By Patrick Clinton
Most ticks are deaf, blind, and slow as Christmas. So how do they find you?
Melissa Shepherd, Lincoln, Nebraska
They smell you. Scientists have learned what sets ticks off by attaching itty-bitty electrodes to the olfactory organs on their legs. At a range of about 20 or 30 feet, a tick can smell the carbon dioxide on your breath. Closer in--a few feet away--it begins to respond to ammonia and other chemicals in your sweat, and closer still, it becomes stimulated
by your body heat.
Basically, there are two kinds of ticks: "Ambush" ticks sit patiently on vegetation, waiting with their forelegs extended, until a likely critter brushes against them. "Hunter" ticks, on the other hand, will actually go after their prey, engaging in hot pursuits of 30 feet or more. And they're faster than you might think. Daniel Sonenshine, author of
The Biology of the Tick, has clocked a tick crossing his desk in under ten seconds.
Impressive, yes, but consider that only one tick in a million finds enough food to survive to adulthood. Nature is inefficient, thank God.
Recently I spotted a CONTINENTAL DIVIDE sign in the middle of North Dakota, on a flat stretch of I-94, near Jamestown. What's going on here?
Peter Salter, Mandan, North Dakota
That sign isn't a hoax, but it has nothing to do with the continental divide, the one that runs down the spine of the Rockies and separates rivers that flow into the Pacific from those that empty into the Gulf of Mexico. The divide you stumbled on is a broad swampy zone that starts up in Saskatchewan and runs across North
Dakota, separating rivers that flow into the Gulf from rivers that flow into Hudson Bay. Call it the Northern Divide, if you want.
But is this a true continental divide? Hard to say. Geologists point out that the Appalachian chain, which is sometimes called the Eastern Divide, isn't a divide at all, since the water on both sides ultimately empties into the Atlantic. But some experts are willing to concede that this Northern Divide may be a little
different, since Hudson Bay is generally considered part of the Arctic Ocean. Still, you want to be conservative in these matters. Once you start creating new divides, cautions a spokesman at the U.S. Geological Survey, "You could just go on ad infinitum."
And the country seems divided enough already.
Do beavers ever get squashed by the trees they're gnawing down?
Tom McQuillen, Dobbs Ferry, New York
"I don't recall having seen one myself," says North Carolina State University wildlife researcher Richard Lancia, "but it is known to happen--and I have seen pictures."
The beaver's reputation as a canny woodsman appears to be a bit overrated. "People think they can cut trees and know where they'll fall," says Lancia. "The fact is, they have no idea." Sure, they can be fairly accurate at dropping timber for their dams, but by the water's edge they usually have gravity on their side: Trees along rivers and lakes already
tend to lean toward the light and open space.
Get beavers away from the shore, however, and they can be downright inept. Sometimes, in thick woods, a beaver will gnaw through the base of a tree that simply won't fall, because the surrounding branches are holding it up. This creates consternation for the beaver while stirring immense merriment in the beaver-research establishment. "They'll go in and
cut it again, and again, and again," says University of Massachusetts biologist Joseph Larson. "There'll be three or four chunks of tree lying around. But the tree's still standing."
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