Metabolic boost puts new sea otter mums at risk of death

When lactating, a sea otter's metabolism can more than double – and this is behind their disproportionate mortality rate around the time of weaning. Belinda Smith reports.

Sea otters pay a hefty price for motherhood – their metabolic rate shoots up more than 50% while suckling a pup.
Joe Tomoleoni

Few scenes are as serene as a baby sea otter snuggling up to its mum floating on her back. But beneath the peaceful exterior, the otter mum's metabolism is going through the roof.

Zoologists from the University of California at Santa Cruz measured the resting metabolic rate of a nursing otter mum and found it rises to chug along up to 50% faster than before her baby was born.

This means she's at much higher risk of dying until her baby is weaned, says lead author Nicole Thometz: "This represents a substantial energetic burden for a species with already high baseline energy demands and minimal energy reserves and is likely one of the underlying reasons why we are seeing high mortality rates."

That sea otters (Enhydro lutris) use more energy after pregnancy has been known to zoologists for a couple of decades. (The same is found in humans, or any other lactating animal – the female must provide food for her baby as well as herself.)

But female sea otters don't put on as much fat for their size during pregnancy as other marine mammals, so they burn through those stores in the first few weeks of their six-month lactation period.

Meeting their pup's demand for food puts them under huge metabolic stress which is why more than half of adult female sea otter deaths in California happen around the end of weaning.

To get a handle on how much higher a mother otter's metabolism rises after pregnancy, Thometz and her colleagues examined two adult female sea otters from California.

They were caught as US Fish and Wildlife considered them to be "unfit for the wild" because they kept hanging around humans.

Both were healthy – "Mollie" was around four years old and weighed 22 kilograms while "Clara" was around three years old and weighed 18 kilograms.

Her daily energy demands more than doubled by the time her pup was four months old.

But Clara was in the early stages of pregnancy, so the pair was brought to the University of California at Santa Cruz for observation.

They were individually placed in a water bath beneath a clear plastic dome and allowed to float on the surface on their back – as they would in the wild.

To track their metabolic rate, the researchers monitored the amount of oxygen the otters used while they were floating and resting for at least five minutes. The more oxygen they consumed, the higher their metabolism was running.

They were tested regularly for a year – from before Clara gave birth until a few months after the pup was weaned.

While pregnant, Clara's resting metabolic rate was around 17% lower than after her pup was weaned. This was so she could stack on the fat reserves – like other mammals do.

After she gave birth, Clara and her pup were popped under the dome as separating the two would be distressing to them both. So the researchers estimated and subtracted the pup's oxygen use to calculate the mother's metabolism.

Two months after giving birth, Clara's resting metabolic rate increased steadily to peak after a month and sat at around 51% of her pregnancy levels for the rest of lactating time. Her daily energy demands more than doubled by the time her pup was four months old.

After weaning her pup, though, Clara's metabolic rate dropped back to levels almost identical to Mollie's.

While the experiment used just two otters, it shows that "reproduction is exceptionally physiologically taxing for female sea otters", the researchers write.

The work was published in the Journal of Experimental Biology.

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