Ontogeny of swimming and diving in northern fur seal (Callorhinus ursinus)pups

Time spent in the water and diving behavior of northern fur seal (Callorhin us ursinus) pups were monitored between birth and weaning at St. Paul Islan d, Alaska. The median age when pups began swimming was 26 days, but prior t o 40 days of age they spent virtually all their time on shore and only rare ly took brief (15-20 min) swims. Pups began spending substantial time in th e water at approximately 40-50 days of age, coinciding both with the early growth of insulating underfur and a seasonal peak in sea surface temperatur e. This suggests that pups had earlier been constrained to remain on shore by their undeveloped thermoregulatory capabilities. Time in the water incre ased up to approximately 100 days of age, when molted pups spent about 35% of their time in the water and swim bouts were several hours in duration. M oreover, the presence of a pup's mother on shore, photoperiod, and precipit ation also influenced the amount of time pups spent in the water. Pups (mea n age = 100 days) dove to very shallow depths (mean = 3 m) for short durati ons (mean = 11 s). Because pups did not gain mass unless suckled by their m others, it is unlikely that they fed extensively while diving prior to migr ation. The pattern of development of swimming and diving in northern fur se als is intermediate between typical phocid and otariid patterns, as is the maternal strategy.