Regulation of inter-sexual relationships within the microcosm of Guinea baboon units

Abstract

Male reproductive success is linked in most animals to their access to fertile females. Resulting competition among males presents a strong selective pressure on male reproductive strategies. In species with female choice males show behaviours aimed at attracting the opposite sex. Displays of quality in various modalities allow females to assess and choose a potential partner. Alternatively, male social investment and support for females, but also into current offspring, can inform females about potential future parental investment. Males compete not only for access to females; they are also under selection to monitor the state and behaviour of females to assess suited mating partners or seize mating opportunities. While in many species, females become the centre of male attention when they reproductively active, in species where males and females form long-lasting bonds males might be permanently incentivised to monitor and control associated females. The extent to which animals monitor their conspecifics and their interactions varies, and the drivers behind the evolution of abilities to gather and process social information are still debated. Non-human primates provide the opportunity to study the link between sociality and mechanisms underlying reproductive strategies and social cognition since they are highly social animals with differentiated relationships and often advanced social skills and knowledge. Further, they present a wide range of social systems, with a remarkable variety of grouping and mating patterns. The general aim of this PhD thesis was to investigate the relationship between Guinea baboon primary males and their associated females. My main focus was hereby, firstly, on testing whether the relaxed social environment of the Guinea baboon society affects males’ attention to social information, and secondly, how primary males distribute social investment among their associated females. Guinea baboons (Papio papio) live in a nested multi-level society. At the core are ‘units’ consisting of one primary male, one to seven associated females, and their offspring. Several units and bachelor males form a ‘party’, which in turn aggregate into ‘gangs’. Females associate with one primary male and show mate fidelity but enjoy relatively high spatial freedom. Male Guinea baboons form strong bonds with other males, support each other in conflicts and show low aggression rates. For my two studies, I collected behavioural data and conducted playback experiments on a wild population of Guinea baboons that ranges close to the Centre de Recherche de Primatologie (CRP) Simenti, a field station of the German Primate Center (DPZ) located in the Niokolo Koba National Park in Senegal. The population comprised approximately 400 individually identified Guinea baboons that belonged to three main parties. In my first study, we investigated whether Guinea baboon primary males keep track of their females’ whereabouts. First, I tested experimentally whether male Guinea baboons respond to play back vocalisation from associated and non-associated females differently. In the main experiment I tested if males keep track of their females’ position. I presented vocalisation of associated females from locations that were either consistent or inconsistent (i.e., violating their expectation) with the actual position of the female. While males seem to be able to recognize their female by voice, as evidenced by stronger responses to calls from unit females than non-unit females, they apparently lack the ability or motivation to track their females’ movements. In the second study we investigate the allocation of social investment in primary males. I analysed proximity and rates of socio-positive interactions for unit-females in relation to their age and reproductive state. Males were closer to and interacted more frequently with females that were reproductively active, and were also more likely to be found in close proximity to lactating females. Males further showed a preference for mature and young adult females over subadult and old females. In summary, the results of my first study fall in line with existing evidence which suggests that the level of competition affects the value of social information and, as a consequence, the motivation to attend to social signals. I further was able to show that Guinea baboon primary males maintain social relationships with all of their associated females but allocate their social investment depending on female short- and long-term reproductive value. Thus, the current reproductive value of a female partner is an influential characteristic even for species living in a relative tolerant and low competitive social system.