ECOLOGY:

A number of critters use echolocation to navigate and catch prey, including bats and dolphins. A narrow sound pulse helps locate far-off prey by reducing extraneous interference.

However, a broad sound pulse helps keep highly navigable nearby prey within earshot. Some insects initiate erratic evasive maneuvers when they pick up ultrasonic vibrations.

Which kind of sound pulse do bats use? It would be evolutionarily advantageous for bats to dynamically use both when pursuing prey, i.e. to be able to dynamically change the width of their echolocation pulse, for ease of detecting far-off prey as well as capturing nearby prey.

They in fact do use both types of pulses. When bats get near their prey, they reduce the frequency of their echolocation pulse by half.

This is commonly thought to be a physical limitation of emitting fast sound pulses. However, reducing echolocation frequency inherently increases the pulse width, suggesting that physical limitations may not be the explanation.

Unfortunately, technical difficulties have prevented scientists from rigorously answering this question. It has now been answered by Lasse Jakobsen and Annemarie Surlykke (University of Southern Denmark) for Myotis daubentonii (Daubenton's bats) and Eptesicus serotinus (serotine bats).

Testing bats.

The scientists trained six bats to catch worms in their laboratory (within an 81 cubic meter flight room), for a total of over 350 trials. You could train me to catch a plate of nachos in the laboratory, but I'd have to pass on worms.

Bat echolocation calls were recorded on an array of microphones. The placement of the microphones in the array depended on whether the scientists were trying to catch calls that were close or far off.

The worms were dangled above the microphones. The direction at which the bats were aiming their echolocation beams was calculated from the bats' vertical and horizontal distance from each microphone.

Dynamic manipulation of echolocation pulse width.

The scientists found that the bats didn't open their mouth (the origin of their echolocation pulses) significantly more during the initial approach relative to the final capture of their prey. This means that mouth size is not the origin of any observed pulse width changes.

Next, they found that, for Myotis daubentonii bats, measured in the horizontal direction, the angle at which the sound pressure decreases by 6 decibels is 40° at 55 kilohertz frequency during the initial approach phase. This angle increases to 90° at 27.5 kilohertz frequency during the terminal phase.

What this says is that the width of the echolocation pulse increases, while its frequency decreases, during the final approach of the bats to their prey. Similar results were observed in the vertical direction, as well as for the Eptesicus serotinus bat (for which the scientists did not obtain vertical measurements).

Further, the bats accurately aimed their echolocation pulse at the prey. Error was less than 3° in either the horizontal or vertical direction, even during the initial (and relatively far-off) approach.

Collectively, this shows that the bats broaden their field of view when they approach their prey, without altering their attention. The scientists note that this resembles using a narrow angle lens for long-distance imaging and a wide-angle lens for short-distance imaging.

Implications.

These results only hold for Vespertilionid bats, which happens to be the most common bat family. On the other hand, bats with a different anatomy, and different prey and hunting techniques, may not be able to dynamically manipulate their echolocation pulse in the same manner, if at all.

Broadly speaking, the acoustic directional flexibility reported herein may nevertheless be important for a wide range of critters and behaviors. It could play a role in feeding, danger alerts, and courtship (e.g. issuing a broad warning vs a personal message), depending on the species, possibilities that are now testable with appropriate experimental modifications.

for more information:
Jakobsen, L., & Surlykke, A. (2010). Vespertilionid bats control the width of their biosonar sound beam dynamically during prey pursuit Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1006630107