Sea otters is without a doubt Michael Phelps from the marine mammal world. They are excellent swimmers, complete with muscular bodies and flippers that allow them to easily cut through the water.
They also resemble Phelps in another way: they are extraordinary in their abilities; able to succeed where other mammals of their size would not have a chance.
Sea otters live in cool waters that can reach temperatures of 32 to 59 degrees Fahrenheit. Typically, mammals living in this type of environment can withstand the cold through blubber or being large.
Previously, researchers were not sure how these relatively small animals (males on average four meters long) managed to maintain a metabolism similar to mammals three times their size. Now, a study was published Thursday in the journal Science points to a distinctive feature of their anatomy as the answer: unique skeletal muscles.
“This study shows how the skeletal muscles of sea urchins are well-suited to generate heat, which is critical for these small marine mammals to survive in cold water,”
What you need to know first – Otters are the smallest marine mammals in the world, and their small body size puts them at a disadvantage when it comes to surviving in the cold waters of the North Pacific.
Many larger marine mammals, such as whales, use fatty deposits known as blubber to retain the heat in their bodies. Lacking such fat, sea otters keep the heat in their tightly packed hair – but even that mechanism is not enough to maintain a body temperature of 98.6 Fahrenheit.
If the sea urchin’s body temperature drops too long below this core temperature, they will die – just like humans.
“Polar and small-body mammals are particularly vulnerable to heat loss and require increased heat production to maintain body temperature,” the study team wrote.
Sea otters again adapted in a different way: their bodies generate a basal metabolic rate that is three times the predicted rate for mammals of the same size.
“Basal metabolic rate is essentially the minimum metabolism required for an animal to maintain basic body function without moving, eating and digesting,” explains Wright.
This increased basal metabolic rate is an adaptation to retain heat in a low temperature environment, also known as thermogenic hypermetabolism.
Prior to this study, researchers did not fully understand the mechanisms underlying the otter’s thermogenic hypermetabolism and how it allows them to thrive in cool environments.
How the discovery was made – Wright and colleagues hypothesized that there could be some connection between the sea otter’s unusually high basal metabolism and its ability to survive in the cold waters of the North Pacific.
Previous studies suggested that sea otters’ basal metabolism was about “three times higher than one would predict based on their size, indicating that their body burns a lot of energy at rest,” says Wright.
Considering that the team speculated that the sea otter’s hypermetabolism was linked to thermogenesis – a function that allows mammals to stay warm by generating heat through certain tissues and skeletal muscles.
“Given how challenging it is for these animals to stay warm, this hypermetabolism is thought to work for thermogenesis,” Wright says.
They put the hypothesis to the test by assessing the respiratory or respiratory ability of sea urchins with different body masses. The researchers used respirometry, which can test the oxygen consumption of live animals.
What is new – Sea urchins heat up through something known as a mitochondrial leak in their skeletal muscles, the study found.
Also known as leakage metabolism, previous studies have shown that this also helps hedgehogs and other mammals stay alive in cold temperatures. Mitochondria are the small organelles in our cells that produce energy that keeps us warm and helps us stay alive.
“These values are also higher than comparable measurements reported for any known mammal.”
The researchers were able to link the leakage metabolism in the sea otter’s skeletal muscles with its breathability – its oxygen consumption rate – which helped explain why its basal metabolic rate is so high compared to other mammals.
“We found that the metabolic capacity of the leak in muscle tissue is high enough to explain the hypermetabolism previously demonstrated in sea urchins,” Wright said.
The sea otter’s leakage metabolic capacity is so high that it surpasses the same capacity in even Alaskan huskies and energetic Iditarod sled dogs. “These values are also higher than comparable measurements reported for any known mammal,” the research team writes.
What’s next – Havoder’s ability to thrive in cool waters has puzzled researchers, but this new research finally reveals how their bodies have adapted to their cold environment. The research helps researchers better understand how small aquatic mammals stay alive in unusually harsh environments.
However, it is difficult to say whether the sea otters’ bodies are a long-term evolutionary adaptation to lower temperatures or a natural response to cold water.
“It is difficult to distinguish between how much of this unusually high leakage of metabolism develops through evolutionary adaptation and how much develops as an acclimatization in response to chronic exposure to cold,” says Wright.
Ultimately, these findings inform about a better understanding of sea urchins – but there is more work to be done, the researchers say.
“This study showed that the skeletal muscle of the sea urchin is suitable for thermogenesis, but was only taken in a single muscle,” says Wright.
Further research should look at “different muscles throughout the body” and other metabolic tissues that may be suitable for thermogenesis, he explains.
In the meantime, we will have to content ourselves with appreciating how cute they are.
Abstract: Basal metabolic rate generally scales with body mass in mammals, and variation from predicted levels indicates adaptive metabolic remodeling. As a thermogenic adaptation to living in cool water, sea otters have a basal metabolic rate that is approximately three times the expected rate; however, the tissue level source of this hypermetabolism is unknown. Because skeletal muscle is an important determinant of metabolism throughout the body, we characterized respiratory capacity and thermogenic leakage in sea urchin muscle. Compared to mammals previously sampled, the capacity for thermogenic muscle leakage was increased and could account for hyperterabolism in sea urchins. Muscle respiration capacity was modestly elevated and reached adult levels in neonates. Premature metabolic development and high leakage indicate that sea otter muscle metabolism is regulated by thermogenic demand and is the source of basal hypermetabolism