Endocrine and behavioral responses to ecological and social challenges in wild Geoffroy's spider monkeys

To maintain homeostasis during external and internal challenges, vertebrates activate allostatic systems, such as the hypothalamic-pituitary-adrenal (HPA) axis. Additionally, animals can use behavioral responses to address such challenges, which may result in the reduction of allostatic processes. We assessed whether HPA activation is influenced by subgroup formation patterns during ecological and social challenges. We selected Geoffroy's spider monkeys (Ateles geoffroyi) as study subjects, because they are characterized by a high degree of fission-fusion dynamics. We expected spider monkeys to address challenges by being in smaller subgroups when food availability is lower, in larger subgroups when perceived predation risk is higher, and either in larger subgroups or in proximity of more individuals when the likelihood of between-group encounters is higher. For 20 months, we collected behavioral data and fecal samples from 30 wild adult and subadult spider monkeys at the Otoch Ma'ax Yetel Kooh reserve in the Yucatán peninsula, Mexico, and assessed food availability in their home range. In support of our predictions, fecal glucocorticoid metabolite (fGCM) concentrations were negatively associated with food availability, and they were lower when in smaller subgroups than in larger subgroups during low food availability. Fecal GCM concentrations were positively associated with alarm call rates and were lower when in larger subgroups than when in smaller subgroups during high perceived predation risk. We found no evidence for an association between fGCM concentrations and the likelihood of between-group encounters. Overall, subgroup formation patterns interact with HPA activity to successfully address ecological challenges in wild Geoffroy's spider monkeys. Our study highlights the importance of assessing ecological challenges and behavioral responses when interpreting HPA activation in the wild.