A female dolphin may carry its deceased calf around for days, until the body is in such a state of decomposition that only the head or part of the body remains. New research published in the journal Zoology suggests that this behaviour is evidence that dolphins grieve for their dead.
The scientific discipline known as comparative thanology examines how animals respond behaviourally, physiologically and psychologically to dead members of their own species.
It is a somewhat tricky field, for the experience of grief and its expression varies widely even between the cultures of our own species, and for many cognitive scientists the jury is still out regarding whether animals have a concrete understanding of death and its finality. Still, the use of the word “grieving” in an animal context has been increasingly accepted since Jane Goodall’s landmark study of wild chimpanzees (Pan troglodytes) in 1986.
Accounts of whales and dolphins caring for or attending dead or dying individuals have been reported since the 1950s, and are observed by cetacean watchers and researchers worldwide. Lead author Giovanni Bearzi from the Dolphin Biology and Conservation Group, based in Italy, along with an international team of colleagues, decided to conduct a comprehensive literature review to investigate patterns and variation in this behaviour in cetaceans.
The team analysed 78 records reported between 1970 and 2016, and adopted a weighted comparative approach to take observation effort into account – mainly because dolphins are much more readily observed than large whale species which tend to be out in deep water and submerged for longer.
Toothed whales were much more likely than baleen whales to attend to their dead. In fact, dolphins accounted for 92.3% of 78 records, and baleen whales only 1.3%. An analysis of relative brain size across the cetaceans found that the taxa with larger relative brain sizes are more likely to interact with their dead. This finding is consistent with the concept that sociality in mammals is closely associated with encephalisation, also known as the “social brain” hypothesis.
While dolphins had many records, killer whales or orcas (Orcinas orca), which are also highly social, had surprisingly low incidences of attending the dead. More systematic reporting may reveal this behaviour in orcas and sperm whales (Physeter microcephalus), but the study suggests it may be less prevalent in species which move in fast swimming pods, or are deep divers.