SIMMONS, S.E.*; CROCKER, D.E.; COSTA, D.P.; Univ. of California, Santa Cruz; Sonoma State University, Rohnert Park; Univ. of California, Santa Cruz: Foraging strategies differ in sexually dimorphic northern elephant seals (Mirounga angustirostris).

Separation of feeding from breeding is a distinctive life history trait of many phocids. Male and female elephant seals differ in their annual foraging and reproductive schedules. Due to this difference and given that sexually mature males are 4-5 times larger than females one would expect differences in their foraging requirements and or foraging ability. Previous studies have shown qualitative differences in foraging location and dive behavior and it has been hypothesized that males dive benthically while females forage mesopelagically. This study used at sea satellite locations from seals to look quantitatively at the foraging behavior of males and females in relation to dive depth and ocean depth. Using satellite tracks and dive data from 6 adult males and 11 adult females between 1995 and 1997, we calculated a diving index (DI) by subtracting mean maximum dive depth (m) from ocean depth (m) at each location. At sea animal behavior was divided into foraging and non-foraging based on transit speed between locations (foraging = transit speed <0.4m/s). When foraging, the mean DI for males (767 ± 586 m, mean ± SD) was significantly less than for females (3,377 ± 1,057 m). Conversely on non-foraging dives there was no significant difference between sexes. This demonstrates that bathymetry is a key factor in male foraging behavior and that there are quantitative differences in foraging strategy between the sexes.