Geographic variation in the foraging behaviour of South American fur seals

ABSTRACT: The implicit assumption of many ecological studies is that animal behaviour and resource use are geographically uniform. However, central place foraging species often have geographically isolated breeding colonies that are associated with markedly different habitats. South American fur seals Arctocephalus australis (SAFS) are abundant and widely distributed colonial breeding central place foragers that provide potentially useful insights into geographic variation in animal behaviour and resource use. However, SAFS movement ecology is poorly understood. To address knowledge gaps and to explicitly test geographic variation in behaviour, we examined the foraging behaviour of 9 adult female SAFS from 2 Falkland Islands breeding colonies separated in distance by 200 km. A total of 150 foraging trips over 7 mo revealed striking colony differences. Specifically, SAFS that bred at Volunteer Rocks undertook long foraging trips (mean ± SD: 314 ± 70 km and 15.2 ± 2.7 d) to the Patagonian Shelf and shelf slope (bathymetric depth: 263 ± 28 m). In contrast, SAFS that bred at North Fur Island undertook short foraging trips (94 ± 40 km and 5.3 ± 2.1 d) and typically foraged near the Falkland Islands coastline (bathymetric depth: 85 ± 24 m). Stable isotope analysis of vibrissae δ13C and δ15N values also revealed colony differences in the isotopic niche area occupied, which indicated that resource use also differed. Contrary to popular models (Ashmole’s halo, hinterland model), colony size was unrelated to distance travelled, and SAFS did not necessarily use foraging grounds closest to their breeding colony. SAFS are likely subject to different selective pressures related to different environmental demands at the 2 breeding colonies. Accordingly, we reason that behavioural differences between breeding colonies reflect different phenotypes, and habitat use is more immediately influenced by phenotype, philopatry and the local environment, rather than density-dependent competition typically attributed to colony segregation in foraging areas.