Sea otters always seem to be having fun, gamboling about in the kelp forests off the coast of California or lazing on their backs on the surface of the waters near Alaska. In truth, though, being a sea otter takes a lot of hard work. Unlike most other marine mammals, sea otters lack a blubber layer that would keep them warm and store energy for the future. Dense fur and a high metabolism help, but adult sea otters still have to eat the equivalent of around 20 to 25 percent of their body mass in food every day to survive.
Female sea otters, though, have it particularly rough. That’s because they spend about six months of the year — every year in adulthood — feeding a pup, sometimes two. That puts huge energy demands on sea otter moms, and also explains why and when some may abandon their young, according to a study published June 15 in the Journal of Experimental Biology.
Nicole Thometz of the University of California Santa Cruz and colleagues wanted to determine how much energy sea otter mothers expend on lactation. The researchers measured the metabolism of sea otter pups that had been born in the wild and taken to the Monterey Bay Aquarium in California for rehabilitation. They also observed free-ranging sea otters. They then calculated how much energy is used by sea otter pups at five stages of life. That’s the amount of energy a sea otter female would provide to her pup through her milk.
A 3-week-old pup increases its mother’s energy needs by 17 percent, Thometz and her team calculated. By four months, the pup requires the equivalent of 78 percent of its mother’s pup-free energy demand. And by the time the pup is weaned at six months, its mom needs some 96 percent more energy to keep her kid fed. At that late stage, a juvenile sea otter is beginning to forage on its own, but it still frequently nurses from its mom, asks her for food and even steals prey from her, making the pup “a substantial burden for [an adult female] until weaning occurs,” the researchers write.
How much of a burden? If a sea otter mom didn’t increase her foraging, “she would lose 29 kilograms in body mass by the time her pup was weaned at 180 days,” the team writes. “This value corresponds to an impossible >100% reduction in body mass and certain mortality.”
These high energy demands may explain why young sea otters sometimes get abandoned. Females have little choice about getting pregnant, and they give birth every year “regardless of current body and environmental conditions,” the researchers note. It’s only after giving birth that she can make the decision of whether she has enough energy stores to devote to a pup — she risks death if she chooses to raise the pup without sufficient reserves. If she’s in poor condition, her best decision may be to cut her losses and leave the pup behind.