Ask Audubon How can squirrels, unlike cats, climb down trees?

--Barbara Helm, Fort Collins, Colorado

While it's been said that a cat's brain is only big enough to get it up a tree, a squirrel's brain, at about the size of a walnut, is four times smaller, so climbing agility has little to do with skull content. The difference is morphological. Mother Nature has seen fit to equip North America's most familiar arboreal mammal with ankles a yogi, not to mention a stranded tabby, would envy.

See for yourself. When you next spot a squirrel making its trunk-borne descent, observe its hind feet. They actually point backward! This unlikely placement is made possible by super-flexible subtalar joints between the anklebone and the heel bone, which allow the back paws to rotate nearly 180 degrees. With the sharp, curved claws of its five powerful toes acting like pitons, the woodland acrobat is able to gain purchase on the bark and "walk" headfirst down the trunk. Squirrels routinely outshine cats not only in the branches but also in performing such gravity-defying feats as shimmying up bird-feeder poles and nimbly negotiating power lines. To say nothing of escaping feline wrath when they flaunt their tree-blazing prowess.

Why do lemmings commit mass suicide?

--Elizabeth Hoyt, Arvada, Colorado

The lemming's legendary compulsion for communal self-destruction, so widely believed that it has become a metaphor for pack behavior run amok, is pure fiction. What is fact is that these busy breeders go through roller-coaster-like cycles of population booms and busts on the order of two or three times a decade. In peak years, aptly called "lemming years," their numbers reach such epic proportions that they literally eat themselves out of house and home. The periodic floods of lemmings onto the tundra after seasons of prolific procreation earned them the Inuit name kilangmiutak, or "one who comes from the sky." At such times, they can multiply a thousandfold, and when they undertake their normal migrations, make rush-hour commuting seem positively breezy. But they're not on some orchestrated death march to the sea, fleeing overcrowding or claustrophobia. More likely, researchers believe, the rodents are simply seeking greener seasonal pastures and en route may fall victim to mishaps. "It's a completely natural strategy," says Peter Turchin, an associate professor of ecology and evolutionary biology at the University of Connecticut. "If they stay in the same place, they will die. They have nothing to lose, although when they migrate, many do die by falling over cliffs or drown crossing rivers and lakes." Inevitably, lemming population explosions are followed by crashes so precipitous that they often raise the specter of extinction and, when graphed, look remarkably similar to the steep slopes from which the creatures supposedly take their fabled flying leaps.

How do frogs survive winter?

--Arnold Yeadon, Furlong, Pennsylvania

Frogs in frigid zones might not be able to turn up the thermostat or flit to tropical hot spots, but with 190 million years of evolution behind them, they have certainly developed the wherewithal to make it through cold spells. Many overwinter by slipping into a deathlike state of dormancy known as torpor.

Being cold-blooded, a frog's internal temperature approximates the temperature of its surroundings. When the mercury drops, the amphibian chills, and its metabolism, heartbeat, and respiration slow down. In low gear, frogs are incapable of responding to external stimuli and vulnerable to freezing to death or being gobbled up by predators. To avoid these scenarios, some species head underground first; others blanket themselves with leaf litter or mud in a lake bottom. A few terrestrial kinds, including wood frogs and spring peepers, employ a seemingly paradoxical strategy: They freeze nearly solid. Rime forming on the skin during an arctic blast triggers the production of a concentrated sugar, which circulates through the bloodstream. This substance acts as an antifreeze, preventing damaging ice crystals from taking shape within the animal's cells and major organs. But everything else, including body cavities, ices up, and vital signs cease. A frost-tolerant peeper can persist for weeks like this, with as much as two-thirds of its body fluids crystallized. It thaws out when warm weather returns, and springs back to life with no serious side effects. Scientists, who have christened this survival trick cryopreservation, are scrutinizing it in the hopes of learning how to freeze live tissues, like hearts, for subsequent use. Frog legs on ice, anyone?

--Carolyn Shea


 

© 2001  NASI

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