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For female sea otters, raising a pup to maturity is like surviving multiple, grueling rounds of playoff hockey. And like the playoffs, not everyone makes it to the end.

The fine line between life and death for mother sea otters is the focus of a new study by researchers at the University of California, Santa Cruz, the U.S. Geological Survey, and the Monterey Bay Aquarium. They wanted to learn how much energy a nursing sea otter has to expend in order to successfully raise a pup from birth to weaning. 

Their findings were published today in The Journal of Experimental Biology.

Unlike whales and seals, sea otters lack a blubber layer to keep them warm in the frigid waters of the North Pacific Ocean, says study coauthor Tim Tinker, a marine ecologist with the USGS Western Ecological Research Center. This also means that a sea otter cannot simply fatten a blubber layer and store up energy ahead of time to nurse a pup through each stage of its infancy—a six-month ordeal.

“Sea otters must rely on their fur coat and their super-high metabolism rate to stay warm,” says Tinker, who assisted lead author Nicole Thometz on the study, conducted as part of her UC Santa Cruz doctoral thesis. “The average adult sea otter has to actively hunt and eat 20 to 30 percent of its body mass in food each day just to meet its energy requirements. We wanted to find out how this energy demand changes for a nursing sea otter, which now has to eat to keep herself alive and produce milk for her pup.”

UC Santa Cruz researcher Nicole Thometz explains sea otter biology to kids.  USGS marine ecologist Tim Tinker outside his lab.
UC Santa Cruz researcher Nicole Thometz explains sea otter biology to children visiting the 2012 USGS Menlo Park Open House; USGS researcher Tim Tinker outside his lab in Santa Cruz, California. (Credit Ben Young Landis/Leslie Gordon/USGS)

To measure this change, the team calculated the daily energy demand of sea otter pups at four stages of their development from birth to weaning, and also of weaned juvenile otters, using data from wild sea otters in California and rescued sea otters being raised at Monterey Bay Aquarium.

And since pups derive their energy almost entirely via their mother’s milk and later, on prey she provides them, calculating a pup’s daily energy demand tells researchers how much additional energy a female sea otter needs to raise a pup.

Age classes used to describe sea otter development, based on work by Susan Payne and Ronald Jameson.
Age classes used to describe sea otter development, based on work by Susan Payne and Ronald Jameson.

The results are staggering. The daily energy demand of a female sea otter increases by 17 percent in the first weeks after the birth of a pup, increasing further as the pup grows through its developmental stages. By the end of these six months of nursing, a mother otter’s daily energy demand increases by 96 percent, and she will have invested nearly 222,275 Calories (930 megajoules) in total towards her pup. 

Some of this investment can be accounted for by increased time spent feeding, and indeed some mothers with older pups spend up to 60 percent of each 24-hour period hunting for prey.

However, when prey resources become scare, females have to dip into their own body’s reserves, burning off what little body fat they have and even losing muscle mass.

“This shows us how much rearing a pup can take its toll on sea otter moms, like a hockey team having to play through four consecutive playoff series with increasingly tougher opponents,” says Tinker. “It is simply exhausting. But in the end, a sea otter mom that has sufficient experience and body reserves can win the Stanley Cup—successfully raising her young and staying alive, that is.”

graph of mother sea otter daily energy demands
Researchers extrapolated the change in daily energy demand of a mother sea otter over the course of her pup's development. Her energy demand essentially doubles as her pup matures. (Figure 4 from Thometz et al. 2014)

Canadian-born ecologist Tinker—who confesses he “never forgave the Los Angeles Kings for stealing Gretzky from Edmonton” —says these energy demand findings can provide important clues to understanding why sea otter populations are having trouble rebounding in parts of California.

Working closely with California Department of Fish and Wildlife, Monterey Bay Aquarium, and UC Santa Cruz, USGS researchers conduct an ongoing suite of projects investigating the status of sea otter populations in California. These include field studies of tagged sea otters, annual range-wide surveys, and analysis of dead sea otters found stranded.

This research has highlighted a perplexing trend: prime-age adult female sea otters are experiencing a high rate of mortality, particularly in the center of the range where sea otter densities are highest.

“Adult female deaths often occur immediately after pup weaning. These otters are underweight and deprived of nutrients, leaving them vulnerable to disease, infections, and even harassment by males,” says Tinker, who reports on the annual survey results. “We’ve seen this ‘end-lactation syndrome’ more frequently over the past 10 years, and this new study finally quantifies the extreme energetic demands that lead nursing sea otters toward this malnourished state.”

For Tinker, his next goal will be to compare these energy demand findings with prey availability for different sea otter populations in California.

He and his colleagues are currently analyzing more than a decade of research on sea otter diets and foraging behavior, habitat use, body condition, health threats, mortality and reproductive success, to determine the driving factors influencing population recovery for this federally listed Threatened species.

--- Ben Young Landis


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