Until recently, hibernation was thought to occur solely among animals in the Northern Hemisphere. Only over the past ten years has the process been documented and studied in animals of the south.
One of the subjects of that study is the eight-inch-long mountain pygmy possum. Some 2,000 of them remain in the wild--living in boulder- strewn fields one mile high in the Australian Alps. Zoologist Fritz Geiser of the University of New England in Australia has been monitoring these animals since last year, when he discovered that, come winter, they curl up into tight balls, cool down, and hibernate. Geiser fitted ten pygmy possums with temperature-sensitive radio transmitters and recorded their temperature for five months. When they hibernate, he found, their body temperature drops from 96 to 35 degrees and their metabolic rate falls to 1 percent of its normal level. The body temperature of bears, in comparison, typically drops only 5 degrees or so, and their metabolic rate slows to a third of normal.
Also unusual are two mouse lemurs from Madagascar. Gerhard Heldmaier, a zoologist from the University of Marburg in Germany, has shown that both the common gray mouse lemur and the western rufous mouse lemur (which until last year was thought to be extinct) routinely enter a state called daily torpor. During normal sleep, a mammal’s body temperature generally drops about half a degree or so. (That’s why people like to cover up with a blanket when they go to sleep.) But in daily torpor, a lemur’s temperature can drop 30 degrees or more and then warm back up to normal, all over a period of six to eight hours.
As the day warms up, the lemurs warm up also, regaining their normal temperature by noon. This might sound like the strategy of a cold- blooded animal, but the lemurs have a much higher metabolic rate than any reptile and can generate their own body heat. Using the sun’s heat, however, is more efficient.
Mouse lemurs are nocturnal and have a normal body temperature of about 98. Heldmaier found that during daily torpor, at around four in the morning their temperature drops precipitously, reaching 68 by eight o’clock. During those hours, they live on a fraction of the energy requirements they normally have, says Heldmaier. He calculates that the mouse lemurs save 40 to 50 percent of their normal energy expenditure.
What’s most striking is that mouse lemurs become dormant under relatively mild conditions. The temperature in tropical Madagascar rarely dips below 50. But during the dry winter season, says Heldmaier, the insects, leaves, and fruits that the lemurs eat become scarce. Hibernation and torpor, Heldmaier says, are not merely northern adaptations to cold but universal energy-saving strategies, and he suspects that many other hibernators remain to be discovered throughout the Southern Hemisphere.
Southern mammals have a particular need for such strategies. El Niño, a periodic fluctuation in ocean temperatures off the western coast of South America, drives disruptive weather patterns throughout southern latitudes. In Australia, says Geiser, we either get a flood or a drought, and nothing in between. So that puts a lot of pressure on animals to develop some metabolic tricks to survive. By going into daily torpor, an animal avoids expending energy searching for food during lean periods.
Many southern mammals also have lower metabolic rates and body temperatures than do their northern counterparts. This was once thought to be a sign of a more primitive thermoregulatory system. But in light of the new findings in the Southern Hemisphere, many biologists are now beginning to view these traits as adaptations to an unpredictable climate rather than as relics of a cold-blooded reptilian ancestry.
While many basic questions about hibernation have yet to be answered, such as how hibernators survive at body temperatures that would kill nonhibernators, the new research has shown that hibernation is not a limited phenomenon. It’s not just animals in cold climates; it’s not just a few species, says Geiser. It’s a general adaptation for many species that face a potential energy crisis.