How the sea otter stays warm: a mystery that was recently solved

The sea otter is one of the smallest marine mammals (1.2-1.5m long, 14-45kg in weight) native to coastal seas of the northern and eastern North Pacific Ocean. Sea otters have no heat-insulating fatty blubber layer like larger marine mammals.

How they generate sufficient heat to maintain core body temperature was a mystery until recently. We now know that they manipulate the biochemistry of their muscle cells to generate the necessary heat – Traver Wright and others wrote about this in Science in July 2021.

Sea otters must maintain a core body temperature of 37 degrees to stay alive but live in waters where temperatures can drop to -1 degree. Larger marine animals, eg whales, rely heavily on fatty blubber to retain body heat but the lean sea otters have no blubber.

Being small in itself confers a further significant heat conservation disadvantage. Body surface to volume ratio increases as volume decreases, a very significant factor when you live in water which transfers heat 23 times more efficiently than air. So, rate of passive heat loss from a large animal like a whale is much less than from the small sea otter.

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Thick coat

The sea otters’ primary heat insulation is an exceptionally thick coat of fur, the densest in the animal kingdom, but this alone is insufficient to maintain necessary core body temperature. The secret to how sea otters maintain body warmth lies in their muscles. Living cells are chemical machines and the totality of their chemical reactions is called metabolism. Skeletal muscle comprises 40 per cent to 50 per cent of bodyweight. Muscle metabolism dominates whole body metabolism and sea otters rely on extreme muscle metabolism to maintain body temperature.

One primary reason for eating is to convert ingested food (carbohydrate, fat, protein) into chemical energy to power all the vital activities of the body. Food is broken down into its basic constituents (sugars, amino acids and fatty acids) and these are transported via the bloodstream around the body and absorbed by the cells of the various tissues. Here they are oxidised (“burned”) in organelles called mitochondria to produce a chemical called ATP, the energy currency of the cell used to power all the cell’s activities.

The mitochondrion can be visualised as two sacks, a larger folded sack enclosed within a smaller sack. This creates two spaces, the inner mitochondrial space and the space between the inner and outer sacks. Energy released from burning food molecules is used to pump charged hydrogen (H) atoms (H ions) from the inner mitochondrial space to the space between the two sacks.

These concentrated H ions now return across the inner sack membrane into the inner mitochondrial space in a controlled manner, spinning a molecular wheel that generates ATP as they cross the membrane. But if H ions leak back across the membrane without spinning the wheel, no ATP is generated and the energy stored in the H ions is released as heat.

Mill wheel

The situation is analogous to the rotation of a mill wheel where the potential energy released as water falls over the wheel, from a higher to a lower level, powers the rotation of the wheel. Wheel rotation occurs most efficiently when the falling water is accurately directed into the collecting buckets on the wheel’s circumference. If the water is not accurately directed, some/much will avoid collection by the buckets, its energy is not harnessed to help turn the wheel.

Something similar happens in the sea otter mitochondria. Many of the H ions pumped across the inner membrane leak back through the membrane without spinning the ATP wheel. The energy of this hydrogen flow is uncoupled from generation of ATP and generates the heat essential to maintaining sea otter body temperature.

To fuel this extremely high rate of metabolism (three times the predicted rate for animals of their size), otters must eat food equivalent to 25 per cent of their body mass per day. Other mammals such as very small mice and hibernating bears can also generate heat energy by this mechanism, but sea otters have developed this capacity to a greater extent than any others.

Sea otters are also one of very few tool-using animals, prying shellfish loose from rocks using small rocks held in their forepaws and breaking them open to eat the flesh by smashing the shellfish against larger rocks. Sea otters were hunted intensively and, although hunting is now illegal, they remain classified as an endangered species.

William Reville is an emeritus professor of biochemistry at UCC