Shrews use a primitive form of sonar to navigate their cluttered habitats of underbrush, according to a new study.
Though scientists have known for decades that shrews emit audible twittering calls, they have been puzzled as to whether they are used for communication, or for contending with the dense hay and grass, or dark tunnels that fill their environment.
Bjorn Siemers of the Max Planck Institute of Ornithology in Germany and a team of researchers captured seven common shrews (Sorex araneus) and nine greater white-toothed shews (Crocidura russula). They tested the animals' behavior in hay layers of varying thickness, and used scent indicators to see whether the shrews changed their calls when they detected the presence of another animal.
The shrews didn't respond to scents placed in their cages, and their calls became more rapid as the amount of hay in their environment increased, both of which point to the calls functioning as a navigational aid.
Bats and dolphins also use sound to echolocate, but they are more refined in their abilities when they emit fast, ultrasonic clicks that help them hone in on prey. Shrews' utterances are much slower by comparison and are confined to the audible range, at frequencies of 5-8 kHz. Previous studies also show that shrews can distinguish closed tunnels from open ones in utter darkness.
"Except for large, and thus strongly reflecting objects such as a big stone or tree trunk, they will probably not be able to disentangle echo scenes," the team wrote in a paper published June 17 in the journal Biology Letters. "This might be comparable to human hearing whether one calls into a beech forest or into a reverberant wine cellar."
Still, the researchers were unsatisfied. They built an "artificial shrew" — really just a small speaker to play real shrews' calls, and a microphone to receive them — and recorded the sound waves as they bounced off nearby grass, moss, and far away brick.
Twittering serves as a sort of long-range X-ray vision, they found, allowing shrews to "see through" nearby clutter and pick up large objects up to 50 centimeters (1.67 feet) away.
"Shrews are very fast animals," Jim Hare of the University of Manitoba in Canada said. "It's essential that they have information about objects at that distance," allowing them to plan escape routes around large objects, or towards the nearest open tunnel for shelter.