Inter-sexual differences in New Zealand fur seal diving behaviour

Abstract

We studied the diving behaviour of adult male and lactating female New Zealand fur seals Arctocephalus forsteri to determine inter-sexual differences in their foraging strategies. Adult male fur seals are significantly longer and heavier than lactating females. Lactating females are central place foragers that regularly provision dependent pups, and consequently females conducted shorter duration foraging trips (5.0 ± 3.7 d) compared to the relatively unconstrained adult males (9.0 ± 3.9 d). Male trip durations were not correlated to variables that measured individuals' dive effort. In lactating females, foraging trip duration was significantly, positively correlated to the number of dives h-1 and negatively correlated with the mean dive duration at night. This indicates that females on longer trips invested relatively more time searching for prey patches, which they utilised on relatively short duration dives. Males and females typically dived at night and appeared to utilise both pelagic and benthic habitats. Males dived deeper (max = 380+ m, mean = 52.1 ± 59.3 m) and for longer (max = 14.8 min, mean = 3.6 ± 2.5 min) than females (max = 312 m and 9.3 min, mean = 41.5 ± 26.8 m and 2.7 ± 1.3 min). Inter-sexual foraging competition is likely to be minimal in this species, because females typically utilised continental shelf waters and males foraged in deeper waters over the continental shelf break, beyond female foraging grounds. Inter-sexual differences in fur seal diving behaviour most likely result from sexual dimorphism, which places different physiological constraints on the diving performance of males and females. © Inter-Research 2005.