Got Milk?

Stealing milk from an unrelated mother is extremely rare amongst mammals, and has seldom been reported for sea lions. Historically, only one recorded case exists of a Steller sea lion pup successfully obtaining milk from an unrelated female, and the incident only lasted 30 seconds. New sightings are therefore captivating for their rarity, and the bearing they have on the poorly understood process of weaning in Steller sea lions.

In a recently published paper, Boyd Porter and Andrew Trites report observing two new occurrences of pups repeatedly suckling lactating females that were not their mothers at a winter haulout site at Timbered Island in Southeast Alaska. One of the sightings involved a mother providing care to an alien pup that was still with its own mother. Surprisingly, the biological mother-pup pair allowed the alien pup to continue suckling without incident, and with no apparent sign of aggression. This case suggests the milk-stealing pup was under fed and needed more milk than its mother could provide.

The second sighting involved a female pup that was never seen with her biological mother, and was considered to be starving due to her deteriorating condition. Porter and Trites observed the pup for 15 consecutive days, during which it made several attempts to steal milk from sleeping mother-pup pairs, typically triggering an aggressive response from the mother. With time, lactating females became increasingly vicious in their attacks, eventually forcing the pup into isolation on the periphery of the haulout, where it starved to death.

Milk stealing rarely occurs without conflict, and such an aggressive response from targeted females is typically to be expected. A possible explanation for this behavior lies in the high energetic cost of providing milk for a single pup. Average daily food requirements of mature females nursing pups are about 70% greater than for females without pups, and captive studies suggest that it is physically impossible for a mother to fully nourish two pups at the same time. With so much effort put into feeding their own, it is hardly surprising that lactating females resent having their milk stolen by others!

Milk stealing, therefore, does not appear to be an effective means of countering starvation for pups. The risks alone of such an endeavor appear to discourage most starving pups from embarking upon this activity in the first place. The only other alternative a pup might attempt is to catch its own fish and wean itself early, yet this option also has its challenges.

On several occasions, Porter and Trites observed immature Steller sea lions swimming on the surface with live prey in their mouths. However, it was unclear whether the young sea lions had caught them or had scavenged them. While the pups mimicked adult foraging behavior throughout this activity, they abandoned and did not consume any of the prey observed.

It is surprising that the pups observed stealing milk did not supplement their intake with fish given the apparent ability of this age group to capture prey. The fact that they did not suggests they may not be behaviorally or physiologically capable of consuming fish. Compared with milk, they may also not be physically capable of consuming enough prey to meet their daily energy needs during this period of rapid growth and development. While the process and exact timing of weaning in Steller sea lions is not known, the research reveals that its occurrence may take place much later in spring or early summer than previously thought.

21 February 2005

Publication:
Suckling attempts during winter by two non-filial Steller sea lion pups (Eumetopias jubatus). Porter, B.T. and Trites, A.W. 2004. Mammalia 63:23-26. abstract Milk stealing and fostering care is rare among mammals. Among pinnipeds, the nursing of offspring that are not their own has been noted for some species of seals, but rarely for sea lions or fur seals. Thousands of hours have been spent observing Steller sea lions in the wild, but only a few successful suckling attempts have been noted. From January to March 1996, we observed two non-filial pups repeatedly suckling lactating females at a winter haulout site at Timbered Island in southeast Alaska. These two observations are noteworthy because of their rarity and the bearing they have on the poorly understood process of weaning in Steller sea lions. The timing of weaning in Steller sea lions has been speculated to occur sometime during winter or spring when pups are 6 months or older. Both mothers and pups we observed were aggressive toward intruding conspecifics and were very protective of their mother’s teats. However, there was a range of individual variation in the tolerance of both mature females and their offspring to the distance they would allow strange pups near the teats. It is undoubtedly advantageous for nutritionally stressed pups to attempt to steal milk, compared with the alternative — starvation. However the potential for injury likely out-weighs any gain in resources and probably deters most young from attempting to approach strange females. The pups we observed stealing milk did not supplement their intake with fish despite the apparent ability of this age group to capture prey. The fact that they did not suggests that they may not have been behaviourally or physiologically capable of consuming fish. Compared with milk, they may also not be physically capable of consuming enough prey to meet their daily energy needs during this period of rapid growth and development. This further suggests that weaning of Steller sea lions pups may occur much later in spring or early summer than many have previously thought.