Evidence of male alliances in spinner dolphins off Samadai reef, Red Sea, Egypt

Abstract

The formation of male alliances is known for primates and some other social mammals. In cetaceans, male alliances have been seen in fission-fusion societies, apparently as a male strategy to gain access to females. Spinner dolphins (Stenella longirostris) are known to vary in their socio-behavioural dynamics in response to environmental variability, ranging from fluid fission-fusion to long-term stable societies. In the Egyptian Red Sea, spinner dolphins are frequently sighted in an offshore reef, Samadai, where they rest during daylight hours after having foraged at night over deep waters. We applied underwater photo-identification mark-recapture technique and direct underwater observations to generate a dataset applicable for behavioural modelling with SOCPROG. At Samadai, spinner dolphins show a great group size variability with high geographic fidelity to the site. The association pattern and apparent group-substructure indicate the existence of male alliances. Males use the area long-term with frequent emigration-reimmigration. Sociogram analyses suggests that most adult males form alliances of variable sizes (3 to 9 members) generally stable across our 2-year study. Underwater videographic data shows that females are mainly consorted by trios (sometimes pairs and singles). At this early stage of our work, it seems that Samadai spinner dolphins alliance structure resembles the multi-level social grouping described for male bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia. However, at Samadai Reef the alliances are only temporarily dismantled to smaller units within the safe waters of the resting area whenever a receptive female is available, and they are formed again, in their original membership, thereafter; somewhat similar to the stable groups/sub-groups of spinner dolphins in remote Hawaiian atolls. The structure of male alliances in Samadai spinner dolphins appears to be a result of a behavioural trade-off between the species-specific ecological needs and cooperation with/against other conspecifics over access to females.