Acoustic analysis in relation to kinship and familiarity for contact calls of adult female golden snub-nosed monkeys living in a multilevel society.

Kinship plays an essential role in the evolution of mammalian social behavior. The mechanisms of kin discrimination have attracted enduring interest, and phenotype matching and familiarity through prior association are the most likely ones among mammals. Previous studies on the mechanisms of kin discrimination based on acoustic signals have obtained inconsistent results. Golden snub-nosed monkeys (Rhinopithecus roxellana) are well-known for their multilevel social structure composed of several distinct units, the formation and stability of which largely depend on genetic relatedness among adult females. Taking advantage of the long-term demographic monitoring of a provisioned free-ranging group in Shennongjia National Park, China, this study provided a systematic acoustic analysis for contact calls of adult females in relation to kinship and familiarity. Results showed that structural similarities of contact calls between individuals did not vary among nonkin, distant kin, or close kin, while the calls exhibited clear individuality. Structural similarities of the calls between individuals significantly increased initially and then showed a tendency to decrease as the length of time co-resident in the same social unit (an index of familiarity) increased. Collectively, the acoustic structure was likely to reflect a dynamic tradeoff between maintaining social bonds and advertising individual identities. In addition, acoustic similarities decreased with age differences between individuals, likely due to age-related morphological changes in vocal apparatus. The results of this study improved the understanding of specific functions of contact calls and provided a basis for investigating kin discrimination mechanisms via acoustic signals in a primate living in multilevel societies.