The foraging benefits of being fat in a highly migratory marine mammal.
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.
Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo, Japan adachi.taiki@nipr.ac.jp.
Taiki
T
Adachi
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, USA.
Jennifer L
JL
Maresh
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, USA.
Patrick W
PW
Robinson
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, USA.
Sarah H
SH
Peterson
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, USA.
Daniel P
DP
Costa
National Institute of Polar Research, Tachikawa, Tokyo, Japan.
Yasuhiko
Y
Naito
Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo, Japan National Institute of Polar Research, Tachikawa, Tokyo, Japan.
Yuuki Y
YY
Watanabe
Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo, Japan National Institute of Polar Research, Tachikawa, Tokyo, Japan.
Akinori
A
Takahashi
D025041:Animal Migration; D000818:Animals; D001522:Behavior, Animal; D050218:Body Fat Distribution; D004242:Diving; D005260:Female; D046048:Seals, Earless / Q000033:anatomy & histology / Q000502:physiology*; D013550:Swimming
accelerometry; buoyancy; cost of transport; elephant seal; foraging behaviour; swimming behaviour