When it's too hot for lizards, you know its really sizzling. But this didn't stop Mark Wooden heading out into the Sonoran Desert when the mercury hit 50°C. Frying alive in the mid-day sun, Wooden realised that everything had gone to ground, except for the round-tailed ground squirrels; 'they were all over the place' he remembers. But how could the diminutive mammals keep functioning in the gruelling conditions? Looking at the their body temperature, Wooden realised that the small creatures had gone heterothermic,regulating their temperature over an enormous range; some even went up to 42°C! But this unorthodox lifestyle must surely come at a cost Wooden thought, or why would the rest of us bother stoking the fire and cooling the flames to stick at a comfortable 37°C? Bundling up a few of the small rodents, Wooden headed home to see if he could find what the squirrels had traded in for heterothermy ().
Safe back in Glenn Walsberg's lab in Tempe, Wooden's biggest problem was keeping the ground squirrels slim! Even on as little as 2 g of food a day, the rodents grew tubby. Wooden suspects that the animal's natural diet is so restricted that it has driven them to heterothermy. He thinks that a homeothermic lifestyle is simply too extravagant for the desert's slim pickings. But as heterothermy is clearly beneficial for the majority of mammals, the desert squirrel must have compensated for the loss by cutting back somewhere else. Knowing that locomotion is one of the first faculties to fail when a homeotherm's temperature fluctuates, Wooden decided to test how the squirrels' muscles fared as their body temperature rose and fell.
Startling the rodents with a hissing sound, Wooden filmed the animals as they scampered for safety along an exercise track. Measuring their speed and the rate that their little legs scuttled along, Wooden was astonished that the animals didn't slow, even when their body temperature plummeted to 30°C or rocketed to 40°C! And when he tested their weightlifting capacity, they all managed to lift 1.3 times their own body weight, no matter what their body temperature.
What was going on? Most muscles only work well at one temperature, but these animals weren't paying attention to those rules. There must be something different about their muscles that protects them from the devastating effects of temperature variations. Wooden is beginning to look closer at the muscles'neurofunction. Although he doesn't know whether their neurons function well over a wide temperature range, when he tested their function in the lab, the squirrels muscles somehow defied logic, functioning as well at 15°C as they did at 37°C!
Getting back to Wooden's first question though is still problematic; why are most mammals homeothermic, when the round-tailed squirrel seems perfectly content without? Wooden is optimistic that by identifying the unique adaptations that permit the rodent's heterothermic life-style, he could identify the crucial factor that made the rest of us select homeothermy. Mean while, Nöel Cowards's classic might need rewriting, 'cause there's no sign of ground squirrels getting out of the mid-day sun!
