Sexual segregation has important ecological implications, but its initial development in early life stages is poorly understood. We investigated the roles of size dimorphism, social behavior and predation risk on the ontogeny of sexual segregation in Antarctic fur seal, Arctocephalus gazella, pups at South Georgia. Beaches and water provide opportunities for pup social interaction and learning (through play and swimming), but increased risk of injury and death (from other seals, predatory birds, and harsh weather), whereas tussock grass provides shelter from these risks but less developmental opportunities. One hundred pups were sexed and weighed, 50 on the beach and 50 in tussock grass, in January, February and March annually from 1989 – 2018. Additionally, 19 male and 16 female pups were GPS-tracked during lactation from December 2012. Analysis of pup counts and habitat use of GPS-tracked pups suggested females had a slightly higher association with tussock grass habitats and males with beach habitats. GPS-tracked pups travelled progressively further at sea as they developed, and males travelled further than females towards the end of lactation. These sex differences may reflect contrasting drivers of pup behavior: males being more risk prone to gain social skills and lean muscle mass and females being more risk averse to improve chances of survival, ultimately driven by their different reproductive roles. We conclude that sex differences in habitat use can develop in a highly polygynous species prior to onset of major sexual size dimorphism, which hints that these sex differences will increasingly diverge in later life.

Antarctic fur seal pup monitoring: Antarctic fur seal pups were captured annually at Main Bay, Bird Island, South Georgia (54.010° S, 38.059° W) as part of a long-term monitoring programme. One hundred pups were selected (by convenience sampling), 50 on the beach and 50 in the tussock grass, each month in January, February and March annually from 1989 – 2018. Each pup was captured by hand, measured, sexed (by examination of genitalia), and weighed to the nearest 100 g (using a hand-held spring balance).

Antarctic fur seal pup GPS-tracking: Thirty-five Antarctic fur seal pups, 19 males and 16 females, were GPS-tracked from the beach habitat at Freshwater beach, Bird Island, South Georgia (54.009° S, 38.052° W) between December 2012 and April 2013. Pups were sexed, measured, weighed, and equipped with a GPS logger (i-gotU GT-600; 37 g; 46 x 41.5 x 14 mm) and a radio transmitter (Sirtrack V2G-152A; 16g, 40 x 20 x 10 mm). GPS loggers were programmed to record locations every five minutes and pups were re-captured and weighed every 3.74 ± 0.076 days until pups weaned or died. Six pups died during the study period (atp 10, 11, 13, 18, 29 and 38).

Data file 'Antarctic fur seal pup monitoring data' contains data for monitored pups from 1989 – 2018.

Data file 'Antarctic fur seal pup GPS tracking data' contains the raw GPS data for all 35 GPS-tracked Antarctic fur seal pups.

Data file 'Mass of GPS tracked Antarctic fur seal pups' contains the pup measurements (in cm) and masses (in kg).