Fig. 2. Body condition and activity timing throughout migration in elephant seals.

Throughout the foraging migration, elephant seals gain body condition and perform drift dives progressively earlier in the day relative to sunrise. (A) After departing the beach, animals lose body condition for approximately 1 month as they transition to offshore foraging grounds and then gain fat throughout their migration. (B) Because of horizontal movement and seasonal changes, daylength gradually decreases throughout the migration. (C) Concomitant with increasing body condition and shortening daylength is a change in rest phenology, such that animals drift progressively earlier in the day relative to sunrise. (D) Animals also forage progressively earlier throughout the trip, although this rate of change is less than the change in drift dive timing. Blue represents nighttime, gray represents twilight, and yellow represents daytime such that the transition from blue to yellow in (C) represents sunrise, whereas the transition from yellow to blue in (D) represents sunset.