Energy Demands Are Sometimes Too Great For Nursing Sea Otters
June 12, 2014

Energy Demands Are Sometimes Too Great For Nursing Sea Otters

Brett Smith for - Your Universe Online

Living in frigid waters without an insulating layer of blubber, sea otters expend large amounts of energy simply trying to stay alive. When a female sea otter gives birth, the additional strain of nursing and raising a pup can often be too much to cope with, especially after receiving a minor wound or infection.

A new report published in the Journal of Experimental Biology examines this "end-lactation syndrome" to understand the energy demands that raising a sea otter pup put on a mother.

"These fundamentally high energy demands are likely the underlying reason why we see so much mortality among prime age females in the middle of their range, where the density of the sea otter population is highest and resources are limited," said study author Nicole Thometz, a biologist at the University of California, Santa Cruz.

Even without raising a pup, the average sea otter must spend 20 to 50 percent of the day foraging for food and typically eating about a quarter of their body weight in seafood. The study team discovered how the daily energy requirements of a mother sea otter jump by 17 percent in the first weeks of the birth of a pup and grow progressively as the pup gets bigger. Ultimately, the mother's daily energy demands are 96 percent greater, almost twice what they are when she isn't being a mom. In other words, she has to find almost double the food on a daily basis to keep herself and her pup alive.

For their study, the researchers examined the energetics of growing sea otter pups that were found stranded in the wild and being raised at the Monterey Bay Aquarium, which uses surrogate mothers to raise the pups until they can be returned to their natural habitat. The team measured the pups’ oxygen consumption to determine their metabolic rates at various activity levels throughout their development.

The scientists also witnessed sea otter pups in the wild to see the amount of time they spent each day relaxing, grooming, looking for food, and various other activities. Incorporating these "activity budgets" in the wild with the metabolic rates assessed at the aquarium made it possible for the researchers to determine average daily energy demands for pups at five developmental phases. The daily energy demands of adult female sea otters not raising pups were determined using previous studies.

The researchers found that it takes about six months for a mother sea otter to raise pups to the point that they can become independent.

"The majority of the pup's calories come from its mother, so we tracked the combined energy demands of mom and pup over time. Given that sea otters have such high baseline energy demands, what these females are doing to raise a pup is really extraordinary," Thometz said.

She added the findings also sadly point to why so many sea otter pups are abandoned in the wild.

“Female sea otters are thought to utilize a ‘bet-hedging’ strategy, either keeping or abandoning a pup post-partum [after birth] depending upon physiological factors,” Thometz said.

”The optimal decision may be to ‘cut losses,’” she noted.