Kin recognition is a key ability which facilitates the acquisition of inclusive fitness benefits and enables optimal outbreeding. In primates, phenotype matching is considered particularly important for the recognition of patrilineal relatives, as information on paternity is unlikely to be available via social familiarity. Phenotypic cues to both paternal and maternal relatedness exist in the facial features of humans and other primates. However, theoretical models suggest that in systems with parentage uncertainty it may be adaptive for offspring to conceal such cues when young, in order to avoid potential costs of being discriminated against by unrelated adults. Using experienced human raters, we demonstrate in a computer-based task that detection of parent-offspring resemblances in the faces of rhesus macaques (Macaca mulatta) increases significantly with offspring age. Moreover, this effect is specific to information about kinship, as raters were extremely successful at discriminating individuals even amongst the youngest animals. To our knowledge, this is the first evidence in non-humans for the age-dependent expression of visual cues used in kin recognition.

In group living animals, especially among primates, there is consistent evidence that high-ranking males gain a higher reproductive output than low-ranking males. Primate studies have shown that male coalitions and sociality can impact male fitness; however, it remains unclear whether males could potentially increase their fitness by preferentially supporting and socializing with females. Here we investigate patterns of male interventions and the effect of coalitions and sociality on male fitness in rhesus macaques (Macaca mulatta) with particular focus on male-female interactions. We combined behavioral collected on Cayo Santiago with genetic data analyzed for male reproductive output and relatedness. Our results revealed that the ten top-ranking males provided the majority of all male support observed. In contrast to other primates, male rhesus macaques mainly formed all-down coalitions suggesting that coalitions are less likely used to enhance male dominance. Males supporting females during and before their likely conception were not more likely to fertilize those females. We also found no evidence that males preferably support their offspring or other close kin. Interestingly, the most important predictor of male support was sociality, since opponents sharing a higher sociality index with a given male were more likely to be supported. Furthermore, a high sociality index of a given male-female dyad resulted in a higher probability of paternity. Overall, our results strengthen the evidence that sociality affects fitness in male primates, but also suggest that in species in which males queue for dominance, it is less likely that males derive fitness benefits from coalitions.