Fig. 4. Resting depth varies with body condition and time of day in elephant seals.

During daytime hours (yellow) when surface light levels and resulting predation risk are higher, seals strategically drift deeper and at lower light levels than during twilight (gray) and nighttime (blue). Seals in superior body condition begin their drifts (A) deeper (note the inverted y axis) and (B) at lower light levels as compared to seals in inferior body condition, presumably due to their positive buoyancy.