How Sea Otters Retain Body Heat Despite Having No Fatty Blubber

Sea otters are among the smallest marine mammals which cope with cold waters. Unlike larger marine creatures like whales, these aquatic otters do not need thick stores of blubber to stay warm.

A recent discovery explains that the otter's secret in retaining body temperature relies not on heat-insulating fatty blubber, but on their muscles. The manipulation of biochemistry in their muscle cells to generate necessary heat had been a mystery until recently, according to The Irish Times.

In order to thrive in waters where temperatures can drop to -1 degree, sea otters must maintain a core body temperature of 37 degrees to survive. Aside from having no thick blubbers, their size also brings them to a disadvantage when it comes to significant heat conservation. However, researchers report in Science in July 2021 how otters meet the challenge of staying warm at sea, which could apply to other marine mammals, as well.

A "game changer" of the marine mammals evolution

Terrie Williams, an ecophysiologist at the University of California, Santa Cruz, who was not involved in the study, commented in ScienceNews that the recent discovery may significantly change our perspective about how mammals in cold oceans develop ways to regulate their body temperature amid the nature of their habitat.

"To me, this is probably one of the clearest pieces of evidence saying, 'Here's how they did it,'" he says.

As the densest in the animal kingdom, there is no way sea otters could have thrived in such an environment with only fur to protect them from losing too much heat. Generally, water transfers heat 23 times as efficiently as air, and their small bodies relative to their volume could lose heat faster regardless of their fluff.

"Being a small-bodied marine mammal in cold waters presents a real thermal challenge," says Traver Wright, a comparative physiologist at Texas A&M University in College Station.

How sea otters maintain body warmth through their muscles

While it wa already known that sea otters rely on an extreme metabolism to maintain warmth, Wright and his colleagues tried to uncover the cellular origins of "that revved-up metabolism for heat generation," Wright says.

Skeletal muscles make up 40 to 50 percent of most mammals' body weight, affecting the whole body's metabolism. The team collected tissue from 21 baby and adult wild sea otters and used a device called a respirometer to measure otter muscle cells' respiratory capacity in different states of oxygen flow. Upon comparing this with other animals, including humans, the otter's oxygen flow exhibited indirect measurement of cells' heat production.

The researchers found that the leaks in mitochondria - the energy-generating part of cells - generate extra heat and cause sea otters' extreme metabolism. To rev up their metabolism, which is basically breaking down food into its basic constituents, otters need to eat more food to make up for that lost energy.

Other mammals with high metabolism, regardless of the size, can also generate heat this way, the study found, although sea otters are much better at it.