Evidence of intralocus sexual conflict: physically and hormonally masculine individuals have more attractive brothers relative to sisters

Intralocus sexual conflict (IASC) occurs when sex-specific selection favors genes that increase fitness in one sex and decrease fitness in the other sex. The current study was designed to explore whether IASC occurs in humans. In a sample of siblings, we identified and measured sexually dimorphic traits and hormones within each sex that are related to fitness and are likely coded for by antagonistic genes: waist-to-hip ratio (WHR) and breast size in women, WHR and bicep size (an index of muscularity) in men, and estradiol (E) and testosterone (T) in both sexes. If these traits and hormones are coded for by genes under IASC, masculine or feminine expression of traits and hormones should differentially predict brothers' and sisters' fitness. Consistent with an IASC model, both men and women who were physically masculine for their sex reported higher mate value brothers relative to sisters. Similarly, in normal-weight individuals, E levels positively predicted the mate value of sisters relative to brothers and T levels positively predicted the mate value of brothers relative to sisters. We found no evidence that individuals with indicators of high genetic quality (i.e., physically masculine men and physically feminine women) share high mate value with all siblings, regardless of sibling sex. Results are novel and demonstrate for the first time that intralocus conflict in humans may influence the fitness of related individuals.     

Brothers and sisters

Data from the Kipsigis of Kenya are used to test two models for how parents invest in offspring, the Trivers-Willard and local resource competition/enhancement hypotheses. Investment is measured as age-specific survival, educational success, marital arrangements, and some components of property inheritance, permitting an evaluation of how biases persist or alter over the period of dependence. Changes through time in such biases are also examined. Despite stronger effects of wealth on the reproductive success of men than women, the survival of sons and daughters is not related to parental wealth. However, a Trivers-Willard effect characterizes educational investment: poor families show a greater concern for daughters’ (vis-à-vis sons’) schooling than do rich families, a trend that has increased over time. In regard to models of local resource competition and enhancement, men’s reproductive success decreases with number of brothers and increases with number of sisters; this pattern of competition with same-sex sibs and cooperation with opposite-sex sibs is not found among women. As predicted from these observations, parents show reduced investment in sons with a large number of brothers, and increased investment in sons with a large number of sisters. By contrast, investment in daughters is entirely unaffected by number of sisters and is influenced only in subtle ways by number of brothers. Levels of investment in relation to sibship size (irrespective of siblings’ sex) are highest for younger children of large sib sets. Discussion of the results in relation to those from other studies leads to three conclusions. First, predictive models for how investment biases vary across societies must consider a broad range of socioecological factors constraining parental options and payoffs. Second, the timing of investment biases within societies will be affected by the value of children and the costs of parental investment. Third, measures of investment appropriate for between-sex and between-class comparisons need careful attention. Each of these issues is brought to bear on the question of why, in contrast to so many other parts of the world, sex preferences are so muted in Africa.     

Costly signaling and cooperation.

We propose an explanation of cooperation among unrelated members of a social group in which cooperation evolves because it constitutes an honest signal of the member's quality as a mate, coalition partner or competitor, and therefore results in advantageous alliances for those signaling in this manner. Our model is framed as a multi-player public goods game that involves no repeated or assortative interactions, so that non-cooperation would be a dominant strategy if there were no signaling benefits. We show that honest signaling of underlying quality by providing a public good to group members can be evolutionarily stable, and can proliferate in a population in which it is initially rare, provided that certain plausible conditions hold, including a link between group-beneficial signaling and underlying qualities of the signaler that would be of benefit to a potential mate or alliance partner. Our model applies to a range of cooperative interactions, including unconditionally sharing individually consumable resources, participating in group raiding or defense, and punishing free-riding or other violations of social norms.     

Costly signaling and torch fishing on Ifaluk atoll

In this paper I evaluate the merit of costly signaling theory (CST) as a paradigm for understanding why men of Ifaluk atoll torch fish. I argue that torch fishing is a handicap that signals men's productivity. Consistent with CST, torch fishing is observed by the predicted audience (women), energetically costly to perform, and a reliable indicator of the frequency a man fishes during the trade wind season. Contrary to expectations of who should benefit from torch fishing and consequently participate, torch fishers are not primarily young and unmarried. Torch fishers, however, are predominately from the matriline that owns the canoe on which they fish, suggesting that torch fishing also signals the productivity of a matriline. Although these results support the possibility that torch fishing is a handicap, no data are presented which demonstrate that torch fishers achieve any gains from sending the costly signal. This shortcoming and other directions for future research on Ifaluk foraging decisions are discussed.     

Costly punishment prevails in intergroup conflict

Understanding how societies resolve conflicts between individual and common interests remains one of the most fundamental issues across disciplines. The observation that humans readily incur costs to sanction uncooperative individuals without tangible individual benefits has attracted considerable attention as a proximate cause as to why cooperative behaviours might evolve. However, the proliferation of individually costly punishment has been difficult to explain. Several studies over the last decade employing experimental designs with isolated groups have found clear evidence that the costs of punishment often nullify the benefits of increased cooperation, rendering the strong human tendency to punish a thorny evolutionary puzzle. Here, we show that group competition enhances the effectiveness of punishment so that when groups are in direct competition, individuals belonging to a group with punishment opportunity prevail over individuals in a group without this opportunity. In addition to competitive superiority in between-group competition, punishment reduces within-group variation in success, creating circumstances that are highly favourable for the evolution of accompanying group-functional behaviours. We find that the individual willingness to engage in costly punishment increases with tightening competitive pressure between groups. Our results suggest the importance of intergroup conflict behind the emergence of costly punishment and human cooperation.