The foraging benefits of being fat in a highly migratory marine mammal

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: Adachi, Taiki, Maresh, Jennifer L., Robinson, Patrick W., Peterson, Sarah H., Costa, Daniel P., Naito, Yasuhiko, Watanabe, Yuuki Y., Takahashi, Akinori
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2014
• Subjects: Elephant Seals
• Online Access: http://dx.doi.org/10.1098/rspb.2014.2120; https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2014.2120; https://royalsocietypublishing.org/doi/full-xml/10.1098/rspb.2014.2120

Foraging theory predicts that breath-hold divers adjust the time spent foraging at depth relative to the energetic cost of swimming, which varies with buoyancy (body density). However, the buoyancy of diving animals varies as a function of their body condition, and the effects of these changes on swimming costs and foraging behaviour have been poorly examined. A novel animal-borne accelerometer was developed that recorded the number of flipper strokes, which allowed us to monitor the number of strokes per metre swam (hereafter, referred to as strokes-per-metre) by female northern elephant seals over their months-long, oceanic foraging migrations. As negatively buoyant seals increased their fat stores and buoyancy, the strokes-per-metre increased slightly in the buoyancy-aided direction (descending), but decreased significantly in the buoyancy-hindered direction (ascending), with associated changes in swim speed and gliding duration. Overall, the round-trip strokes-per-metre decreased and reached a minimum value when seals achieved neutral buoyancy. Consistent with foraging theory, seals stayed longer at foraging depths when their round-trip strokes-per-metre was less. Therefore, neutrally buoyant divers gained an energetic advantage via reduced swimming costs, which resulted in an increase in time spent foraging at depth, suggesting a foraging benefit of being fat.