Forming strong social bonds can lead to higher reproductive success, increased longevity and/or increased infant survival in several mammal species. Given these adaptive benefits, understanding what determines partner preferences in social bonding is important. Maternal relatedness strongly predicts partner preference across many mammalian taxa. The role of paternal relatedness, however, has received relatively little attention, even though paternal and maternal kin share the same number of genes, and theoretically similar preferences would therefore be expected for paternal kin. Here, we investigate the role of maternal and paternal relatedness in female affiliation in Assamese macaques (Macaca assamensis), a species characterized by a relatively low male reproductive skew. We studied a wild population under natural conditions using extensive behavioral data and relatedness analyses based on pedigree reconstruction. We found stronger affiliative relationships and more time spent grooming between maternal kin and paternal half-sisters compared to non-kin, with no preference of maternal over paternal kin. Paternally related and non-related dyads did not form stronger relationships when they had less close maternal kin available, but we would need a bigger sample size to confirm this. As expected given the low reproductive skew, affiliative relationships between paternal half-sisters closer in age were not stronger than between paternal half-sisters with larger age differences, suggesting that the kin bias towards paternal kin was not mediated by age similarity. An alternative way through which paternal kin could get familiarized is mother- and/or father-mediated familiarity.

This study is part of a long-term research project on a population of Assamese macaques at the Phu Khieo Wildlife Sanctuary. All individuals could be reliably identified based on natural variation in appearance. We collected data on one group in 2007/2008 and 2010/2011, on two groups in 2013/2014, and on four groups in 2015/2016 and 2016/2017. This study focused on adult females (n=59), with females defined as adult in the mating season of their first conception. For 25 of these females the mother and exact age are known from direct observation. The other 34 females were already present in the population when the project started in 2006, so no information on their mother was available and age was estimated based on morphology and behavior. To collect observational data we followed the study groups from dawn to dusk, from sleep tree to sleep tree. During 40 minutes continuous focal animal observations, we recorded frequencies and durations along with actor and receiver of affiliative behaviors (proximity, body contact and grooming). To measure the strength of affiliative relationships among adult females, we used a modified dyadic composite sociality index based on the frequency and duration of proximity, body contact and grooming. To assess relatedness and assign dyads to kin classes we genotyped all 136 adult individuals of the population at 17 microsatellite loci and one mtDNA locus (details see manuscript). We estimated relatedness using the triadic maximum likelihood estimator, and assigned dyads to kin classes based on parentage analyses using the softwares Colony and Cervus. This study was conducted completely non-invasively and adhered to the ASAB/ABS Guidelines for the Use of Animals in Research. The Department of National Parks, Wildlife and Plant Conservation (DNP) and the National Research Council of Thailand (NRCT) authorized data collection and export of samples with a benefit sharing agreement (permit numbers: 0004.3/3618, 0002.3/2647, 0002/17, 0002/2424, 0002/470).