Social relationships and death-related behaviour in aquatic mammals: a systematic review

Abstract

Some aquatic mammals appear to care for their dead, whereas others abandon their live offspring when conditions are unfavourable. This incredible variety in behaviours suggests the importance of comparing and contrasting mechanisms driving death-related behaviours among these species. We reviewed 106 cases of aquatic mammals (81 cetaceans and 25 non-cetaceans) reacting to a death event, and extrapolated 'participant' (, , and ) and 'social' characteristics (, ,, and) from published and unpublished literature. A multiple correspondence analysis (MCA) was performed to explore the relationships between these characteristics and death-related behaviours, with species clustered based on MCA scores. Results showed that both cetaceans and non-cetaceans react to death but in different ways. Non-cetaceans, characterized by a short maternal investment, were observed to protect the dead (defending it from external attacks), while cetaceans spent much longer with their offspring and display carrying (hauling, spinning, mouthing with the carcass and diving with it) and breathing-related (lifting and sinking the carcass) activities with the dead generally in association with other conspecifics. Our work emphasizes the need of increased documentation of death-related cases around the world to improve our understanding of aquatic mammals and their responses to death.This article is part of the theme issue 'Evolutionary thanatology: impacts of the dead on the living in humans and other animals'.

Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates

“…Many authors have called for more and better data on animals' responses to the dead (including a number of contributors to the present issue-e.g. Watson & Matsuzawa [130]; Reggente et al [168]; Anderson [73]) to advance comparative thanatology-greater taxonomic breadth, more quantitative descriptions and more systematic phylogenetic comparisons. While fully supporting these calls, we also advocate controlled experiments to probe the sensory and cognitive bases of the detection of death and its associated psychological states.…”

“…Thanatological behaviour among cetaceans (whales, dolphins and porpoises) is also becoming increasingly well documented and shows many parallels with primate and proboscid data. The vast majority of reports (compiled in Reggente et al [168], see also [27] for a recent review) concern interactions with dead calves or juveniles; carrying their carcasses has been documented in various dolphin and whale species. Indeed, the behaviour has been observed worldwide and in a range of environments including open oceans, bays and inlets, and rivers [169].…”

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

“…While this review aims to provide a broad overview, I pay particular attention to primates for their evolutionary relationship to humans, cetaceans, and birds for their convergences with humans in cognition, and eusocial insects (Hymenoptera: ants, bees, wasps, and Isoptera: termites) for their convergences with humans without complex cognition. While the selection of species may appear an unsystematic collection of anecdotes, this is largely a reflection of the discipline at present; these are the taxa which have received the most attention, first as anecdotal reports by field researchers and then, with taxa specific reviews (e.g., Bearzi et al, 2018;Bush and Clayton, 2018;Reggente et al, 2018;Watson and Matsuzawa, 2018). This has had the unintended effect of making the discipline fairly "siloed."…”

“…Remaining near or guarding (repelling others) can protect a vulnerable individual from attacks by conspecifics and predators. It has been observed in chimpanzees [Pan troglodytes (Anderson, 2016;Watts, 2019)], gorillas [Gorilla beringei (Porter et al, 2019;Watts, 2019)], marmosets [Callithrix jacchus (Bezerra et al, 2014)], ring-tailed lemurs [Lemur catta (Nakamichi et al, 1996)], snub-nosed monkeys [Rhinopithecus roxellana (Yang et al, 2016)], elephants [Loxodonta africana (Douglas-Hamilton et al, 2006)], giraffe [Giraffa camelopardalis (Bercovitch, 2013;Strauss and Muller, 2013)], peccaries [Pecari tajacu (de Kort et al, 2018)], dingos [Canis dingo (Appleby et al, 2013)], mongooses [Helogale parvula (Rasa, 1983)], and pinnipeds (Reggente et al, 2018). This behavior includes waiting for a conspecific that cannot keep up, standing over a conspecific that is unable to move, or chasing away conspecifics and predators (citations above).…”