Body design

To understand bat body adaptations for flight, it may be instructive to examine bird bodies. Bird bodies are designed for mass reduction. They do this in a number of ways. They have lost teeth and the accompanying heavy jaws and jaw musculature over evolutionary time. They have thin, hollow, and strong bones. Many bones are fused or reduced in size. The long bony tail of their ancestors has been greatly reduced to the small vestigial pygostyle. Birds have a series of air sacs in the body that serve to reduce weight. They do not have a urinary bladder to store urine nor do they have a urethra. The kidneys excrete uric acid into the cloaca where it is mixed with intestinal contents to produce the white guano associated with birds. Birds have lost one ovary, and lay eggs so they do not have to carry a fetus. The most distinctive feature of birds is their feathers, which provide lift, insulate them against heat or cold, streamline the body, and reduce mass.

Bats, as mammals, must address these weight reduction issues differently. In general, bats are much smaller in size than birds. Most bats belong to the suborder Microchiroptera (the insectivorous bats or microbats, also called the "true bats") and range from 0.07 oz (2 g) (Kitti's hog-nosed bat, perhaps the smallest mammal) to 8.1 oz (230 g), but fewer than 50 species weigh more than 1.8 oz (50 g). The larger flying foxes (Megachiroptera) may reach 56.4 oz (1,600 g) with wingspans of 6.5 ft (2 m), but they are never as large as the largest birds. Bat bones are thinner and lighter than those of most mammals, but not as light as bird bones. Bat bones have marrow in the shafts, whereas bird bones are hollow. Several bones in the bat skeleton (ulna, caudal [or tail] vertebrae) have been reduced, while several have been lost altogether (fibula, caudal vertebrae in fruit bats). The distal phalanges have less mineralization and a flatter cross-section than normally found in mammal bones, which provides more flexibility in the wing frame. Birds, on the other hand, have more mineralized bones that are somewhat more brittle. If present in the bat wing, these could actually break under the stresses on the wing frame during flight. Bats have not lost any organs as birds have. Bats still retain teeth. To compensate for the extra skull mass, they have a short neck that helps to keep the center of gravity in the middle of the torso. The bat body as a whole has been shortened and some of the vertebrae have fused, making for a stiff backbone. The diets of bats are high-energy foods, such as insects, fruit, or nectar, that pass through the gut quickly so as not to load the animal down with bulky fiber. This high-energy diet also meets the energy requirements for flight. Bats, because they are mammals, have fur instead of feathers. Fur has some limited lifting properties, produces rough surfaces that change airflow, and has some malleability for streamlining, but it is inferior in those properties to feathers. Fur does insulate, but not as efficiently as feathers.

The most important difference between bats and birds is that birds are daytime flyers and bats are nighttime flyers. As nocturnal flyers, bats face problems not faced by birds. The first problem they had to solve is navigation in a visually limited environment. Other problems bats must solve are getting sufficient oxygen and nutrients to tissues and thermoregulation. Bats have dealt with these problems very successfully.

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