Paternal kinship among adult female rhesus macaques (Macaca mulatta)

The impact of maternal kinship on social behaviour has been studied in detail for many primate species, but it is difficult to assess the importance of kin selection in shaping the evolution of social behaviour when studies are limited to maternal kin, completely ignoring paternal kinship. This thesis aimed to investigate the extent of paternal kinship and its impact on the social relationships among adult females in one group of free-ranging rhesus macaques (Macaca mulatta) living on the island of Cayo Santiago. The main findings can be summarised as follows: Firstly, in order to access the extent of paternal kinship, paternity has been analysed for all infants of the study group born between 1993 and 1998. Results revealed that male reproduction was highly skewed over the study period as few males have sired a high number of offspring, but the majority of potential sires have sired no or few offspring. This created a kinship structure in which nearly all animals born during the study period had at least one paternal half-sibling in close age proximity, i.e., either of the same age (74%) or within a two-year age difference of themselves (15%). Assuming that male reproductive success is also skewed in other primate species, then the importance of paternal kinship in primate societies should be re-emphasised. One of the most important consequences of male reproductive skew is that many individuals will have more paternal than maternal half-siblings during their life time. Secondly, in order to access the impact of paternal kinship on the social relationships among adult females, focal data on affiliation and aggression have been collected on 34 adult females with respect to their social partners who were either their maternal half-sisters, paternal half-sisters or unrelated females. The present study confirmed that the closest affiliative relationships characterise maternal half-sisters. Probably the most important result of this study was the finding that adult females were significantly more affiliative with their paternal half-sisters than with their non-kin. The recognition of paternal sisters was more pronounced among females of the same age than among females of different age, with a decrease in affiliation as the exact age difference (measured in years) increased among paternal half-sisters. This indicates that age proximity had an additional regulatory effect upon affiliative behaviour. However, evidence for paternal kin discrimination was only found with respect to affiliation, but not with respect to dyadic aggression suggesting context-dependent kin discrimination. Thirdly, when more kin categories were included in the analysis, adult females showed a strong bias towards maternal kin in comparison to paternal kin. This bias towards maternal kin when the degree of relatedness was held constantly suggests, that maternal kinship had a larger impact on the social relationships among adult female rhesus macaques than paternal kinship at least in the study group. Both affiliation and aggression declined with decreasing degrees of relatedness, but distant kin still differentiated each other from non-kin contradicting the existence of a relatedness threshold. Fourthly, paternal kinship was finally investigated with respect to coalition formation. A coalition is formed when an individual intervenes in an ongoing conflict between two opponents in order to support one party against the other. Female rhesus macaques intervened most often on behalf of their maternal half-sisters. In addition, unrelated female peers supported each other more often than unrelated female non-peers. Females did not support their paternal half-sisters more often than non-kin, but data may indicate indirect evidence for paternal kin discrimination as females tended to target their paternal half-sisters less often than non-kin. This finding might be due to the fact, that paternal half-sisters can be very different in rank, while maternal half-sisters are of adjacent rank, implying that a low-ranking female cannot provide actual help to her paternal half-sisters, as she may risk a higher probability of retaliation when intervening in a conflict between two higher-ranking opponents. As a compromise, females may instead avoid to target their paternal half-sisters, suggesting that constraints to an individuals’ own competitive ability play an important role in coalition formation. Maternal and paternal half-sisters showed a stronger trend in reciprocity and interchange than non-kin and also provided a higher proportion of costly interventions towards each other. Finally, the results of the present study strongly suggest that familiarity among individuals can arise through association in early development by at least two alternatives: (i) mothers mediating familiarity among their offspring (which are maternal half-siblings) caused by the close mother-offspring relationship during lactation and (ii) age proximity is mediating familiarity among age mates (including both paternal related and unrelated peers) as peers go through important life history stages such as infancy, menarche, pregnancy or motherhood at similar times while females of different age do not. In any case, paternal half-siblings additionally need a mechanism such as phenotype matching to discriminate paternal half-siblings from non-kin even within their peer group