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.