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SQUIRRELS CHOOSE TO COOL BY WARMING
kathryn{at}biologists.com
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When the days draw in and the temperature begins to fall, many mammals prepare to survive the cold winter when food is scarce by dropping their metabolism and becoming torpid. But some animals can also slip into torpor when their environment becomes hypoxic, dropping their temperature and metabolic rate until the oxygen levels return to normal. Glenn Tattersall is intrigued by the physiological changes that overcome the ground squirrels as the oxygen level drops. He wondered if the animal's drop in temperature is because the tiny creatures reset their body temperature to a lower level, or because they simply can't make enough heat to stay warm when they reduce their metabolism. In this issue of the J. Exp. Biol. (p. 33), Tattersall describes how the animals modulate their temperature as they descend into torpor and recover later, which suggests that the ground squirrel's cooling is a matter of choice, rather than the physics of their matter.
Tattersall decided to see whether the squirrels regulated their body temperature in response to an environmental factor that he had control over: oxygen. He decided to monitor the squirrel's surface temperature and metabolic rate as he varied the oxygen level in a respirometer. The biggest problem was building a squirrel-sized respirometer from material that didn't absorb their infrared signature; until Tattersall finally hit on the idea of using a Ziploc bag to make an infrared window! Dropping the oxygen to 7%, he watched the squirrel's metabolic rate fall as their bodies began radiating heat. Within moments of the oxygen decline, the animal's ears, nose and feet began glowing, as the animal's diverted their blood supply to rapidly dump heat into the environment and cool their core.
However, when he returned the oxygen level to 21%, the squirrel's surface showed no sign that the tiny creatures had reignited their central heating. Tattersall repeated the experiments at several environmental temperatures, and noticed that the colder the ambient temperature, the greater the drop in the squirrel's metabolic rate. He explains that this suggests that as the oxygen level falls, the squirrels suddenly change their temperature set point, dropping their metabolic rate and turning off heat generation. Once the animal's metabolic rate has fallen, it must dump body heat as fast as possible.
Although adult humans appear not have the metabolic flexibility to change their temperature set point, newly born babies might. Many premature infants have severe breathing problems soon after birth and often become hypoxic. Tattersall explains that if this is the case, the tiny babies could have a lower temperature set point like the ground squirrels, so clamping their body temperature at 38°C could face struggling infants with a severe metabolic challenge. So understanding how North American ground squirrels choose to cool could help clinicians treat some of their most vulnerable patients.
References
Tattersall, G. J. and Milsom, W. K. (2003).
Transient peripheral warming accompanies the hypoxic metabolic response in the
golden-mantled ground squirrel. J. Exp. Biol.
206, 33-42.
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