You gotta love bats!

 kw: book reviews, nonfiction, science, natural history, bats

Why does this bat have its mouth open? It's not in anticipation of biting the moth. Rather, it is shouting! Most bats navigate with echoes, and they make chirps and clicks at very high frequencies to "see" in the dark. They also use echoes to locate and hunt prey. On this final approach, the bat is "shrieking" a very fast series of clicks to track every flutter of the moth, so as to scoop it into the tail membrane (you can see the membrane is already cupped for the capture). Then it tips its head in and bites.

This incredible image is credited to Michael Durham on a FaceBook page of the U.S. Fish and Wildlife Service. This is a Little Brown Bat, Myotis lucifugus, the most common bat in North America.

I spent a month in Nevada one summer. There was a bright light on a tall pole near the campground. Moths and other insects would fly around and around in the light, and bats would swoop through to catch them. From time to time my friends and I would toss a small rock up there. Often, a bat would swoop in, then dodge aside once it got close enough to discern that it wasn't an insect. Now, many years later, a schoolyard near our house has a running track alongside a patch of forest. In spring or autumn, if my wife and I take a walk on the track in late twilight, we will sometimes see bats flittering about overhead. We don't see the insects they are hunting, but the bats are obviously able to detect them.

I like bats. Alyson Brokaw adores bats. In her book The Weird and Wonderful World of Bats: Demystifying These Often-Misunderstood Creatures, Dr. Brokaw brings us as close to bats as she can, without actually handing one to us (which I suspect she would do if she could). The fourteen chapters of the book describe as many aspects of bats' natural history, from their voices to their other senses such as smell and vision, to their flight and how their design makes them such maneuverable flyers, to their long lives and memories, and the kinds of places they prefer to roost. It is a lot to cover, particularly because among the 1,400 species of bat, all these characteristics are quite variable. For example, not all species of bats roost in caves; perhaps less than half do. Some like to tuck up under the eaves of houses. Some make little tents of leaves in large-leaved trees.

I began thinking about echolocation as a substitute for sight. Our vision is sharp because light has a short wavelength, around 1/2,000th of a millimeter. By contrast, when we hear a sound off to the side, we can point in the general direction, but even when the sound is high-pitched, like a gunshot, our apprehension of the direction is not as accurate as if we saw the puff of smoke, for example. The high frequencies of a sharp "crack" are in the 10,000 to 20,000 Hz range. The speed of sound is 343 meters per second, though that varies a little with temperature and humidity. The wavelength of 20,000 Hz is about 17 mm, or about 2/3 of an inch. The distance between our ears is about 7 inches, some 180 mm. Thus, the angular accuracy of our hearing, when a high-pitched sound is sort of in front of us (or behind, but not to either side so much) is a little under three degrees. By contrast, the sharpness of human vision is mostly in the range of 1/30 to 1/60 of a degree, hundreds of times more accurate.

Bat voices range from 10,000 Hz to 200,000 Hz, but few species make chirps or clicks higher than 120,000 Hz. We can hear the "lower" frequencies, and I have heard bat chirps. But I was hearing only a small fraction of the sound. The wavelength of a 120,000 Hz sound is just under 3 mm, or about 0.11 inch. As it happens, with their ears only 12-25 mm apart, bats' angular discrimination is also in the range of a couple of degrees. But there is another aspect to this. Sound with a 3 mm wavelength will efficiently bounce off an insect as small as half that dimension, which includes mosquitos. While it is not true that a little brown bat catches 1,000 mosquitoes every night—the author debunks that item effectively—, on average the bat will eat a few hundred insects nightly, including mosquitoes. It just depends on what is available. So, as a bat homes in on an edible target, by the time it is within a half meter the 3-degree discrimination angle means the bat knows where the critter is within a "circle of confusion" of about an inch (~25 mm), a distance that shrinks rapidly as it gets closer. It can scoop it right up and snack away!

Near the end of the book we learn what it takes to put up bat boxes, the bat version of bird houses, so they have more places to roost. Just as only a few bird species will use a bluebird-sized bird house, so we must do a little research, perhaps with the help of a naturalist at a nearby natural history museum or agricultural college; once we know what the local species are, we can determine what kind of box to make or purchase, and where to put it. I was surprised to learn that bats that like to use bat boxes don't like them to be attached to trees. That makes sense; too many bat predators can climb trees. A bat box on a tall pole or just under the eaves above a smooth wall will be more popular.

There is a lot we can do for the sake of bats, which do a lot for us, particularly insect control. The first step is to learn what they are really like. This book is a good first step. Most of us saw too many monster movies, where bats or batlike beasts influenced us. To a bat, a human is a big, noisy monster, and they keep their distance! You or I may never love bats the way Dr. Brokaw does, but it is worthwhile to like them, at least a little!