Living in the cold can be difficult for animals. As the body cools, organs, including the brain and muscles, slow down. The body temperature of animals such as reptiles and amphibians depends more on their ambient temperature, but mammals can increase their metabolism and use more energy to warm their bodies. This allows them to live in colder areas and stay active at night or during the winter months when temperatures drop.
Although scientists know that mammals can increase their metabolism in the cold, it is not clear which organs or tissues use this extra energy to produce more heat. Staying warm is especially challenging for small aquatic mammals such as sea otters. Therefore, the researchers wanted to know how they adapted to survive the cold. For a new study, several experts in the field of human metabolism and marine mammals collaborated. Heidi Pearson of the University of Southeastern Alaska and Mike Murray of the Montgomery Bay Aquarium were in the group.
Understanding the energy consumption of animals that have adapted to living in cold conditions may also provide clues to the manipulation of human metabolism.
Metabolism of otters
It is difficult to stay warm, especially for mammals that live in water, because water removes heat from the body much faster than air. Most marine mammals have a large body and a thick layer of fat that acts as an insulator.
Sea otters are the smallest marine mammals and do not have this thick layer of fat. Instead, they are insulated by the fur, which has the highest density among mammals (one million hairs per 5.6 cm2). However, fur needs high maintenance and should be cleaned regularly. About 10% of the daily activity of otters is spent on protecting the air insulation layer trapped in their fur.
Dense fur alone is not enough to keep otters warm. To produce enough body heat, their resting metabolism is about three times higher than that of mammals of similar size. However, this high metabolism rate is costly.
Sea otters need to eat more than 20% of their body mass each day to get enough energy to meet their high needs. In contrast, humans eat about two percent of their body mass. (About 1/3 kg of food per day for a person weighs about 70 kg.)
Sea otters eating dandelion crabs in Monterey Bay, California
Where does the heat come from?
When animals eat, the cells in the body cannot use the energy in the food directly to perform their functions. Instead, food is broken down into simple nutrients such as fats and sugars. These nutrients are then transported by the blood and absorbed by the body’s cells.
Inside cells, there is a component called mitochondria in which nutrients are converted to ATP, a high-energy molecule that is used as an energy source.
The process of converting nutrients to ATP is similar to the process of converting a stored water barrier into electricity. When water flows through the dam, it generates electricity by turning the blades connected to the generator. If the dam leaks, some water (or stored energy) is lost and can not be used to generate electricity. Similarly, leaky mitochondria are less efficient at producing ATP from nutrients. Although the energy leaked into the mitochondria cannot be used to do the job, it produces heat that warms the ottoman’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% of most mammalian body mass. Muscles use a lot of energy during exercise and produce a lot of heat. No doubt you have experienced this condition when warming up during exercise and when shivering in the cold.
To find out if muscle metabolism helps keep otters warm, researchers studied small samples of otters’s muscles of various sizes and ages (from newborn puppies to adult otters). They placed muscle samples in small chambers designed to monitor oxygen consumption, which is a measure of energy expenditure.
By adding different solutions that stimulate or inhibit different metabolic processes, the researchers determined how much energy the mitochondria use to produce ATP and how much energy can be converted to heat. The researchers found that the mitochondria in the otters’s muscles could leak a lot and allow the otters to increase the heat inside their muscles without physical activity or vibration. The energy lost in the form of heat helps them survive the cold. It is noteworthy that newborn puppies have the same metabolic ability as adults; Even if their muscles are not yet mature enough for swimming and diving.
A mother ottoman feeds her baby manually and gives him crab pieces
More comprehensive results
Researchers say studying them clearly shows that muscle is important for tasks beyond movement. Because muscles make up a large portion of body mass, even a small increase in muscle metabolism can dramatically increase an animal’s energy expenditure.
This finding has important implications for human health. If scientists can find ways to safely and reversibly increase skeletal muscle metabolism at rest, doctors could probably use it as a tool to reduce the growing rate of obesity by increasing the amount of calories a patient can burn. Conversely, reducing skeletal muscle metabolism can conserve energy in patients with cancer or other debilitating diseases, and it can also reduce the food and resources needed to support astronauts on long-haul space flights.
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