Inferring prey size variation from mandible acceleration in northern elephant seals

Prey size is an important factor for predators as it affects prey quality (energy content) and hence total energy gain. However, it remains challenging to obtain information about prey size from free‐ranging marine predators. Here, we developed a method that estimates prey size using mandible acceleration in captive northern elephant seals and then applied it to 34 free‐ranging seals. In captive seals, the number of feeding‐related acceleration signals were positively related to prey size category (<15 cm). In free‐ranging seals, smaller number of acceleration signals occurred frequently in both mesopelagic (200–1,000 m) and bathypelagic layers (>1,000 m), suggesting that seals foraged mainly on smaller prey (possibly <15 cm). However, the quantity of larger acceleration signals increased in the bathypelagic layers, suggesting that seals were more likely to forage on larger prey (>15 cm) at deeper depths. These results suggest that seals might compensate for higher energetic costs of deeper‐diving by targeting larger prey. Although our study has practical limitations ( e.g ., calibrating prey size in captive conditions), our method allows concurrent inference of prey size and foraging behavior, being potentially useful to investigate how predators adjust their behavior in response to the changes in the foraging environment.