New Zealand fur seals return to the same breeding sites each year (2), with the males arriving in late October to establish territories prior to the arrival of the females (1). Territories are defended through vocalisations, threatening displays and in about one in three encounters, actual physical fighting, where the combatants wave their necks from side to side and attempt to inflict bite wounds on the face, neck and shoulders (1) (7). The adult females arrive at the breeding sites from late November to December, and shortly after (usually just two to three days) give birth to the young that were conceived a year before during the previous breeding season (7). Around six to eight days after giving birth, the females mate again, but continue to attend to the pups onshore, while periodically undertaking foraging trips that become longer and longer as the pups get older. The pups are usually weaned at around ten months old, with females reaching sexual maturity at around four to six years and males at around five to six years. However, it is unlikely that a male will command a breeding territory until at least eight to ten years of age (1) (2) (7). As polygynous breeders, dominant males will mate with multiple females in a single breeding season, with each male territory comprising an average of five to eight females (1) (2). The gestation period is nine months, but an initial three month period of delayed implantation, where the fertilised egg does not implant in the uterine wall, results in the year long interval between mating and giving birth (2).
During the non-breeding season, from March to September, the breeding sites are occupied by pups and young juveniles, whilst adult females alternate between periods at the breeding sites and periods foraging at sea. Similarly, adult males spend substantial time ashore in non-breeding colonies, where a hierarchy is established based on size (7). The New Zealand fur seal has a varied diet comprising squid, octopus, and fish, and occasionally even birds, such as penguins and shearwaters (1) (2) (7). Although it normally dives in search of prey for no longer than one or two minutes (2), some individuals have been recorded at depths of almost 400 metres, staying underwater for over 14 minutes at a time (1). At sea, the New Zealand fur seal actively grooms, and will raft up to sleep in a variety of postures including the jug-handle position typical of many southern fur seal species (1) (6). Predators of the New Zealand fur seal include killer whales, sharks, sea lions and possibly leopard seals at the sub-Antarctic islands (1).