Changes in social behavior and fecal glucocorticoids in a Japanese macaque (Macaca fuscata) carrying her dead infant.

Maternal care towards dead infants has been observed in many wild and captive mammals, but the consequences of this behavior for social interactions and the physiological status of the mother remain elusive. Here, we report changes in rates of aggression and grooming time, and fecal glucocorticoid (fGC) levels in a free-ranging female Japanese macaque (Macaca fuscata) that carried her dead infant for 20 days. Our observations revealed that when carrying the dead infant, the mother showed increased rates of grimace, avoidance, and human-directed behaviors, along with reduced allogrooming time and fleeing from other individuals. Postpartum fGC levels were comparable to those of non-pregnant and non-lactating females, suggesting that the energetic costs and stress of carrying a dead infant are low. Our findings indicate that carrying a dead neonate can have three profound consequences on the mother: increased fearful behavior, decreased allogrooming, and a rapid return to baseline fGC levels. We hypothesize that dead infant-carrying may have evolved as a strategy to mitigate stress from infant loss. These findings have implications for our understanding of grief in nonhuman primates and can impact management protocols surrounding deaths in captive social groups.