Don't bat an eye or you might miss Southeast's rarer winged wonders

Posted: Sunday, May 20, 2007

If you're out on the trails in the evening this spring, you might catch a glimpse of bat; but you actually don't need to venture far from town. One can see bats occasionally flying about in the glow of street lights.

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Southeast Alaska doesn't have many bats, and only one species (the little brown bat) is fairly common. They can be spotted flitting over Montana Creek, Cowee Creek, and other local streams.

Bats typically congregate in caves and mine shafts in winter and become dormant. Emerging in spring, the females move to nursery colonies in attics, behind loose shingles, in cracked or hollow trees or other secluded spots.

I'm told that bats have even been found roosting under rocks high in the intertidal zone.

Mating occurs in fall or winter, but a female stores sperm from the male until spring, when a single egg gets fertilized. A couple months or so later, one young is born and is carried around by its mother for several days.

Later, the young bat is left in the roosting area, hanging upside-down by its toes, the characteristic resting position of all bats.

An exception occurs when females give birth; then they hang right-side-up by their thumbs and catch the breech-born young in their cupped tail membranes.

Contrary to myth, bats do not want to get tangled in your hair. I couldn't persuade my mother of that. Occasionally a bat would get into the living room in our old house by creeping down through the walls from the attic.

Then my normally calm mom would go berserk, rampaging around the room, swinging a tennis racket, until the poor confused bat was ultimately subdued.

Bats are the major predator of night-flying insects. This predator-prey relationship is so important to both sides that each has evolved special adaptations for catching (the bats) or eluding (the insects).

Insect-eating bats emit very high frequency calls inaudible to humans, then catch the echoes much like an airborne radar system. The call is extremely loud, so the bats have adaptations, especially in the bones of their middle ears, to keep from deafening themselves.

When an insect is detected, the calls increase in frequency and pitch as the bat approaches and pinpoints its prey. The prey isn't generally caught in the bat's mouth, but in a pouch formed by the membrane between the hind legs or in the wing.

Bats can catch hundreds of insects per hour, or as much as half their body weight per night. Lactating females have to catch even more, to make the energy needed to nurse their young. They may catch more than their own body weight in insects each night.

The insect prey, however, are not helpless. They have evolved counter-adaptations to the bats' echo-locating system.

In effect, there's an evolutionary arms race going on. As the echo-locating system improves, the insects eventually improve their evasion tactics. Then bats then get better at catching the escape artists, which then get better at escaping.

In order to escape, insects must hear the high sounds from cruising bats. The prey insects have ears but, curiously, their ears may be located on many different body parts: legs, mouthparts, wings, thorax, and abdomen. It depends on the type of insect

Great sensitivity to ultrasound, however, is generally found only in insects chased by bats, such as night-flying grasshoppers, katydids, beetles, moths, mantises, and lacewings.

If these prey insects detect a bat before the bat detects them, they typically change their flight direction. If, however, a bat has pinpointed them and is closing in for attack, the insects adopt more dramatic maneuvers.

Lacewings and katydids go into immediate but passive falls, mantises do accelerating power dives, and moths do a series of rapid spiral turns. These evasive tactics work about half the time, so there is a real payoff in terms of survival.

Some insects reduce the risk of capture by emitting their own ultrasound that appears to throw off the bats.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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