Northern elephant seal development: The transition from weaning to nutritional independence

Summary

• 1. The 2 1/2-month period from weaning to departure from the rookery was studied in northern elephant seal pups at Año Nuevo Island, California. After being weaned at 4 weeks of age, pups fast completely while learning to swim and dive before going to sea in search of food.

• 2. Pups increase their birthweight by a factor of 3–7 times on mother's milk alone. Males weigh more than females at birth and at weaning, and are nursed an average of one day longer.

• 3. Some newly weaned pups prolong the suckling period by stealing milk from nursing females or by being adopted by foster mothers. Males are more persistent and more successful at milk stealing than females and, consequently, exceptionally large weaners are always males. These sex differences are predicted from the manner in which adults maximize their reproductive success. An advantage in size and condition benefits a male much more than a female. Molting and canine eruption occur later in males than in females and these differences may be an adaptation in the former which facilitates milk stealing.

• 4. Sexual differentiation is evident in the earliest social interactions. Males interact more frequently than females and their bouts last longer. Behavioral components in the interactions of males and females resemble those of fighting adults of each respective sex.

• 5. Weaners begin entering the water when 6–7 1/2 weeks old. They learn to swim and dive gradually. As swimming proficiency increases, they venture further away from their birthplace. They are excellent swimmers and can stay underwater for 15 min by the time they leave the rookery at 3–3 1/2 months of age.

• 6. Although pups are born over an 8-week period, the majority of them depart the rookery over a 3-week period. Pups weaned late in the breeding season are stimulated to enter the water by those weaned earlier. Departure from the rookery is correlated with the onset of coastal upwelling and the latter is correlated with food abundance in the ocean.

• 7. Few weaned pups die before leaving the rookery. Mortalities result from injuries inflicted by adult males and females, trauma or drowning due to storms and high surf, overheating, accidents, and shark predation.

• 8. Survival after the first period at sea to 7 months of age is at least 50%. Thereafter, the survival rate declines gradually to a minimum of 12% at 3 years of age. There are no sex differences in survival to age 2. Pups born early in the breeding season exhibit a higher survival rate during the first year than those born later in the season; females born early in the season exhibit the highest parity rate in their cohort at age 3.

• 9. Early sex differences are interpreted as subserving the different adult reproductive strategies of each sex.