The development of diving behavior in juvenile Weddell seals: pushing physiological limits in order to survive

In juvenile phocids, the successful transition from nursing to independent foraging is contingent upon the development of adequate diving skills withi n the limited time between weaning and the depletion of body reserves. Yet, because juvenile seals are unable to remain submerged for as long as adult s, owing to their smaller size, higher metabolic rates, and lowered oxygen stores, their behavioral options are likely constrained. To determine how s uch limitations might influence foraging strategies, we studied the develop ment of diving behavior and physiology in Weddell seal (Leptonychotes wedde llii) juveniles, using a combination of time-depth recorders, satellite-lin ked dive recorders, and morphological and physiological measurements (mass and blood chemistry). Time-depth recorder data indicated that the average d epth, duration, and frequency of dives made by pups increased rapidly in th e period from birth through weaning, but slowed soon thereafter. While prew eaning increases in these parameters were correlated with seal age, postwea ning increases in dive capacity were gradual and were probably the result o f slower changes in mass and body composition. In weaned pups and yearlings , dive frequency and lime underwater increased with age and (or) mass. Desp ite their smaller size and lower absolute-energy requirements, the amount o f time juveniles spent in the water was similar to that spent by adults. Ho wever, because juveniles were unable to remain submerged as long as adults and because most foraging dives were deep, juveniles were unable to spend a n equivalent amount of time at the foraging depths. This difference was evi dent even though juveniles dove much closer to their anaerobic threshold th an did adults. These findings support the hypothesis that the foraging effi ciency of younger seals is reduced relative to that of adults, owing to phy siological and morphological constraints on aerobic dive duration, and sugg ests that low juvenile survival might result from behavioral constraints.