Published in Biology Letters, the researchers found that bereaved macaque mothers spent less time resting (sleep, restful posture, relaxing) than the non-bereaved females in the first two weeks after their infants’ deaths.

Researchers believe this physical restlessness could represent an initial period of ‘protest’ among the bereaved macaque mothers, similar to that observed in studies on mother-infant separation in primates. The ‘protest’ phase, however, was not followed by an extended period of ‘despair’ or characterised by other behavioural markers of grief common in human behaviour.

With little empirical research into animals’ responses to death, this study represents the first systematic study into whether primate mothers show similar behavioural responses to death as seen in human grief.

The results provide a fascinating contribution to the emerging study of evolutionary thanatology, the study of death, bereavement, and grief across different species.

The researchers observed the behaviour of 22 macaque mothers at the Caribbean Primate Research Center on Cayo Santiago, an island off the coast of Puerto Rico. Half of the macaques observed (11) had recently lost an infant (on average 16 days prior), while the other non-bereaved half acted as a control group.

Behavioural observations were categorised into resting, feeding, grooming and displacement behaviours (locomotion, pacing, self-grooming, self-touching) and were recorded by the researchers over a 16-day period on smartphones using the CyberTracker software.

Co-author Dr Alecia Carter (UCL Anthropology) said: “Following the loss of an infant, we had expected the macaque mothers to spend more time resting, as is common among bereaved humans. What we actually observed was the opposite. The bereaved macaque mothers spent less time resting in the first two weeks after an infant’s death and there was no difference of their time spent foraging, grooming, or doing displacement behaviours.

“The mothers’ period of restlessness was surprisingly short, but this short-term response can also be seen in primate mother-infant separation studies, which show a relatively short period of ‘maternal disturbance’ after separation from their infant.”

Lead author MSc student Emily Johnson (UCL Anthropology) said: “Death is an inevitable part of life and how we respond to death can vary greatly, even among humans. We wanted to explore how the behavioural response to death, the experience of grief, differs between primates and humans. Is grief a uniquely human experience?”

For the study, the researchers considered grief and bereavement to be different things. While bereavement describes the state of losing someone to death, grief describes the negative emotional behavioural response.

Emily said: “Anthropologists have long questioned whether animals are capable of experiencing grief, and many pet owners will report their beloved cat or dog grieves after the death of a fellow pet. But these are reports from owners who are often themselves grieving. Grief-like behaviour, such as reduced activity, could simply be because they have lost a playmate.

“Our findings show no behavioural markers of grief in the way humans understand it among the macaque mothers, so we recommend further study in this area and greater data collection on primates’ responses to bereavement.”