The effects of allogrooming and social network position on behavioural indicators of stress in female lion-tailed macaques (Macaca silenus)

Allogrooming serves an important social function in primates and confers short term benefits such as parasite removal and stress-relief. There is currently mixed evidence as to the immediate impact of allogrooming on an individual’s stress levels, which may be influenced by their role in the grooming dyad, position in their social network, or their relationship with their grooming partner. In this study of seven captive adult female lion-tailed macaques (Macaca silenus) in a mixed sex group at Chester Zoo, UK, we found evidence to support a tension-reduction function of allogrooming. Focal animal sampling showed that the duration of self-directed behaviour (SDB), which indicates moderate to high levels of stress, was significantly lower in the five-minute period following allogrooming than the five-minute period preceding it for both recipients and groomers. However, when compared to match-control periods, both SDB rates and durations were significantly lower across all individuals in the five-minute period both before and after allogrooming, indicating that although allogrooming reduces stress, it is also more likely to occur when individuals are already in a relatively relaxed state. The rate and duration of SDB post-grooming did not correlate with the strength of a dyad’s bond (based on proximity). This suggests that it is the act of allogrooming itself, rather than the identity of the partner, that reduces stress for both parties. Analysis of the proximity network highlighted a clear cost to social integration; node strength, a measure of the number and strength of an individual’s direct relationships, positively correlated with the duration of self-directed behaviour, suggesting that more gregarious individuals may experience higher levels of stress. These findings add to the growing body of literature that examines the effect of the individual social environment on primate stress levels, and also highlight the need to further investigate the link between social integration and the stress experienced by group-living animals.