Kinship and sociality in wild Assamese macaques (Macaca assamensis): are they related?
by Delphine De Moor
Date of Examination:2020-02-28
Date of issue:2020-12-04
Advisor:Dr. Oliver Schülke
Referee:Dr. Oliver Schülke
Referee:Prof. Dr. Julia Fischer
Persistent Address:
http://dx.doi.org/10.53846/goediss-8346
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Abstract
English
Social bonds are found in social species spanning the entire mammalian kingdom. The formation of such bonds can be adaptive: strong links have been established between social integration and increased survival and/or reproductive success in a diverse range of taxa. A crucial step in understanding how animals benefit from forming social bonds is to understand what drives social partner preferences, and how partner choice can contribute to fitness consequences. One factor that has repeatedly been shown to have a profound influence on the social life of animals is kinship. Social mammals tend to associate, affiliate and cooperate with their relatives more than with unrelated group mates. The bulk of this research, however, has focused on maternal kinship in females. Much less is known about the role of paternal kinship in the development of social bonds and about the importance of kinship in male social bonding. A key issue for the development of paternal kin biases is that many mammal females mate promiscuously so that paternity is concealed. Nevertheless, there is accumulating evidence showing that individuals can and do discriminate their paternal kin from non-kin. One mechanism proposed for paternal kin recognition is familiarity through age proximity. In species with relatively high reproductive skew and relatively short alpha male tenure, infants born into the same age cohort are likely paternal kin, and they might become familiarized with each other through growing up together. Based on both theoretical grounds and the limited data available, however, is seems that the role of age proximity for paternal kin recognition might have been overestimated. Additional limitations constrain the development of kin biases in males. First, males face strong competition for access to fertile females, which is expected to hinder the formation of male relationships. Second, males usually disperse from their natal group, leaving most of their kin behind. For these two reasons, male social bonds were originally assumed to be restricted to the rare male-philopatric species in which males would have familiar kin available, and indirect fitness benefits would tip the balance in favour of risky cooperation in the contest for access to females. Counter to this idea, strong male bonds have also been reported in male-dispersing species. The question now is whether those bonds are restricted to the few close kin that post-dispersal males have available in their group, or whether factors other than kinship might underlie male partner choice in bonding. In this thesis, I investigated how relatedness affects social bonding in wild Assamese macaques (Macaca assamensis), combining extensive behavioural data with relatedness analyses based on pedigree reconstruction. Assamese macaques are characterized by male dispersal and a relatively low reproductive skew, which allowed me to address two key questions that have remained largely unanswered so far: “Can paternal kin biases in affiliation develop in a species in which age proximity is not a reliable cue for paternal relatedness?” and “Are strongly bonded post-dispersal males generally closely related?” My results show that female Assamese macaques biased their affiliation towards their paternal half-sisters, independent of age proximity and maternal kin availability. With this, I show that females did not just form strong bonds with their paternal half-sisters to compensate for a lack of close maternal kin, and that the role of age proximity as a cue for paternal relatedness might be less important than originally assumed. Instead, I propose that in primates, paternal kin might be recognized through the stable male-female associations that mothers typically form with the likely fathers of their offspring (i.e. mother- or father-mediated familiarity). For male Assamese macaques, the results of the role of relatedness on bonding are more ambiguous. Post-dispersal males formed stronger bonds with the few close kin they had available than with the average non-kin. However, strong bonds were not exclusively formed with kin, and non-kin partners were chosen over available close kin partners in some cases. Relatedness seems to be only one of several factors influencing male bonding. Because bonds mediate partner choice in cooperation, which can provide males with substantial direct fitness benefits through increased reproductive success, competence and compatibility between partners might be more important than kinship. In conclusion, kinship affects animal sociality beyond maternal kin biases in females. Nonetheless, kinship is only one piece of the puzzle, and individuals likely choose their partners based on a complex set of interacting, context-dependent decision rules. To better understand when and why kin biases develop, more data are needed from species with a wider variety of dispersal patterns and mating systems. Only then can the variation in kin biases be explained in light of the interspecific demographic constraints.
Keywords: kin selection; social bonds; nepotism; relatedness; cooperation; coalitions; kin discrimination; kin biases; paternal kin; maternal kin; Assamese macaques; social relationships; Cercopithecines; Macaca assamensis; primates