model concurs with field data that found a different monopolization potential between Bornean and Sumatran flanged males and a lower proportion of flanged males in the population in Sumatran orangutans. Pronounced developmental arrest is linked to very low adult mortality, which explains why it is so limited in its taxonomic distribution.

A model for warfare in stratified small-scale societies: The effect of within-group inequality

In order to predict the features of non-raiding human warfare in small-scale, socially stratified societies, we study a coalitionary model of war that assumes that individuals participate voluntarily because their decisions serve to maximize fitness. Individual males join the coalition if war results in a net economic and thus fitness benefit. Within the model, viable offensive war ensues if the attacking coalition of males can overpower the defending coalition. We assume that the two groups will eventually fuse after a victory, with ranks arranged according to the fighting abilities of all males and that the new group will adopt the winning group’s skew in fitness payoffs. We ask whether asymmetries in skew, group size and the amount of resources controlled by a group affect the likelihood of successful war. The model shows, other things being equal, that (i) egalitarian groups are more likely to defeat their more despotic enemies, even when these are stronger, (ii) defection to enemy groups will be rare, unless the attacked group is far more despotic than the attacking one, and (iii) genocidal war is likely under a variety of conditions, in particular when the group under attack is more egalitarian. This simple optimality model accords with several empirically observed correlations in human warfare. Its success underlines the important role of egalitarianism in warfare.