More allogrooming is followed by higher physiological stress in wild female baboons

Social bonds increase fitness in a range of mammals. One potential pathway by which social bonds increase fitness is by reducing the exposure to physiological stress, i.e., glucocorticoid (GC) hormones, that can be detrimental to health and survival. This is achieved through downregulating hypothalamic-pituitary-adrenal (HPA)-axis activity. While long-term measures of social (grooming) bonds are often negatively correlated with HPA-axis activity, the proximate role of physical touch through allogrooming remains an open question in the sociality-health-fitness debate. Demonstrating potential anxiolytic benefits of grooming in the wild is hindered by methodological limitations. Here, we match accelerometer-identified grooming in wild female chacma baboons (Papio ursinus) to non-invasive faecal GC concentrations (fGCs). Consistent with previous work, we found a negative overall correlation between individual averaged fGCs and grooming rates. However, when time-matching grooming to fGCs, we found both more giving and receiving grooming was followed by higher fGCs. This upregulation of HPA-axis activity suggests that maintaining social bonds and its associated fitness benefits, may come at a shorter-term physiological cost. This finding sheds new light on a ubiquitous social behaviour typically considered ‘relaxing’ and suggests sociopositive contact can trigger physiological stress.