RESUMEN
Because senescence impairs the ability of older males to compete successfully for mates, male reproductive strategies are expected to change with age. The terminal investment hypothesis proposes that older males, who could die soon, should take greater risks to obtain mating opportunities. Another possibility is that older males avoid such risks, adopting alternative reproductive tactics, such as increased affiliation with females, increased reliance on coalitions or sexual coercion to continue to compete with younger animals. We tested these hypotheses in wild chimpanzees, Pan troglodytes schweinfurthii, of the Kanyawara community, Kibale National Park, Uganda, where old males sire offspring at relatively high rates. Our data set included >40 000 incidents of male aggression and >5800 copulations observed between 2005 and 2017. We found that, even as their dominance status declined, old males maintained relatively high copulation rates, especially with established mothers. There was no evidence for terminal investment in response to ageing. Males became generally less aggressive as they aged. Neither did old males form affiliative bonds with females, nor use sexual coercion more frequently, as alternative reproductive tactics. Old males did, however, participate in coalitionary aggression at higher rates than young males and increased the proportion of their aggression that was coalitionary over time. Coalitions were positively associated with mating success, particularly for low- and middle-ranking males. These results support the hypothesis that ageing male chimpanzees use coalitions as an alternative reproductive tactic. The lack of evidence for terminal investment in response to ageing appears to reflect a broader mammalian pattern in which males who rely on fighting to secure mating opportunities avoid excessive risk taking as their formidability wanes.
RESUMEN
Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships.
RESUMEN
Humans prioritize close, positive relationships during aging, and socioemotional selectivity theory proposes that this shift causally depends on capacities for thinking about personal future time horizons. To examine this theory, we tested for key elements of human social aging in longitudinal data on wild chimpanzees. Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but they also socialize more with important social partners. Further, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over their life span. Our findings indicate that social selectivity can emerge in the absence of complex future-oriented cognition, and they provide an evolutionary context for patterns of social aging in humans.
Asunto(s)
Envejecimiento/psicología , Pan troglodytes/psicología , Conducta Social , Conducta Agonística , Animales , Femenino , Masculino , Apego a ObjetosRESUMEN
OBJECTIVES: Sodium, a vital micronutrient that is often in scarce supply for tropical herbivores, is sometimes found at high concentration in decaying wood. We tested two hypotheses for chimpanzees: first, that wood-eating facilitates acquisition of sodium; second, that wood-eating occurs in response to the low availability of sodium from other dietary sources. MATERIALS AND METHODS: We studied the behavior of more than 50 chimpanzees of all age-sex classes in the Kanyawara community of Kibale National Park, Uganda. We quantified the sodium content of dietary items, including wood samples from tree species that chimpanzees consumed or did not consume. To assess variation in sodium intake, we used 7 years of data on time spent feeding on plant foods, 18 months of data on rates of food intake by adult females, and 20 years of data on meat-eating. RESULTS: Major dietary sources of sodium were wood, fruits and meat. Chimpanzees consumed wood primarily from decaying trees of Neoboutonia macrocalyx (Euphorbiaceae), which had substantially higher sodium content than all other dietary items tested. Wood-eating was negatively correlated with fruit-eating. Females ate wood more often than males, while males had a greater probability of consuming meat at predation events. DISCUSSION: We propose that females ate wood more often than males because females had reduced access to meat, their preferred source of sodium. This hypothesis suggests that the need for sodium is a motivating reason for chimpanzees to consume both meat and wood.