Alliance-specific habitat selection by male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia

Abstract

Individuals and groups within the same population may differ in their use of resources. Also referred to as niche specialization, such differences can be documented through direct or indirect observation of resource or habitat use. Here, we examined selective habitat use in alliance-forming male Indo-Pacific bottlenose dolphins, Tursiops aduncus , in Shark Bay, Western Australia. Using 6 years of sighting data (2001–2006), we calculated the home ranges of 17 male alliances (comprising 3–14 individuals each). We defined five habitat types in the study area and measured the proportion of each habitat type in the home range of each alliance. Habitat selectivity was examined using selection ratios of used and available habitat within alliance home ranges. Since home range is also a form of habitat selection, we also examined selection ratios of alliances within defined subareas within the study area. Subareas were the combined home ranges of subsets of alliances that were linked by general location, distinctive habitat features and seasonal movement patterns. During each sighting, the predominant group behaviour was recorded. We analysed data from all sightings (which included all behavioural types: resting, travelling, socializing and foraging), as well as a restricted set of foraging groups only. For all sightings, alliances were selective of habitat within each of three subareas (northern, central, southern), and 14 alliances were selective of habitat within their home range. For foraging sightings, alliances within all three subareas were selective with respect to foraging habitat, and seven alliances showed significant selection of foraging habitat within their respective home ranges. Some alliances with broadly overlapping ranges foraged in different habitats. Alliance-specific habitat use may be influenced by ecological and social factors. Our results raise the possibility that the considerable variation in alliance size within this population may be influenced by the distribution of food as well as the distribution of females. • Niche specialization can occur among individuals and groups within populations. • Male dolphin alliances (each of 3–14 members) cooperate to secure oestrous females. • Alliances inhabit overlapping ranges with similar proportions of available habitats. • Alliances with overlapping ranges and similar habitats forage in different habitats. • Thus food, as well as females, may explain marked differences in alliance tactics.