Leaky muscle, hotter otter

Abstract

Just the thought of diving into 5°C water is enough to start many of us shivering, but many marine mammals spend their lives submerged in chilly ocean waters. While most of these aquatic animals have thick layers of insulating blubber, sea otters have only their dense fur to keep out the cold. But as the smallest of the sea-going mammals, sea otters lose body heat to cold water so rapidly that fur isn't enough to keep them warm. Instead, these animals must generate excess body heat to keep their temperatures up. Traver Wright of Texas A&M University, USA, and the University of Texas Medical Branch, USA, with colleagues from Texas, Alaska and California, investigated the source of this body heat to understand what allows sea otters and other mammals to thrive in such challenging environments.Skeletal muscle is important for generating body heat through contractions like shivering, but in some cases it can also warm things up by ‘leaking’ energy from metabolism. Cellular metabolism involves converting food into chemical energy that can be used by the body for many different purposes. However, muscles can short circuit this energy-generating cycle to warm the body by diverting the proton gradient that usually drives the production of chemical energy to generate heat instead during the final steps of metabolism. Wright and his team hypothesized that because muscle is such a large and metabolically active tissue, muscle energy leak could be the key feature helping sea otters to keep warm. To test their prediction that sea otter muscle is leakier than that of other mammals, the team measured the metabolic activity from muscles of sea otters across a range of sizes and ages.The researchers found that sea otter muscles produce amounts of stored chemical energy similar to that of the muscles of other mammals, but the amount of additional energy leaking out as heat was higher than seen in any other mammals of a similar size. This relatively high energy loss supports the hypothesis that muscle leak is important for maintaining body temperature in these sea otters. The team also found that sea otters had similar rates of muscle metabolic activity and leak regardless of size or age, including the youngest pups. This suggested that high metabolic heat production is important at all stages of life.Wright and his colleagues then used published measures of sea otter muscle mass to calculate how much all the leaky muscles combined could contribute to whole-body heat production. They found that muscle leak could account for the energy demands of keeping sea otters warm at larger body sizes, but is not enough to explain the energy demands of the smallest sea otters. Instead, these pups may need to rely on heat generated from other tissues, actively shivering, or cuddling close to their mothers to stay warm.Overall, leaky muscles can explain how high metabolic rates help to protect sea otters from the cold. The detection of this heat source in even the youngest sea otter pups suggests that it is an important adaptation for life in frigid waters. For these mammalian mariners, it's the leaks that keep them seaworthy.