Life in the cold can be difficult for animals. When the body cools, the organs including the brain and muscles slow down.
The body temperature of animals like reptiles and amphibians mainly depends on the temperature of their surroundings – however, mammals can increase their metabolism and use more energy to warm their bodies. This allows them to live in colder areas and stay active when temperatures drop at night or during the winter months.
Although scientists know that mammals can increase their metabolism when it is cold, it is not clear which organs or tissues use this extra energy to produce more heat.
Staying warm is a particular challenge for small aquatic mammals like sea otters, so we wanted to know how they adapted to the cold.
We assembled a research team with expertise in both human and marine mammalian metabolism, including Heidi Pearson from the University of Alaska Southeast and Mike Murray from the Monterey Bay Aquarium. Understanding energy use in animals adapted to life in the cold can also provide clues about manipulation of human metabolism.
Metabolism of the sea otter
It is particularly difficult for aquatic mammals to stay warm because water dissipates heat away from the body much faster than air. Most marine mammals have large bodies and a thick layer of fat or fat for insulation.
Sea otters are the smallest marine mammals and do not have that thick layer of fat. Instead, they are isolated by the thickest fur of any mammal, with up to a million hairs per square inch.
However, this coat is very easy to care for and requires regular grooming. About 10 percent of a sea otter’s daily activity is maintaining the insulating layer of air in its fur.
Thick fur alone is not enough to keep sea otters warm. To generate enough body heat, their resting metabolism is about three times that of most mammals of similar size. However, this high metabolic rate comes at a price.
In order to meet the high demand for energy, sea otters have to consume more than 20 percent of their body mass in food every day. In comparison, humans eat around 2 percent of their body mass – around 1.3 kilograms of food per day for a person weighing 70 kilograms.
Where is the heat coming from?
When animals eat, the energy from their food cannot be used directly by the cells for their work. Instead, the food is broken down into simple nutrients like fats and sugars. These nutrients are then transported in the blood and taken up by the cells.
Inside the cell are compartments called mitochondria, in which nutrients are converted into ATP – an energy-rich molecule that acts as the cell’s energy currency.
Converting nutrients to ATP is similar to converting stored water into electricity through a dam. When water flows out of the dam, it generates electricity through rotating rotor blades that are connected to a generator – much like the way the wind turns the rotor blades of a windmill. If the dam leaks, some water – or stored energy – is lost and cannot be used to generate electricity.
Similarly, leaky mitochondria are less efficient at making ATP from nutrients. Although the leaked energy cannot be used for work in the mitochondria, it generates heat to warm the sea otter’s body.
All tissues in the body use energy and produce heat, but some tissues are larger and more active than others. Muscles make up 30 percent of the body mass of most mammals. When they are active, the muscles use a lot of energy and produce a lot of heat. You have undoubtedly seen this before, regardless of whether it gets hot during exercise or shivers when it is cold.
To find out if muscle metabolism helps keep sea otters warm, we examined small samples of sea otters muscle that range in size and age from newborn pups to adults. We placed the muscle samples in small chambers that monitor oxygen consumption – a measure of how much energy is being used.
By adding different solutions that stimulated or inhibited different metabolic processes, we determined how much energy the mitochondria could use to make ATP – and how much energy could flow into a heat-producing leak.
We discovered that the mitochondria in the muscles of sea otters could be very leaky, allowing otters to increase the heat in their muscles without exercising or shivering.
It turns out that sea otters’ muscles are good at being inefficient. The energy “lost” as heat in converting nutrients into motion enables them to survive the cold.
Notably, we have found that newborn pups have the same metabolic skills as adults, even though their muscles have not yet matured for swimming and diving.
Our research clearly shows that muscles are essential for more than just movement. Because muscles make up such a large proportion of body mass, even a small increase in muscle metabolism can dramatically increase an animal’s energy expenditure.
This has important implications for human health. If scientists can find ways to safely and reversibly increase resting skeletal muscle metabolism, doctors could potentially use it as a tool to reduce the rates of obesity increase by increasing the amount of calories a patient can burn.
Conversely, reducing skeletal muscle metabolism could save energy in patients suffering from cancer or other wasteful diseases, and could reduce the food and resources needed to assist astronauts on long-term space flights.
Traver Wright, Research Assistant in Health and Kinesiology, Texas A&M University; Melinda Sheffield-Moore, Professor of Health and Kinesiology, Texas A&M University; and Randall Davis, Regents Professor, Department of Marine Biology, Texas A&M University.
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