Sea Otter Moms Risk Lives to Raise Babies

Sea otter moms invest so much energy in raising their pups that they risk their own survival, a new study suggests.

The marine mammals spend about 930 megajoules—a measure of energy—to bring up baby, which is the equivalent of burning through 133 percent of their body mass.

A sea otter and her one-week-old pup in Monterey Bay, California, in 2009. Photograph by Suzi Eszterhas, Minden/Corbis

Sea otters have huge energetic requirements even when they aren’t raising young. The animals, found along coastal areas in the North Pacific, consume a quarter of their body mass in food each day, in part because their small bodies do not hold heat very well and they lack the insulating layer of blubber that is found in many other marine mammals.

Not surprisingly, mothers with pups need even more food. But until now, how much more was an open question.

The new study, published June 11 in the Journal of Experimental Biology, revealed that females with six-month-old pups need to consume nearly twice as much food as females with no pups in order to satisfy the family’s daily energy needs. In an effort to accomplish this, some mothers spend up to 14 hours per day foraging for food.

“This shows you how hard these females are working,” said study leader Nicole Thometz, a biologist at the University of California, Santa Cruz. (See National Geographic’s pictures of animal mothers and babies.)

Some mothers don’t get enough energy and end up losing weight, which “makes them more vulnerable to things like infection and disease,” said Thometz.

Life on the Edge

For the study, Thometz and her colleagues measured the energetic demands of sea otter pups at the Monterey Bay Aquarium in California. Those data, in combination with information on the behavior of wild sea otters, were then used to estimate the total energetic burden placed on sea otter mothers.

The findings surprised Thometz. “Knowing how high their energetic costs were already, I wasn’t sure how much they could go beyond that.”

Daniel Ardia, a biologist at Franklin and Marshall College in Lancaster, Pennsylvania, agreed, noting that this study reveals “a remarkable level of maternal investment.

“It suggests that sea otters are a species living on a thin margin, placing them at risk at a time of rapid global change.” (See video: “Sea Otters in Danger.”)

Tough Choices

Areas that are densely populated with sea otters, such as the central California coast, seem to be particularly challenging places to raise pups due to increased competition for food. 

Biologists in California have noted that as otter populations have slowly rebounded in recent decades, following catastrophic declines due to hunting in the 18th and 19th centuries, an increasing number of females appear to be thin and unhealthy after raising their pups. 

Other females are forgoing the full demands of motherhood, abandoning their pups shortly after birth or weaning them at an earlier age. (See: “Sea Otters Strike a Blow for the Environment?”)

In lean times, giving up on their pups lets females prioritize their own survival, perhaps leaving the door open for future breeding attempts.

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Meet the Author
Katie Langin holds a Ph.D. in ecology and was a 2014 AAAS Mass Media Fellow at National Geographic.