UC Santa Cruz

Female sperm whales (Physeter macrocephalus) are top predators in mesopelagic ecosystems, integrating chemical information about ecosystem structure through their diet. By studying proxies for diet and habitat use, we may be able to learn about how sperm whales' foraging and environment change through time. We measured stable isotopes of carbon (δ13C) and nitrogen (δ15N) from individual growth layer bands from the teeth of 10 female sperm whales, to track changes in diet and habitat use from ca. 1930 to 1960, and to investigate relationships with major environmental events. While El Niño events can strongly affect food webs, particularly in the Eastern Tropical Pacific, we found no clear linkage between bulk δ13C and δ15N records and El Niño records, possibly due to high variability among dentinal isotopic records or weak effects of El Niño events during the 20th century. However, we found that bulk δ13C and δ15N records fall into three temporal patterns, suggesting distinct groupings of whales with clearly differentiated life-long foraging strategies. Average bulk δ13C and δ15N values for each tooth were positively correlated, and we found individual whales generally separated in isotopic space according to temporal pattern groupings.

To determine if whales from each temporal pattern foraged in different regions with distinct isotopic baselines, we measured δ13C and δ15N values from individual amino acids (AAs) in a subset of samples. Amino acid isotope results clearly indicate that the bulk isotopic trend is due to baseline differences, as opposed to differences in diet or ecosystem structure. Specifically, our results indicate that whales from each of our identified groupings used different geographic regions, but had similar trophic positions, because essential- and source-AA isotope values correlated with bulk isotopic values, while both non-essential- and trophic-AAs had no relationship to bulk δ13C and δ15N values, respectively. Considering the bulk isotopic records together with CSIA data, we suggest that female sperm whales inhabiting the eastern Tropical Pacific likely had three different life-long foraging strategies under similar large-scale environmental constraints. Together, these results provide novel insight into social bonds among female sperm whales, since each social group shared the same habitat and diet over their life-time, but had separate trophic niches between adjacent social groups possibly due to environmental gradients.