Abstract
Sexual segregation seems to be common in bottlenose dolphins, whereby males and females live in different pods that mix mainly for mating. Male dolphins often use aggressive behaviour to mate with females, while females with calves may have different activity and dietary requirements to males and different susceptibility to predation. We investigated the degree of spatial and social sexual segregation in Indo-Pacific bottlenose dolphins (Tursiops aduncus) in a subtropical estuary in Australia. Based on surveys completed over three years, dolphin groups were mostly mixed-sex or female. Mixed-sex groups were found in larger groups in mostly deeper water, whereas, female groups were foraging across all water depths in smaller groups. Aggressive coercive behaviour by males towards females was high, occurring mainly in deeper water, at higher tides, and outside the breeding season. Habitat use by female dolphin groups suggests that shallow tributaries may provide a sanctuary from aggressive males, access to suitable prey items and density for mothers and their calves, or a combination of these factors.
Highlights
Sexual segregation for much of the year is a widespread phenomenon among social mammals including, ungulates [1], bears [2], kangaroos [3], seals [4,5] and cetaceans [6,7,8,9]
Sexual segregation theory divides the categories of sexual segregation into social segregation where sexes live in separate groups outside of the breeding season, habitat segregation where sexes differ in habitat use, and spatial segregation where they occupy different areas within the same habitat [10]
Our study aimed at quantifying the types of dolphin groups most commonly found throughout the different seasons, what types of habitat they use, and, if spatial or social sexual segregation occurs, which variables might explain these patterns
Summary
Sexual segregation for much of the year is a widespread phenomenon among social mammals including, ungulates [1], bears [2], kangaroos [3], seals [4,5] and cetaceans [6,7,8,9]. Sexual segregation theory divides the categories of sexual segregation into social segregation where sexes live in separate groups outside of the breeding season, habitat segregation where sexes differ in habitat use, and spatial segregation where they occupy different areas within the same habitat [10]. Our study aimed at quantifying the types of dolphin groups most commonly found throughout the different seasons, what types of habitat they use, and, if spatial or social sexual segregation occurs, which variables might explain these patterns. The main predators of bottlenose dolphins in the study site are bull sharks (Carcharhinus leucas) that are found mainly in the deeper estuary channels during the day; an area that dolphins and females with young calves should avoid [14]. It has been proposed that females with calves might avoid male harassment [15] and avoid areas frequented by males. It has further been proposed that females might avoid males due to direct or indirect effects on the frequencies of antagonistic interactions: females might avoid males of all age classes either because males engage in agonistic acts in the presence of females [11] or because male presence causes an increase in female-female aggression as has been shown for Roosevelt elk [12,13]
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