The response of their vocal and hearing structure to this specialized use is truly amazing. There are no more unique faces in the mammal kingdom than those of the bats. Most bats have enormous ears with ridged and channeled interiors that probably have much to do with amplifying faint sounds. Set in front of the ear is a narrow, upright protuberance known as the tragus. Farther down the face, in the region of the nose, are other strangely shaped skin structures including the “nose leaf.” As yet the functions of these appendages are not entirely known, but it is suspected that at least part of their purpose is to beam the squeaks along a definite line and thus help orient the bat with its surroundings. With such an efficient system to guide it, the bat has small need for eyes. The expression “blind as a bat” is misleading, however, because most bats, in spite of their relatively small eyes, can see rather well. Since most southwestern bats are insectivorous, with the exception of a very few species along the Mexican border which are considered fruit eaters, the question arises as to how they exist during the winter months when insects are not to be found. There are two common methods by which animals avoid such a lean period: by migration and by hibernation. Bats employ both. Some species are thought to fly as far south as Central America. Others group together in caves and hang in a deep torpor all winter. In this state of inactivity their body temperatures may fall to within one degree of their surroundings, and their rate of metabolism sometimes falls to one-eighteenth of that during active periods. As a rule, bats prefer a cool place for hibernation, because the cooler the temperature the slower the rate of metabolism. Body temperatures as low as 33.5° F. have been recorded in hibernating bats. The temperature must not fall below freezing, or the animals will perish. During this period of inactivity bats have been known to lose up to one-third of their weight. Because of their secretive habits and nocturnal periods of activity, bats have few enemies other than man that are capable of making any serious inroads on their numbers. Consequently the birth rate is quite low in most species. Many have no more than one young each year; and the red bat, which bears up to four young, seems to be the most prolific in the United States. There is great variety in the methods by which different species care for the young. Some mothers leave the babies hanging to the roof of the cave while they go on their nightly search for food; others carry the young clinging tightly to their fur. The young mature quickly. They are usually able to fly within a month after they are born.