Growing their separate ways: the ontogeny of sexual segregation in Antarctic fur seals

Sexual segregation occurs in a diverse array of taxa in the animal kingdom and has important ecological implications. Several hypotheses have been proposed to explain sexual segregation in adults, including size dimorphism, social behaviour and predation risk, but its initial development remains poorly understood. We aimed to quantify the ontogeny of sexual segregation in Antarctic fur seals, Arctocephalus gazella (a highly polygynous and sexually dimorphic species) to investigate the underlying drivers and ecological consequences of this phenomenon. All fieldwork was conducted at Bird Island, South Georgia. Three-hundred pups were sexed within beach and tussock grass habitats annually from 1989 – 2018. Thirty-five pups (19 males and 16 females) were deployed with GPS tags and tracked between December 2012 and April 2013, and 45 juveniles (26 males and 19 females) were deployed with Global Location Sensors (GLS loggers) and tracked between 2007 and 2014. Whiskers were also collected from 40 adults (20 males and 20 females) and stable isotope values were determined along each whisker. Analysis of pup habitat use revealed that males had a higher association with riskier habitats than females, and travelled further at sea toward the end of lactation. Sexual segregation became more pronounced as seals developed, with male juveniles foraging significantly further south than females. Stable isotopes along adult whiskers also indicated that males spent more time foraging south in maritime Antarctica during each annual cycle and that females had two main foraging strategies, with 30 % of females foraging north of the Polar Front and the remainder to the south of it. This sexual segregation likely developed from intense reproductive selection pressures, whereby reproductive success is more varied in males than females, so males prioritise growth (at the expense of increased risk) whereas females prioritise survival. The resulting niche partitioning relaxes competition which elevates population carrying capacity, but also exposes the sexes to different area-specific stressors. Studying the ontogeny of sexual segregation enhances knowledge about selective forces influencing animal behaviour with key implications for ecology, evolution and conservation.