Abstract
Sex-specific niche differentiation is common in marine vertebrates, but how this varies long-term is poorly understood. Here we investigated interannual variation in sexual segregation among breeding northern gannets Morus bassanus, wide-ranging central-place foragers with slight sexual dimorphism. Over 11 breeding seasons, we used GPS tracking and/or stable isotopes to test for sex differences in foraging trip characteristics (range, duration and timing); spatial distribution; habitat selection; and carbon and nitrogen isotopes in blood. When combining data from all years, females foraged further and for longer than males, yet despite this, the foraging areas of the sexes almost completely overlapped. Males and females selected foraging habitats that differed in terms of oceanography but not fishing vessel density. We also detected temporal segregation: females were more likely to be at sea during the day than at night, while males were more likely to be at sea during the night. However, foraging behaviour quantified by all GPS analyses varied interannually, with sex differences detected in some years but not others. Finally, males had consistently higher red blood cell δ13C and δ15N than females across all years, which was not driven by size dimorphism, instead likely by prey choice or very fine-scale habitat selection. We conclude that environmental variation influenced short-term sex differences in movement, but sex differences in stable isotopes that integrate behaviour over longer periods reveal more consistent differences. Our results suggest that inferences drawn from single-year studies may not relate to general patterns, highlighting the importance of long-term studies and combining methods.
Highlights
Male and female animals frequently occupy separate foraging niches (Mysterud 2000, Catry et al 2005, Wearmouth & Sims 2008), hypothesised to be driven by a combination of competitive displacementMar Ecol Prog Ser 661: 1–16, 2021 and sex-specific specialisation (Catry et al 2005)
Sexspecific foraging plays a major role in structuring species distributions by reducing intraspecific competition (González-Solís et al 2000, Catry et al 2005) and allowing the sexes to differ in activity budgets or nutritional requirements related to their reproductive roles (Ruckstuhl & Neuhaus 2002)
The degree of sexual segregation can vary over time, both between breeding stages (Phillips et al 2004) and across the annual cycle (Castillo-Guerrero & Mellink 2011, Besel et al 2018), but few studies have examined this over long periods
Summary
Male and female animals frequently occupy separate foraging niches (Mysterud 2000, Catry et al 2005, Wearmouth & Sims 2008), hypothesised to be driven by a combination of competitive displacementMar Ecol Prog Ser 661: 1–16, 2021 and sex-specific specialisation (Catry et al 2005). Seabirds are a useful model for studying sexual segregation as they are socially monogamous and share parental duties (Lack 1968), restricting both sexes to the colony such that they compete during breeding Their longevity and the large scale of their foraging trips allows us to explore segregation over a range of ecological conditions. Studies have revealed sex differences in broad-scale space use, fine-scale habitat use, activity patterns, diet and trophic position (Solís et al 2000, Bearhop et al 2006, González- Harris et al 2013, De Pascalis et al 2020). Such studies are generally conducted over 1−3 breeding seasons Determining the extent to which sex differences are maintained over multiple years provides insight into the possible influence of extrinsic and intrinsic factors that underpin such differences
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