Foundations of variation in male aggressiveness and tolerance between chacma baboons (Papio ursinus) in Botswana and Guinea baboons (P. papio) in Senegal

Content Provider	Semantic Scholar
Author	Kalbitzer, Urs
Copyright Year	2014
Abstract	The fitness limiting resource for most male primates is the indivisible fertilization of females. Therefore, males commonly compete aggressively with each other over access to receptive females. Among species, however, there is considerable variation in aggressiveness, and in some species males may even show cooperative behaviors, which is often associated with more relaxed and tolerant social relationships. The proximate and ultimate reasons for this variation may depend on several aspects of a social system, but are not yet entirely understood. Baboons (Papio spp.) constitute a well-suited model for investigating the foundation of male aggressiveness and tolerance since different species show considerable variation in male-male competition and other aspects of their social systems. Male chacma baboons (P. ursinus) in southern Africa and male Guinea baboons (P. papio) in western Africa probably represent the opposite extremes in terms of male aggressiveness and tolerance in this genus. The purpose of my thesis was therefore to investigate several aspects of variation in male dominance relationships in these two species. Chacma baboons live in stable multi-male multi-female groups in which mature males disperse into neighboring groups and aggressively compete for a high rank. High ranks confer ‘priority of access’ to fertile females and therefore determine individual reproductive success. Guinea baboons, in contrast, live in a multi-level society in which closely related males often remain in the same group. Male social relationships seemed to be characterized by lower rates of agonistic behaviors, high spatial tolerance, and cooperation; thus aggressiveness did not appear to represent the main factor regulating the access to females in this species. Comparable quantitative data to corroborate these assumptions about differences in male aggressiveness between the two species were still lacking, however. Thus, one aim of my thesis was to compare aggressiveness and spatial tolerance between male chacma baboons living in the Moremi Game Reserve in Botswana, and male Guinea baboons living in the Parc National de Niokolo Koba in Senegal. Differences in male-male competition are reflected in male reproductive trajectories which are associated with testosterone patterns in primates. In addition to these long-term patterns, the ‘challenge hypothesis’ predicts that short-term increases in testosterone levels promote aggression in the context of mate competition. The second aim of my thesis was therefore to investigate the link between testosterone and dominance-related aggression by comparing testosterone patterns between male chacma and Guinea baboons in relation to age, aggression and dominance relationships. The competitive regime may also affect physiological costs, or allostatic load for individuals of different social status. These costs are reflected in inter-individual variation in glucocorticoid levels. The third aim of my thesis was therefore to obtain more information about the effects of male social relationships on allostatic load by comparing variation in glucocorticoid levels in relation to dominance status between chacma and Guinea baboons. Among individuals of the same species, aggressiveness is a trait with a strong heritability. It is therefore likely that interspecific variation is also linked to differences in genetic architectures. The two length polymorphisms 5-HTTLPR – in the gene of the the serotonin transporter - and MAOALPR -in the gene of the monoamine oxidase A - affect the activity of the serotonin neurotransmitter system and have been linked to variation in aggressiveness. Thus, interspecific variation in aggressiveness in baboons may be linked to different alleles in these two loci. The fourth aim of my thesis was to compare 5-HTTLPR and MAOALPR alleles among five species of baboons. By providing the first comparable data on behavior of male chacma and Guinea baboons, I could show that male chacma baboons were more often involved in agonistic interactions than male Guinea baboons, while male Guinea baboons showed a higher spatial tolerance towards other males and sometimes even exchanged affiliative behaviors. Furthermore, male chacma baboons formed linear hierarchies with highly consistent dominance relationships, while the linearity of hierarchies among male Guinea baboons was generally low. These observations seem to be consistent with previous studies on male social relationships in both species, indicating that the two species indeed vary in the intensity of male contest competition. Testosterone levels were not correlated with age or dominance ranks in either species, but there was a trend that testosterone correlates with aggressiveness in chacma baboons. This is consistent with previous observations in this species during periods in which high ranks are challenged. This was not the case during this study but a recent fission event in the study group may have resulted in a similar situation driven by regular encounters between the two groups. In Guinea baboons, testosterone was not correlated with expressed agonistic behavior, which could be explained by the assumption that agonistic behaviors are less important to obtain mating opportunities in this species. High ranking male chacma baboons showed higher levels of glucocorticoids than subordinate males. Again, this is consistent with previous observations during unstable periods, suggesting that the unusual event of group fission resulted in an unstable period. In Guinea baboons, glucocorticoid levels were not correlated with male dominance position further supporting the view that ranks are less important among male Guinea baboons. A comparison of 5-HTTLPR and MAOALPR alleles among five species of baboons suggested that 5-HTTLPR genotype is not linked to interspecific variation in behavior as this locus was monomorphic in most species. A comparison among several papionin species indicated that this locus may have been under selection during the historical dispersal of macaques into Asia. The significance of this variation, however, remains to be investigated but may be related to environmental variation between different species. By contrast, MAOALPR was polymorphic and the pattern of different alleles broadly mapped onto assumed behavioral variation among baboons. Overall, variation in this locus seen between and within species makes further investigation of this polymorphism in baboons promising to understand the genetic foundation of behavioral variation in primates. In conclusion, with my thesis I provided insights into the proximate causes and costs of variation in male dominance relationships among different species of baboons. Behavioral variation among baboons may be linked to the MAOALPR genotype. The investigation of this locus with regard to neurotransmitter activity and behavior in baboons may reveal further insights into the proximate mechanisms underlying variation in aggressiveness in primates. Behavioral observations and measurements of glucocorticoid levels suggest that male reproductive strategies in Guinea baboons incur fewer ‘obvious’ costs. Studying the mechanisms regulating the access to receptive females in this species, and comparing the reproductive success among different individuals may ultimately reveal further insights into the evolution of reproductive strategies and variation in aggressiveness and tolerance among primate males.
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