For anyone interested in exercise physiology, whether it's in mice or men,the tool of choice is nearly always the treadmill; just set the treadmill rolling and measure the animal's metabolic rates as it scampers along the track. But that was until Mark Chappell and his co-workers at the University of California, Riverside, wondered whether forcing an animal to run at set speeds can really tell us much about the energetics of voluntary running, and whether the costs of voluntary exercise in various climatic conditions might vary. Knowing that the North American deer mouse has to cope with a range of temperatures across seasons, Chappell and his colleagues wondered how different thermal environments would affect the metabolic costs of roaming free (p. 3839).
But Chappell explains that measuring the rodents in their mountain top homes wasn't practical; the mice are nocturnal and spend much of the winter running in tunnels under snow! Fortunately, Chappell had access to a lab-based deer mouse population, and when given a chance the creatures happily spent time running in exercise wheels. So the team decided to measure the animal's metabolic rate in a specially designed sealed cage where the mice had free access to a wheel and Chappell could continuously monitor their oxygen consumption as they exercised. Placing the customised respirometer in an incubator also allowed the team to vary the ambient temperature to see how the rodents fared. Chappell admits that it wasn't clear whether his new respirometer could successfully measure the rodents' metabolic rates, but remembers that he was relieved `when we plotted our first positive relationship between running speed and oxygen consumption, and knew it was going to work'.
After testing 32 deer mice exercising at 3, 10 and 25°C over periods of a day or more, the team were surprised to find that the animals did not stick to a narrow range of `preferred' running speeds, as humans and horses do. The team also found that the mice spent more energy overall as the ambient temperature dropped. Chappell explains that large mammals use heat produced by exercise to keep warm when the temperature falls. The deer mice were also saving energy by substituting some exercise heat for other forms of heat production, such as shivering, which could clearly come in useful in chilly conditions.
While exercise heat might be energy-saving at some temperatures, the deer mice did not seem overly concerned about economy; they did not prefer to run at high speeds, which have the lowest transport costs. The little creatures were clearly not pushing themselves, since Chappell found that the rodent's maximum voluntary running speed was 4 to 5 km h–1,considerably lower than their maximum sprint speed of 13 km h–1. `We also didn't see any `wind sprints', Chappell recalls, `the mice almost never chose to run at speeds that required maximum oxygen consumption'.
But Chappell is still puzzled by what motivates deer mice to run. The team hope that they will one day understand what motivates voluntary running in mice and other small rodents, and that their novel setup may eventually provide the key to many other questions that forced-exercise methods cannot answer.