Using stable isotopes to investigate foraging variation and habitat use of sperm whales from northern Peru

Female sperm whales Physeter macrocephalus are top predators in mesopelagic ecosystems, integrating chemical information about ecosystems through their diet. Proxies for diet and habitat use may be useful to learn about how sperm whales' foraging behavior and environment change through time. We measured stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) from individual growth layer groups from the teeth of 10 female sperm whales, to track changes in diet and habitat use from ca. 1926 to 1960. We found that bulk delta C-13 and delta N-15 records fell into 3 temporal patterns, which may indicate different ontogenetic changes in diet, habitat, or both. Average bulk delta C-13 and delta N-15 values for each tooth were positively correlated, and individual whales generally separated according to temporal patterns. To determine the underlying driver of the bulk relationship, we measured delta C-13 and delta N-15 values from individual amino acids (AAs) in a subset of samples. AA isotope results indicated that the bulk isotopic trend was due to baseline differences. Specifically, whales from each identified pattern likely used different feeding regions, but had similar trophic positions. This conclusion is supported by the relationships between bulk and compound- specific AA isotope values for both nitrogen and carbon. We suggest that these female sperm whales inhabiting northern Peruvian waters had 3 different lifelong foraging strategies, having the same trophic position but feeding overall in different regions. These results provide novel insights into social bonds among female sperm whales, since whales with similar foraging patterns likely shared the same habitat and diet over their lifetime, whereas whales with different foraging strategies had separate trophic niches.