Macaque mothers' responses to the deaths of their infants.

Although it is understood that all humans grieve the death of close social partners, little empirical research has addressed animals' responses to death. In this study, we collected quantitative data on the behaviour of 11 bereaved rhesus macaque (Macaca mulatta) mothers at Cayo Santiago to the natural deaths of their infants and matched, non-bereaved controls. Our research focused on behavioural signs of grief, including loss of appetite, lethargy, increased stress and social withdrawal, highlighting that such responses are documented in the human literature, but could be found in mammalian taxa. Using mixed models, we found that, contrary to prediction, bereaved mothers spent less time resting than the non-bereaved control females in the first two weeks after their infants' deaths. There were no other behavioural markers of grief. We conclude that mothers showed a short-term behavioural response to their bereavement that does not match human's prolonged 'despair' grief. We propose that mothers' behavioural responses might be a form of 'protest' grief, as is seen in primate infants when separated from mothers and in humans, or do not grieve. We hope to advance the field of comparative thanatology by providing a framework and novel predictions for future studies in this area.