Social bonds provide multiple pathways to reproductive success in wild male chimpanzees

In most male mammals, fitness is strongly shaped by competitive access to mates, a non-shareable resource. How, then, did selection favor the evolution of cooperative social bonds? We used behavioral and genetic data on wild chimpanzees (Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania, to study the mechanisms by which male-male social bonds increase reproductive success. Social bonds increased fitness in several ways: first, subordinate males that formed strong bonds with the alpha male had higher siring success. Independently, males with larger networks of strong bonds had higher siring success. In the short term, bonds predicted coalition formation and centrality in the coalition network, suggesting that males benefit from being potential allies to numerous male rivals. In the long term, male ties influenced fitness via improved dominance rank for males that attain alpha status. Together, these results suggest that male bonds evolved in chimpanzees by affording both short- and long-term pathways to reproductive success

Sexually Coercive Male Chimpanzees Sire More Offspring

SummaryIn sexually reproducing animals, male and female reproductive strategies often conflict [1]. In some species, males use aggression to overcome female choice [2, 3], but debate persists over the extent to which this strategy is successful. Previous studies of male aggression toward females among wild chimpanzees have yielded contradictory results about the relationship between aggression and mating behavior [4–11]. Critically, however, copulation frequency in primates is not always predictive of reproductive success [12]. We analyzed a 17-year sample of behavioral and genetic data from the Kasekela chimpanzee (Pan troglodytes schweinfurthii) community in Gombe National Park, Tanzania, to test the hypothesis that male aggression toward females increases male reproductive success. We examined the effect of male aggression toward females during ovarian cycling, including periods when the females were sexually receptive (swollen) and periods when they were not. We found that, after controlling for confounding factors, male aggression during a female’s swollen periods was positively correlated with copulation frequency. However, aggression toward swollen females was not predictive of paternity. Instead, aggression by high-ranking males toward females during their nonswollen periods was positively associated with likelihood of paternity. This indicates that long-term patterns of intimidation allow high-ranking males to increase their reproductive success, supporting the sexual coercion hypothesis. To our knowledge, this is the first study to present genetic evidence of sexual coercion as an adaptive strategy in a social mammal

An intergenerational androgenic mechanism of female intrasexual competition in the cooperatively breeding meerkat.

Female intrasexual competition can be intense in cooperatively breeding species, with some dominant breeders (matriarchs) limiting reproduction in subordinates via aggression, eviction or infanticide. In males, such tendencies bidirectionally link to testosterone, but in females, there has been little systematic investigation of androgen-mediated behaviour within and across generations. In 22 clans of wild meerkats (Suricata suricatta), we show that matriarchs 1) express peak androgen concentrations during late gestation, 2) when displaying peak feeding competition, dominance behaviour, and evictions, and 3) relative to subordinates, produce offspring that are more aggressive in early development. Late-gestation antiandrogen treatment of matriarchs 4) specifically reduces dominance behaviour, is associated with infrequent evictions, decreases social centrality within the clan, 5) increases aggression in cohabiting subordinate dams, and 6) reduces offspring aggression. These effects implicate androgen-mediated aggression in the operation of female sexual selection, and intergenerational transmission of masculinised phenotypes in the evolution of meerkat cooperative breeding

jtfeld/fem_soc_offspring_surv: submission release

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Predictors and consequences of gestation length in wild chimpanzees

ObjectivesEnergetics are widely recognized to influence timing of birth in humans and other eutherian mammals, yet considerable variation exists in the relationship between energetic constraints and gestation length. In humans, poor nutrition and short inter-gestational intervals (IGIs) are associated with shorter gestations. In other mammals, lower energy availability is usually associated with longer gestations. We investigated the predictors of gestation length, and the impact of gestation length on offspring survival, in chimpanzees, humans’ closest living relatives.Materials and MethodsWe used 50 years of demographic and behavioral data to estimate gestation lengths in the wild chimpanzees of Gombe National Park, Tanzania, and then used ecological and demographic data to explore the predictors and consequences of gestation length in our sample.ResultsGestation lengths were shorter for females in their early 30s (relative to younger and older females), and after short IGIs. Other predictors potentially associated with maternal energetic condition and maternal investment were not associated with gestation length. We also found that shorter gestation lengths corresponded to lower offspring survival.DiscussionLike humans, chimpanzees had shorter gestations after short IGIs, and short gestations were associated with higher offspring mortality. We consider competing explanations for the conflicting relationships between energetics and gestation length across eutherian mammals in light of these results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/175114/1/ajpa24601.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175114/2/ajpa24601_am.pd

Male presence grid

Lists each date of study period and presence/absence of each adult male. This allows the calculation of relative ranks on each date based on Elo scores

Female presence grid

Lists each date of study period and presence/absence of each adult female. This allows the calculation of relative ranks on each date based on Elo scores

Chimpanzee females queue but males compete for social status

Dominance hierarchies are widespread in animal social groups and often have measureable effects on individual health and reproductive success. Dominance ranks are not static individual attributes, however, but instead are influenced by two independent processes: 1) changes in hierarchy membership and 2) successful challenges of higher-ranking individuals. Understanding which of these processes dominates the dynamics of rank trajectories can provide insights into fitness benefits of within-sex competition. This question has yet to be examined systematically in a wide range of taxa due to the scarcity of long-term data and a lack of appropriate methodologies for distinguishing between alternative causes of rank changes over time. Here, we expand on recent work and develop a new likelihood-based Elo rating method that facilitates the systematic assessment of rank dynamics in animal social groups, even when interaction data are sparse. We apply this method to characterize long-term rank trajectories in wild eastern chimpanzees (Pan troglodytes schweinfurthii) and find remarkable sex differences in rank dynamics, indicating that females queue for social status while males actively challenge each other to rise in rank. Further, our results suggest that natal females obtain a head start in the rank queue if they avoid dispersal, with potential fitness benefits

Optimized elo rating R code

Commented R code used for modeling Elo scores, corresponding to models 1-3 in manuscript. Includes code for import of sample data files to allow rerunning of models