Whale and dolphin behavioural responses to dead conspecifics

Abstract

The scientific study of death across animal taxa-comparative thanatology-investigates how animals respond behaviourally, physiologically and psychologically to dead conspecifics, and the processes behind such responses. Several species of cetaceans have been long known to care for, attend to, be aroused by, or show interest in dead or dying individuals. We investigated patterns and variation in cetacean responses to dead conspecifics across cetacean taxa based on a comprehensive literature review. We analysed 78 records reported between 1970 and 2016, involving 20 of the 88 extant cetacean species. We adopted a weighted comparative approach to take observation effort into account and found that odontocetes (toothed cetaceans) were much more likely than mysticetes (baleen whales) to attend to dead conspecifics. Dolphins (Delphinidae) had the greatest occurrence of attentive behaviour (92.3% of all records), with a weighed attendance index 18 times greater than the average of all other cetacean families. Two dolphin genera, Sousa and Tursiops, constituted 55.1% of all cetacean records (N=43) and showed the highest incidence of attentive behaviour. Results of analyses intended to investigate the reasons behind these differences suggested that encephalisation may be an important predictor, consistent with the "social brain" hypothesis. Among attending individuals or groups of known sex (N=28), the majority (75.0%) were adult females with dead calves or juveniles (possibly their own offspring, with exceptions), consistent with the strong mother-calf bond, or, in a few cases, with the bond between mothers and other females in the group. The remaining records (25.0%) involved males either showing sexual interest in a dead adult or subadult, or carrying a dead calf in the presence of females. Because an inanimate individual is potentially rescuable, responses to dead conspecifics-especially by females-can be explained at least in part by attempts to revive and protect, having a clear adaptive value. In some cases such responses are followed by apparently maladaptive behaviour such as the long-term carrying of, or standing by, a decomposed carcass, similar to observations of certain terrestrial mammals. Among the possible explanations for the observed cetacean behavioural responses to dead conspecifics are strong attachment resulting in a difficulty of "letting go"-possibly related to grieving-or perhaps individuals failing to recognise or accept that an offspring or companion has died. Our current understanding is challenged by small sample size, incomplete descriptions, and lack of information on the physiology and neural processes underpinning the observed behaviour. We provide research recommendations that would improve such understanding.

Responses to death and dying: primates and other mammals

“…Other studies are conducted from a more explicitly applied perspective, for example investigations of the repellant properties of necromones for "problem" species (e.g., sharks: Stroud et al 2014;cockroaches: Rollo et al 1994), or potential distress in laboratory rodents/livestock witnessing conspecifics being euthanized/slaughtered (Boivin et al 2016;Anil et al 1996Anil et al , 1997. In parallel, noninvasive, observational studies are accumulating, looking at thanatological responses in various species, including nonhuman primates (see below) and notably cetaceans, resulting in attempts to organize the data and reveal mechanisms and possible adaptive functions (for recent reviews of cetacean research see Bearzi et al 2018;Reggente et al 2018).…”

Why Care: Complex Evolutionary History of Human Healthcare Networks

“…In addition, some healthcare behaviors, like grooming, are widespread across taxa (Lehmann et al, 2007;Octavio Lopez-Riquelme and Luisa Fanjul-Moles, 2013;Bush and Clayton, 2018), while others are restricted to only a few relevant species. For example, lifting a sick conspecific to breathe at the surface of a body of water (Bearzi et al, 2018) is only necessary in aquatic mammals. Understanding the complex transmission dynamics produced by different types of behaviors and their distribution across species is central to understanding how healthcare behaviors evolve.…”

Primate thanatology and hominoid mortuary archeology

“…More widely, the accumulation of thanatological observations in nonhuman animals is revealing how some behaviors once thought to be 'sophisticated' and perhaps even exclusive to humans, are in fact widespread (McComb et al 2006;Heinze and Walter 2010;de Waal 2013;Chouvenc et al 2011;Renucci et al 2011;López-Riquelme and Fanjul-Moles 2013;King 2013;Anderson 2016;de Kort et al 2017;Bearzi et al 2018;Gonçalves and Biro 2018;Gonçalves and Carvalho 2019). Examples include various methods of corpse disposal-described and systematically studied especially in eusocial insects and linked in particular to chemical cues-and post-mortem transport and care of dead infantsobserved especially in primates and cetaceans and studied from the perspectives of strong emotional bonds between individuals as well as those species' understanding of death.…”