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' (age class, sex, relationship and decomposition) and 'social' characteristics (escorting, calf dependence, alloparental care, herding and dispersal patterns) from published and unpublished literature.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'.

Keywords: aquatic mammals; behaviour; death; multiple correspondence analyses; sociality.

Figure 1.

The first factor plane (dimension 1 and dimension 2) of the MCA which explains 30.6% of the total inertia within the dataset. Species are coloured by their cosine squared (cos²) value with larger values, suggesting a stronger association with each axis. The categories that represent the greatest contribution to both the dimension 1 and dimension 2 axes are shown in black (see table 1 and electronic supplementary material, tables S4 and S3 for a description of each category).

Figure 2.

The results of the hierarchical clustering dendrogram using scores derived from the first eight axes of the MCA. Clusters were determined by finding the optimum level of inertia gain (p < 0.05).