Sensory Systems And Echolocation
Contrary to popular myth, bats are not blind. In fact, the large eyes of many species suggest that they have well-developed vision. Like most mammals, they have keen senses of taste and smell, the latter being useful in locating food items, and in identifying roost sites and other bats, including family members. Bats also have excellent hearing. Many species use a wide range of vocalizations to communicate with one another. Some species hunt for food by listening to the sounds of their prey moving about.
The most remarkable sensory adaptation of bats is their capacity for echolocation. This sensory ability allows bats to maneuver in total darkness, using echoes of their ultrasonic calls to detect objects in their vicinity. Efforts to understand how bats can fly in complete darkness date back to the late eighteenth century, when the Italian scientist Lazarro Spallanzani (1729-1799) conducted experiments that included denying bats the use of their senses of smell, touch, and vision. He observed that bats lost their way only if their head was covered by a small sack, and concluded that bats must have a sixth sense, not shared with humans.
A Swiss scientist named Charles Jurine reported in 1794 that if a bat's ears are blocked, it cannot maneuver. Spallanzani heard this report, and taking the experiment a step further, showed that bats with brass tubes inserted into their ears can only navigate when the tubes are open. He then concluded that bats must somehow "see" with their ears. How this could occur was not explained until the 1930s, when the echolocation pioneer Donald Griffen (then an undergraduate at Harvard University) detected ultrasonic signals produced by bats in the lab, using a microphone capable of picking up sounds above 20 kHz-ultrasound. In a series of experiments, Griffen showed that bats used the echoes of their calls to locate obstacles, and he coined the term "echolocation" to describe this sensory ability.
It is now known that some other animals are also able to echolocate, including whales, dolphins, shrews, and some birds such as cave swiftlets. It is also known that some bats, including all but one of the flying foxes, are not able to echolocate. (The only megachiropteran genus that can echolocate is Rosettus, whose sounds are produced by tongue clicks.) Evidence indicates that bats can echolocate using reflected sound as effectively as we see with our eyes, using reflected light. Bats do this by sensing the time elapsed between the production of the sound (by means of their larynx) and the return of its echo, thus gauging the distance of objects near them. Of course, they do not perform these computations in a conscious way, any more than a person willfully determines the frequency of incoming light waves to perceive an object as blue or green. Rather, the bat brain carries out the necessary functions in a split second, providing them with a continuously updated "picture" of their surroundings.
What bats "see" in this way might best be imagined as something like what a human visitor might see in a darkened disco, where a strobe light is flickering to illuminate dancers and other objects in a pulse-like fashion; every time the strobe light flashes, the observer gets a brief update on the position of nearby things. A faster pulse rate in the strobe means that more information about these objects can be conveyed to the observer. The same is true, more or less, for bats, which vary their calling rate depending on what they are doing. The rate for a bat on a routine cruise is 5-10 calls per second; as it locates and closes in on a flying insect the call rate increases, and it finally accelerates briefly to more than 200 per second in a terminal "feeding buzz" that pinpoints the location of the food item. However, such a high rate of calling is only suitable for near targets; if an object is too far away, the outgoing signal gets mixed up with those returning from other distant objects. In addition, echolocation takes considerable energy; the intensity of the calls of some bats, if they were of a frequency audible to humans, would make them as loud as the beeping alarm of a home smoke detector. Expending the energy required for the feeding buzz does not pay off during an ordinary commute, and so the calling rate at this time is relatively low.
The brain of an echolocating bat carries out some astonishing perceptual feats, and solves problems similar to those facing engineers during the early development of radar technology. For example, there is the problem of signal attenuation: sounds progressively degrade as they get farther away from their source. Pulses must be loud enough to survive degradation, but a loud sound can overwhelm the sensitive receiver structures needed to pick up the return signal. Through their evolution, bats solved this problem in a similar way as the early radar engineers: they are equipped with a send/receive switch that momentarily disconnects the receiver function just as the loud outgoing pulse is produced; the receiver is then reconnected in time to receive the echo. The switching is accomplished by muscles that attach to the bones of the inner ear; when the muscles contract, these bones do not transmit sound well. When the muscles relax, the ear returns to its normal sensitivity. Further signal attenuation happens within the brain itself, as neurons responsible for sound perception block the transmission of messages to higher regions of the brain at the moment that a bat vocalizes. In addition, special echo-detector cells in the brain respond more intensely to the second of two separate sound pulses, which is an excellent way of picking up the echo of a vocalization.
There is also the problem of sorting out echoes returning from near, medium, and distant objects; how can a bat tell the distance between itself and its next meal? They accomplish this by means of frequency modulation, so the sorting can be done by differences in pitch (frequency). If a bat utters a downward-sweeping whistle call, an echo returning from a more distant object will be older and thus higher in pitch compared with echoes from closer objects. The bat thus has a standard for comparison: lower pitch means close by, higher pitch means farther away. In addition, bats can measure the speed of a moving target by means of the Doppler effect; this is the phenomenon responsible for the change in pitch of an ambulance siren as it moves toward and then passes a stationary observer. To do this, the bat's brain compares the pitch of an echo with that of the original call; they can do this reliably even in the midst of hundreds of echolocating colleagues engaged in a midnight feeding frenzy. All of this is accomplished automatically and instantaneously, with no more conscious effort than a person might exert while watching images on television.
Of course, the echolocation system does not guarantee a perfect success rate while hunting. Some flying insects, mice, and other potential prey can detect the echolocation signals of bats and then take evasive action.
Bats also use their senses of sight and smell to find food. Their other senses are also important in recognizing other bats, including their offspring, and perhaps also in identifying roost sites.