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

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 delta C-13 and delta N-15 values ranged from -17.8 to -14.5 parts per thousand and from 8.5 to 22.3 parts per thousand, respectively. The delta N-15 values of defecated squids were about 3 parts per thousand lower than values of the sperm whale, corresponding to a trophic difference of one level. There was a significant difference in both the delta N-15 and delta C-13 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 delta N-15 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 delta C-13 than groups with a more offshore or pelagic influence, a general characteristic of this isotope in marine habitats.