Elephant seals vary time spent in light and darkness to optimize risk/reward trade-offs

While scientists have known that creatures may adjust the timing of their daily routines based on starvation and predation, these shifts have only previously been measured based on data from a population at a single point in time. Now, using data collected as 71 elephant seals undertook their foraging migrations across the North Pacific Ocean, researchers report a view of how these animals divide their time between light and darkness to optimize tradeoffs between risks and rewards based on 7 months of data per seal, collected between 2004 and 2012. Their findings refute a hypothesis about how seals prioritize feeding. To better understand how seals divide their time between light and darkness, Roxanne Beltran and colleagues used high-resolution biologgers to continuously measure elephant seals as they journeyed for 7 months across the North Pacific Ocean, monitoring shifts in their body fat, time spent in light and darkness, movement both across the ocean and to different depths, and exposure to risk. They observed that as the seals became plumper, they were more likely to engage in low-risk rest at night rather than high-reward hunting, refuting a hypothesis that feeding is always a higher priority than rest for these seals. Seals with more body fat also prioritized safety over energy conservation by resting where the water was considerably darker, more than 100 meters deeper than leaner seals. Additionally, for each hour increase in daylight time, the seals moved their bedtime earlier by about 11% of the day length. This suggests that predation risk in the daylight, rather than circadian rhythm or the timing of the seals' meals, determines their preferred resting time. The authors note that such findings, which illuminate the daily routines of elephant seals and other marine mammals, may help inform future conservation efforts. "As humans continue to threaten the ability of wild animals to forage efficiently and sleep safely, we must work to understand how physiology and ecology interact to shape the timing of fundamental behaviors," Beltran et al. write.

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