An outstanding characteristic of shrews is their need for a constant supply of food. Because all small animals lose heat quickly, they must eat almost constantly to replace this loss. Some species will eat their own weight in food as often as every 3 hours. An outstanding exception is the water shrew, which can do without food for as long as 2 days without starving to death. Since most shrews live in or near the water, they find ample food in the insects, spiders, minnows, and small mammals which live in moist locations. The group is as ferocious as it is voracious. Most shrews do not hesitate to attack animals outweighing them several times. It has been said that if shrews were as big as squirrels they probably would even attack man.
In the mountains of Utah, Colorado, and northern New Mexico the northern water shrew (Sorex palustris) may be encountered. It is somewhat larger than the vagrant shrew and will not be seen away from water. Gray below and black above, it is wonderfully camouflaged, whether in water or on land. It, like other shrews, has long whiskers known as vibrissae. Land shrews use these whiskers as tactile organs to help them follow the dark maze of their runways. Water shrews are thought to use them as sense organs in place of eyes to pursue the minnows, tadpoles, and water bugs they eat. Actually, the water shrew resembles a large water bug as it darts about below the surface surrounded by the silvery bubbles of air imprisoned in its fine fur.
Bats
Order Chiroptera (Latin: chir, hand, and optera, wing)
The special treatment accorded bats in this book is not given them by choice. It results from an inability to so clearly describe any one or two species chosen that the layman might be able to distinguish these from their numerous and equally interesting relatives. When one considers that numerically bats are thought to compare favorably with birds, that there are a great number of species divided into many genera, and that the four-State area with which we are concerned is invaded, so to speak, by eastern, northern, western and Mexican species besides having several of its own, it soon becomes apparent that this group can be described here only in the most general way. If some of the popular superstitions about bats are contradicted here, it is to be hoped the reader will find the facts no less interesting.
The adaptation for which bats are best known is their ability to fly. This specialized talent is shared by no other type of mammal. It is made possible by considerable modification of several structures of the body, that of the forelimbs being the most extreme. The bones of both the upper and lower forelegs are considerably lengthened, but cannot compare with the extreme elongation of the digits. The clawlike protuberance from the front of the wing corresponds to the thumb. The wing membrane stretched across the “fingers” is attached to the side of the body and to the hindlegs as far as the ankle. Most bats have another wing membrane, called the interfemoral membrane, which joins both hind legs, and in many species it also embraces the tail. The wing membranes look and feel somewhat like thin leather. Running through them is an intricate system of blood vessels. These not only supply nourishment to the membrane but also act as a radiator in cooling the blood stream during the strenuous physical labor involved in flight. The principles of flight are similar to those used by birds; that is, the wings are partially folded on the upstroke and fully extended during the down beat. This maneuver produces a rustling sound that is clearly audible in the quiet of a cave. In fact, if thousands of bats are disturbed at the same time it becomes a low roar.
The fact that bats are nocturnal, and at the same time lead an aerial life which necessitates flying through labyrinths plunged in total darkness, has been the cause of much research as to the means by which they can do this. It is now definitely known that they depend on a sonar system where, by emitting shrill cries, they are guided by the echoes rebounding from nearby objects. These “squeaks” range within a frequency of from 25,000 to 75,000 vibrations per second, which is too high for the human ear to register. The sounds are uttered at rates from about 10 per second when the bat is at rest to as many as 60 per second when it is in flight and surrounded by the many obstacles to be found in a cave. Fantastic as this performance seems, it is matched by a theory that tiny muscles close the bat’s ears to each squeak and open them again to hear only the echo.
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.
Despite much recent scientific study, bats are still among our least known creatures. Their insectivorous diet surely makes them of great importance to man. Beyond this, their immense numbers indicate that ecologically they must have tremendous influence on any area in which they live.