Environmental and physiological determinants of successful foraging by naive southern elephant seal pups during their first trip to sea

The ability to forage successfully during their first trip to sea is fundam ental to the ultimate survival of newly weaned southern elephant seals (Mir ounga leonina). However, there is considerable variation in the body mass a nd fat content of seal pups at weaning, which results in some individuals h aving larger energy and oxygen stores than others, which may confer advanta ges on them. The diving behaviour of 21 newly weaned seals was studied usin g satellite relayed data loggers. Seals were captured at Macquarie Island i n December 1995 and 1996, approximately 4 weeks after weaning. Two groups o f seals were specifically targeted: a heavy group from the top quartile of weaning masses (n = 6) and a light group from the lower quartile (n = 15). Most of the seals made dives in excess of 100 m depth and 5 min before fina l departure from the island. However, for the first 60-80 d, all of the sea ls exhibited behaviour quite distinct from the patterns reported for older conspecifics, and made relatively shallow (100 +/- 39 m; mean +/- SD) and s hort (5.7 +/- 1.23 min) dives. During this time the seals spent 74.3 +/- 12 .6% of each day diving, and the depth of the dives did not follow any diurn al pattern. The diving behaviour of all seals changed abruptly when they st arted on their return to land. During this time their behaviour was more li ke that of adults: they made deeper (159 +/- 9 m) and longer dives (9.01 +/ - 1.69 min) than previously, and the dives showed a strong diurnal pattern in depth. There is no obvious explanation for this change in behaviour, alt hough its abrupt nature suggests that it is unlikely to have been due to ph ysiological changes in the seals. The size of the seals at weaning was an i mportant influence on diving behaviour. Heavy weaners made significantly de eper (130 +/- 40 m) and longer dives (7.36 +/- 0.55 min) than light weaners (88 +/- 32 m and 5.04 +/- 0.64 min, respectively). This indicates that sma ller seals are constrained to some extent by their physiological capabiliti es, which perhaps requires some individuals to adopt different foraging str ategies.