Air Superiority

from North American Wildlife

 

       Sixty million years ago, bats were the only mammals capable of sustained flight. They held the claim until 1783, when the first sheep flew. But the sheep cheated. It enlisted the aid of two other mammals, Joseph and Étienne Montgolfier, who set it aloft in the first successful flight of a hot-air balloon. A duck and a rooster accompanied the sheep on the trip.

      Of course, birds had invented flight 80 million years before bats, and reptiles 20 million years before them. And all vertebrates are latecomers to the art of flying compared to the insects, which had taken wing some 200 million years earlier. But the mammals repeated one of nature’s most successful experiments with the novelty born of ignorance. And indeed, nothing else flies quite like a bat.

       Bats probably evolved from early shrew-like mammals that scampered over tree trunks and branches in pursuit of insects. One theory proposes that webs of skin stretching between the forelimbs and the rib cages of these insectivores may have been used to cup flying insects. Later, these flaps may have allowed the proto-bats to glide from branch to branch, eventually evolving into full, flapping wings.

      Bats do not fly as efficiently as birds. They have never developed two of the birds’ greatest assets: feathers and hollow bones. Nor do bat skeletons have the deep keels and large attached breast muscles of the birds. They are not particularly good at gliding or soaring, which all of the long-distance avian fliers use to their advantage. Still, it beats walking. While a bat flying from one point to another consumes calories at many times the rate of a mouse scampering the same distance, it gets there so much faster that in the end, it uses far less energy.

       What the bats lack in endurance, they make up for with maneuverability— both in the air and when roosting. Bats can chase individual insects through forest canopies and can hover, dip, and swoop in pursuit of their prey with an ease unmatched by birds. The absence of a bony keel and their broad flat bodies enables them to squeeze into narrow crevices for day roosting and hibernation.

       Almost all of the bats of North America eat only insects. In the temperate zone, where fruits and flowers are available only seasonally, there simply isn’t enough food to build the fat reserves necessary to survive northern winters. Some pollen- and fruit-eating bats do inhabit the southern United States and Mexico, while fishing and vampire bats live only in the tropics, but the range of insectivores such as the little brown bat extends as far north as central Alaska.

      The bats are a highly successful order of mammals. One in four mammalian species is a bat; only the rodents have more. Although only 40 of the world’s 1000 species live in North America, in the warmer parts of the United States and Mexico they are among the most numerous of vertebrates. There is a cave in Texas where 20 million Mexican free-tailed bats roost each night.

      Their success stems from their flying abilities, in particular their ability to fly at night. By feeding after dark, bats have almost sole access to an enormous food source: nocturnal flying insects. During the day, birds are the chief hunters of flying insects. But hunting after dark is hazardous for a low-flying bird, and trying to see something as small as an insect after twilight is just about impossible. Before bats, the insects had the night sky to themselves.

      But the bats had already developed a trick that allowed them to overcome both these problems before they ever got off the ground: sonar. Like some shrews today, the proto-bat may have used echolocation to find its insect prey. With active sonar, or echolocation, modern bats can navigate obstacles and locate insects as small as a mosquito in complete darkness.