Bulk and Compound Specific Isotope Analysis Provide Insights into Habitat Use and Diet Variability Among Female Sperm Whales (Physeter macrocephalus) from the Eastern Tropical Pacific

Sperm whales (Physeter macrocephalus) have a strong social structure and females form sympatric clans that span thousands of kilometers and encompass thousands of whales. We investigated variability in foraging habitat and diet through the life history of ten female sperm whales from the Eastern Tropical Pacific (ETP), using coupled bulk and compound specific amino acid stable isotope measurements made on annual tooth growth layers. We identified three broad temporal patterns in annual records of bulk nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values in sub-groups of individuals: 1) constant, 2) increasing, and 3) decreasing isotopic values between ca.1930 and 1960. These different isotopic patterns suggest that whales sampled may belong to different clans. In addition, average bulk δ¹⁵N and δ¹³C values differed among individuals and appear to be group-specific. This suggests that whales from different sub-groups foraged in different habitats and/or had differing diets. To differentiate between these interpretations, we conducted compound specific amino acid isotope analysis on dentin samples from six whales. Amino acid proxies for baseline δ¹⁵N values indicate that bulk variability among whales is due to δ¹⁵N differences at the base of the food web, indicating different feeding areas. The δ¹³C values of essential amino acids, which are positively correlated to bulk δ¹³C values, support this conclusion. Together, the compound specific and bulk annual records suggest that whales with higher isotopic values likely foraged in regions closer to the coast throughout their lives, while whales with lower isotopic values likely foraged further offshore. Overall, our data suggest clear spatial segregation of clans within a similar ocean region. Since molecular analysis cannot yet distinguish genetic variability among whale clans, we suggest that stable isotope analysis can help to identify clans and their foraging ecology, based on habitat use.