Behavioural Ecology Traits of Elusive Deep-Diver Whales Unravel a Complex Social Structure Influenced by Female Philopatry and Defence Polygyny

Abstract

Knowledge of the role of individual associations has provided an insightful understanding of the structures of animal societies, especially in highly social mammals such as primates. Yet, this is unbalanced towards marine mammals, particularly to beaked whales, due to their elusive nature. In addition, information on the fundamental drivers of the social structure of these deep-diving animals is still scarce. Here, the hypothesis of female defence polygyny was tested in Blainville’s beaked whales (Mesoplodon densirostris) and discussed within the context of marine and terrestrial organisms displaying similar patterns, by (i) estimating residency times to obtain information on the movements into and out of the area, (ii) analysing social networks to assess individual association metrics, (iii) measuring the strength of the associations to assess the existence of preferred or avoided relationships among individuals, and (iv) modelling different social structures to address temporal patterns in social relationships. Using a 9-year photographic dataset derived from the pelagic habitat, individual associations were inferred based on likelihood techniques. This approach allowed to infer on the species’ social structure in relation to age class, sex, residency status, and spatio-temporal patterns, which can be a good practice to be applied for other taxa. Heterogeneity in capture probability and residency times was observed between age-sex classes, with adult females exhibiting long-term site fidelity. This suggests different habitat roles and spatial structuring within this social organisation. Strong and long dyadic associations occurred between adult females and immatures, contrarily to between males, and the best-fitting models of the temporal patterns suggested long-lasting and temporary associations. The present findings unravel a complex social structure stratified by age-sex class and influenced by female philopatry and defence polygyny, like an unimale group mating system, which varies from other beaked whales but is similar to some birds, pinnipeds, or non-human primates.

Nurturant behavior towards a dead calf in a Cuvier's beaked whale

“…Deep‐diving toothed whales of the family Ziphiidae are among the least known of all marine mammals due to a combination of factors such as their oceanic offshore habitat, their ability to make deep and prolonged foraging dives, short surface bouts and elusive behavior (Aguilar Soto et al, 2006; Baird et al, 2006; Hooker et al, 2009, 2019; Tyack et al, 2006). The extremely close morphological resemblance between members of this family (especially among the genus Mesoplodon ), make species recognition at sea difficult, contributing to our limited knowledge of basic aspects of their distribution, behavior, biology, and ecology (e.g., Badenas et al, 2022; Fedutin et al, 2015). Several species have recently been described including the Deraniyagala's beaked whale, Mesoplodon hotaula (Dalebout et al, 2014), Sato's beaked whale, Hyperoodon minimus (Yamada et al 2019), Ramari's beaked whale, M .…”

“…The Cuvier's beaked whale is one of four species of beaked whales that have been the subject of longitudinal studies using photo-ID methods in different marine areas (e.g., Bahamas, Canary Islands, Madeira, Hawaii, Mediterranean, etc. ; Badenas et al, 2022;Hooker et al, 2019). These studies have highlighted the fact that this species does not typically exhibit strong social bonds, with whales being encountered alone or in small groups (Claridge, 2006;Dinis et al, 2017;McSweeney et al, 2007).…”