Cooperative feeding in common dolphins as suggested by ontogenetic patterns in δ¹⁵ N bulk and amino acids

Understanding the effect of stage-specific traits on species feeding habits can reveal how natural selection shapes life strategies. Amino acid (AA) nitrogen stable isotopes (δ¹⁵ N) provide multiple proxies of habitat baseline values and diet that can improve our understanding of species feeding strategies relative to their animal metabolism. We evaluated the effect of body length as a proxy for life stage and sex on the feeding habits of the common dolphin Delphinus delphis delphis using δ¹³ C and δ¹⁵ N in bulk tissue and AAs δ¹⁵ N from skin samples collected for almost two decades. For bulk δ¹³ C and δ¹⁵ N data, we used SIBER analysis to compare isotopic niches by sex and life stage. For AA δ¹⁵ N data, we developed a hierarchical Bayesian model (HBM) to estimate indices of trophic status (Δ¹⁵ N and trophic position). The model reflected the natural hierarchical structure of AA data by partitioning variability into three sources: between laboratory replicates, within dolphins and among dolphins. Estimates of Δ¹⁵ N based on all trophic and source AAs were more precise for each dolphin, less variable among dolphins and on average 2.4‰ higher than indices based on single trophic (Glx) and source (Phe) AAs. Precision was further increased when information was shared among individuals through random effects or regression models. Estimates of trophic position showed similar patterns. Both Δ¹⁵ N and δ¹⁵ N_bulk isotopic niches showed no difference by sex, suggesting that males and females have similar feeding habits and may not segregate. However, lower Δ¹⁵ N values for weaning calves and smaller juveniles discriminate them from adults, whereas δ¹⁵ N bulk isotopic niches do not. A trophic discrimination factor (TDF_Tro-Src ) of 3.1‰ was required for reasonable estimates of trophic position for these dolphins. Together, the lack of δ¹⁵ N differences between sexes, low variation between juveniles and adults and knowledge of common dolphins' social organization support intraspecific feeding cooperation as an important strategy to feed in the highly dynamic marine environment. Our study also presents an efficient way to analyse complex AA δ¹⁵ N data using HBM to investigate foraging behaviour in long-lived marine species difficult to study in the wild.