Sperm whale feeding variation by location, year, social group and clan: Evidence from stable isotopes

Abstract

We studied the diet of sperm whales Physeter macrocephalus by measuring carbon and nitrogen isotope ratios in 106 sloughed skin samples and 10 squid beaks (Histioteuthidae spp.), the latter collected from sperm whale defecations. Samples were collected during 8 studies conducted between 1989 and 2000 in the South Pacific Ocean. We examined diet variation across region, year, social group and vocal clan. The isotopic signatures of groups and acoustic clans of sperm whales were compared using a nested analysis of variance (ANOVA) and the absolute distances between each pair of samples were calculated. The δ13C and δ15N values ranged from -17.8 to -14.5‰ and from 8.5 to 22.3‰, respectively. The δ15N values of defecated squids were about 3‰ lower than values of the sperm whale, corresponding to a trophic difference of one level. There was a significant difference in both the δ15N and δ13C values between groups (nested within clans and the studies) and clans (nested within studies). Most of the variation was between studies. The latitude at which the samples were collected was inversely related to the δ15N values. We suggest that the differences in diet between the groups from different clans are mainly caused by characteristic behaviour of the clans and differential use of micro-habitats; i.e. groups from a clan with a generally more benthic or inshore distribution had higher δ13C than groups with a more offshore or pelagic influence, a general characteristic of this isotope in marine habitats. © Inter-Research 2007.