Abstract
Sexual segregation (sex differences in spatial organisation and resource use) is observed in a large range of taxa. Investigating causes for sexual segregation is vital for understanding population dynamics and has important conservation implications, as sex differences in foraging ecology may affect vulnerability to area-specific human activities. Although behavioural ecologists have proposed numerous hypotheses for this phenomenon, the underlying causes of sexual segregation are poorly understood. We examined the size-dimorphism and niche divergence hypotheses as potential explanations for sexual segregation in the New Zealand (NZ) sea lion (Phocarctos hookeri), a nationally critical, declining species impacted by trawl fisheries. We used satellite telemetry and linear mixed effects models to investigate sex differences in the foraging ranges of juvenile NZ sea lions. Male trip distances and durations were almost twice as long as female trips, with males foraging over the Auckland Island shelf and in further locations than females. Sex was the most important variable in trip distance, maximum distance travelled from study site, foraging cycle duration and percent time at sea whereas mass and age had small effects on these characteristics. Our findings support the predictions of the niche divergence hypothesis, which suggests that sexual segregation acts to decrease intraspecific resource competition. As a consequence of sexual segregation in foraging ranges, female foraging grounds had proportionally double the overlap with fisheries operations than males. This distribution exposes female juvenile NZ sea lions to a greater risk of resource competition and bycatch from fisheries than males, which can result in higher female mortality. Such sex-biased mortality could impact population dynamics, because female population decline can lead to decreased population fecundity. Thus, effective conservation and management strategies must take into account sex differences in foraging behaviour, as well as differential threat-risk to external impacts such as fisheries bycatch.
Highlights
Sexual segregation, defined here as differential space and resource use by males and females [1], is a significant element of the life history strategy of many animals [2]
Sex differences in the foraging locations of juvenile New Zealand (NZ) sea lions has conservation implications; the majority of female foraging grounds overlap with fisheries operations and female juvenile NZ sea lions are more susceptible than males to fisheries impacts through bycatch and resource competition
This study provides more support for the niche divergence hypothesis than the size-dimorphism hypothesis, it is possible that decreasing intraspecific competition acts to maintain sexual segregation, rather than being the causal mechanism
Summary
Sexual segregation, defined here as differential space and resource use by males and females [1], is a significant element of the life history strategy of many animals [2]. The foraging ranges of female wandering albatross (Diomedea exulans) have larger overlap with fisheries activities than males, resulting in higher bycatch mortality of females [4]. This sex-biased mortality has a drastic impact on population dynamics since female population decline can result in a reduction in the fecundity of a population [3]. Sexual segregation is a behavioural and ecological phenomenon commonly observed in a wide variety of taxa ranging from fish to birds to mammals, the fundamental causes are poorly understood [3]
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