Abstract
Little is known about the social structure of male sperm whales (Physeter macrocephalus) after they leave their natal units. While previous studies found no evidence for preferred associations among males, the observation of mass-strandings consisting exclusively of males, suggest that they have strong social bonds. To investigate the social associations among male sperm whales, we used half weight index of association, permutation tests and standardized lagged association rate models on a large photo-identification database collected between 2006 and 2017 in Nemuro Strait, Japan. Our results suggest that while male sperm whales are not as social as females, they do form long term associations, have preferred companionship, and forage in social proximity to each other. The best-fitting model to the standardized lagged association rate showed that associations among males last for at least 2.7 years and as most males leave the area after 2 years, associations may last for longer. Twenty dyads were observed associating over more than 2 years, for a maximum 5 years. One dyad was observed associating on 19 different days and clustered on 7 different days. Male associations may function to enhance foraging or to fend off predators. Such relationships seem to be adapted to a pelagic habitat with uncertain resource availability and predation pressure.
Highlights
Among mammals, male relationships tend to be competitive [1], and consistently strong association rates are relatively rare
Physeter macrocephalus, form all-male groups, the factors promoting group formation among males may be quite different from those in species where males associate with related males to improve mating success
Female sperm whales and their offspring live in stable social units at low or mid-latitudes [6], and most females remain within their natal unit throughout their life [7]
Summary
Male relationships tend to be competitive [1], and consistently strong association rates are relatively rare. Males form strong relationships to defend estrous females, or access to female groups or territory (e.g., chimpanzee Pan troglodytes: [2]; bottlenose dolphin Tursiops sp.: [3]; lion Panthera leo: [4]; cheetah Acinonyx jubatus: [5]). Most of these associated males are kin. Males leave their natal unit before sexual maturity (~6–16 years) and are described as forming “bachelor schools” consisting of males of about the same age, generally at high latitudes outside
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