THE INFLUENCE OF CLIMATIC SEASONALITY ON THE LIFE-CYCLE OF THE PRIBILOF NORTHERN FUR-SEAL

Weather conditions recorded from 1956 to 1986 on St. Paul Island, Alas ka, were probed to establish their influence upon the northern fur sea l's life cycle (Callorhinus ursinus). Air temperatures, wind speeds, a nd relative humidity levels were seasonally decomposed and compared wi th the timing of pupping and migration. Most pups are born in early Ju ly when air temperatures and relative humidity approach their highest annual levels and wind speeds are at their lowest. Weather conditions favor growth and survival of pups from July to September but are unfav orable in June. A rapid deterioration in weather through October and N ovember corresponds with the fall migration of pups and lactating fema les. The data suggest the pivotal event in the fur seal's life cycle i s the timing of birth and survival of nursing pups. As such, the ultim ate determinant of the precisely timed fur seal life cycle appears to be climatic seasonality during the breeding season.