The Demands of Sea Otter Motherhood Prove Costly

Motherhood is exhausting work, and none know this better than sea otter moms, whose exhaustive efforts can at times prove fatal.

Sea otter moms work so hard to raise their pups that they are depleted physiologically, and zapped of so much energy that they cannot survive the stress of a minor wound or infection - an occurrence known as "end-lactation syndrome." Scientists believe it accounts for high mortality rates among female sea otters in some areas.

University of California, Santa Cruz biologist Nicole Thometz set out to quantify the energy demands of a growing sea otter pup, and find out just how costly raising one can be for their mothers. The findings are published in the Journal of Experimental Biology.

 Even without a pup, adult sea otters have remarkably high energy requirements, eating about a quarter of their body weight in seafood every day. And as the smallest marine mammals, living in cold coastal waters with thick fur but no insulating layer of blubber, sea otters need to maintain a high metabolic rate to stay warm. It's no surprise that motherhood can push female otters past their limit.

"These females have to accomplish a huge energetic task every year," Thometz said in a statement.

Thometz and her colleagues found that the daily energy demands of a female sea otter jump by 17 percent in the first few weeks after the birth of a pup, and grow steadily as the pup gets larger. Eventually, the mother's daily energy demands are 96 percent higher - nearly twice what they are when she doesn't have a pup. In other words, she has to find almost twice as much prey every day to keep herself and her pup fed.

"These fundamentally high energy demands are likely the underlying reason why we see so much mortality among prime age females," Thometz added.

Raising a pup takes about six months before they are weaned and independent enough to feed on their own. The findings explain why female sea otters are so often found in poor condition - weak and skinny - at the end of the lactation period, and why some mothers abandon their pups before they are fully grown.