Several animals such as bats and whales use echolocation to aid them during navigation and foraging. These animals emit calls that produce echoes when the sound waves bounce off an object in their environment. The different characteristics of these echoes, such as the amplitude or the time it takes for the sound wave to reflect, provide the animal with valuable information about the characteristics of the surrounding objects.
The biosonar of bats consists of short high-frequency sound pulses. When hunting, bats emit faster and shorter pulses as they get closer to their prey, increasing feedback frequency during the final approach. The directionality of the sonar should also be important; a directional sonar would be effective in locating insects in front of it, while filtering ‘noise echoes’ from peripheral objects. However, as the bat nears its prey, a broader beam of sound would be more beneficial, as the field of view covered by a directional beam is reduced at short distances.
A recent study by Lasse Jakobsen and Annmarie Surlykke from the University of Southern Denmark investigated whether insectivorous bats have the ability to adjust the sound beam's width according to the proximity of their prey. To do this, they trained six vespertolionid bats to feed off tethered mealworms inside a large room. They then recorded their calls as they hunted for the mealworms using 12 microphones arranged in a cross. The multimicrophone array was located at the back of the room, behind the mealworm. This configuration allowed the scientists to determine the bat's position using the differences between the time of arrival of the sound waves at each microphone, and the direction of the sound beam by calculating the vertical and horizontal angles from the bat's position to each microphone.
During the initial hunting phase, the bats emitted highly directional echolocation beams with a half-amplitude angle of approximately 40 deg horizontally and 45 deg vertically. As the bats closed in on the mealworms, the beam broadened dramatically, and the half-amplitude angle of the beam more than doubled. The bats were able to do this by decreasing the sound frequency by about an octave, from 55 to 27.5 kHz. In 1989 Elisabeth Kalko and H. U. Schnitzler from the Universität Tübingen in Germany noted that Myotid daubentonii lowers the frequency of its echolocation pulses during the moments preceding the catch. They speculated that this change in frequency represented a physiological constraint and that the bats were physically unable to produce high frequency pulses at the high repetition rates emitted during the final approach; Jakobsen and Surlykke's study dismisses this hypothesis by pointing out that many other bats are capable of producing pulses at these high repetition rates without a concomitant decrease in frequency, and proposes a functional significance to the phenomenon.
By having a narrow but highly directional beam of sound aimed directly at the prey during the beginning of the pursuit, the sound beam is optimized to locate insects positioned in front of the bat, while filtering noise resulting from surrounding objects. As the bat closes in on its prey, a wider sound beam translates into a wider detection angle, preventing prey from escaping by quickly flying outside the bat's field of view. Future research will likely reveal that the biosonar of other echolocating species also exhibit dynamic control of the width of the sound beam in response to changes in their surroundings.
