News

2011.0328

NTNU Life Science Professor Chung-Hsin Wu Investigates Taiwan Bats’ Echolocation Ability

<p> </p> <p> </p> <div><font size="3">In the past, scientists discovered that many species of bats produce high-pitched sounds to locate prey and navigate their way. Chung-Hsin <em><span>Wu, chairman of Bat Association of Taiwan and professor of the </span><font size="3">Department of Life Science of National Taiwan Normal University, has been studying bats for 10 years. Professor Wu found that bats know how to capture ultrasound waves emitted by themselves, which enables them to fly and catch prey without colliding with other bats.</font></em></font></div> <div> </div> <div> <p><strong><font size="3">Echolocation helps bats navigate in darkness</font></strong></p> </div> <div><font size="3">Birds dominate the sky during daytime so bats choose to be active at night, during which there are fewer enemies and competitors. Bats fold their wings and sleep with an upside-down hanging position during daytime. When darkness falls, swarms of bats come out of caves to hunt. </font></div> <div> <p> </p> <p><font size="3">The Italian scientist Lazzaro Spallanzani dissolved the mystery of how bats can catch insects in total darkness. Spallanzani discovered that bats can steer around things in darkness while owls cannot avoid bumping into obstacles, so he presumed bats have a mysterious sixth sense. Later, a Swiss scientist discovered that plugging one of the ears of a bat makes it lose sense of direction and the ability to navigate. Scientists therefore postulated that bats “use their ears to see things”.</font></p> </div> <div><font size="3">In 1930, Donald R. Griffin, a student from Harvard University, used special equipments that could transfer bats’ ultrasounds to crack the mystery of how bats can see their way in darkness. Griffin later coined the term “echolocation”.</font></div> <div> <p> </p> <p><strong><font size="3">The ability of echolocation can be acquired through practice, thereby increasing chances of catching insects without being interfered by sounds sent by other bats </font></strong></p> </div> <div><font size="3">In order to understand bats’ echolocation behavior, Professor Wu used special ultrasound detectors to record bats’ sounds and used computers to analyze the sounds. He also put Taiwan bats through an MRI machine to better understand their physical structure. </font></div> <div><font size="3">According to Professor Wu, most of the insect-eating bats can emit ultrasounds that are inaudible to humans. The sound waves bounce back off of obstacles and are collected by bats’ ears, determining where prey or obstacles are. While in flight, bats use their throat muscles in rapid contractions to produce high-pitched sound waves, which are then emitted from their noses and mouths. The waves would bounce back from objects in front of them to bats’ ears. Through echolocation, bats are able to determine locations and characteristics of objects ahead.</font></div> <div> <p> </p> <p><font size="3">One thing about echolocation used to perplex scientists: while flying at high speed, how does a bat tell its reflected sound waves from those sent by other bats? Professor Wu reckons that bats, like humans, make presumptions about things. And bats’ brains can distinguish sound waves. When a bat produces a long sound through its nose, it expects the reflected sound to be a long sound. Thus each bat will search for a particular type of sound wave, thereby avoiding interfering or colliding with one another while going after prey. </font></p> </div> <div><font size="3">Are bats born with the ability of echolocation? According to Professor Wu, the reason why young bats cannot catch as many insects as adult bats do is that young bats’ echolocation ability still need to be sharpened through practice. Only by accumulating experiences can young bats improve their insect hunting success rates.</font></div> <div><font size="3">Bats’ ultrasound echolocation ability not only enables them to prey on insects at night but also allows them to live deep inside dark caves, which offer them protection from natural enemies. That also explains why bats flock together in caves, because inhabiting in such environments reduce the chances of being preyed upon.<span>   </span></font></div> <div> <p> </p> <p><strong><font size="3">FM bats and CF-FM bats prefer different environments to fly and hunt</font></strong></p> </div> <div><font size="3">Bats emitting ultrasounds can be divided into FM (frequency-modulated) bats and CF-FM (constant-frequency-frequency-modulated) bats. The two types of bats fly and hunt in drastically different environments. FM bats prefer to fly in open environments or at fringes of forests while CF-FM bats are usually active in thick forests.</font></div> <div> <p> </p> <p><font size="3">While beaming out sound waves to locate objects, FM bats gradually shorten duration of calls to avoid overlap of emitted sounds and reflected sounds. Professor Wu postulates the reason FM bats exhibit such behavior may be that they cannot deal with overlap of emitted signals and echoes. On the contrary, CF-FM bats, living in dense forests full of obstacles, are believed to be capable of tolerating outgoing sounds and returning echoes. Cases in point include Formosan horseshoe bats and Formosan leaf-nosed bats. These two species of bats can send out CF-FM ultrasounds. After collecting echoes bounced back from objects, bats can determine properties of objects by using its accumulated experiences. Bats plunge into hunting when the object in front was detected as prey but steer clear of the way if the object is judged to be an obstacle.</font></p> </div> <div><font size="3"> </font></div> <div><strong><font size="3">Noctuid moths’ deception strategy and anti-bat-detection nerve system</font></strong></div> <div> <p> </p> <p><font size="3">Bats’ echolocation system fails at times despite its extreme precision. For example, noctuid moths have the ability to detect bats’ ultrasounds, for they have mechanoreceptors located between chest and abdomen that allow them to detect bats’ ultrasonic chirps 30 meter away, providing them with ample time to get away from the predator. Noctuid moths adopt different strategies based on the detected intensity of bats’ ultrasonic waves. While being closely chased, the noctuid moth frequently changes flight directions or folds its wings and drops, disguising itself as a piece of falling leaf and deceiving the bat into giving up chase. By using such trick, the noctuid moth can be spared the fate of being hunted down.</font></p> </div> <div><font size="3">In addition, some noctuid moth can quickly pick up bats’ echolocation signals and generate vibration through its vibrator located in foot joints. By so doing, a string of ultrasonic waves is beamed to interfere with the bat’s echolocation. Some species of moths can even use a strategy similar to anti radar detection, absorbing bats’ signals by their fur.</font></div>