When it comes to locating a meal, insect-eating bats generally employ one of two foraging tactics: capturing prey in the air or snatching it from a substrate. Accordingly, the animals use different kinds of echolocation during these activities. Whereas aerial hunters tend toward longer calls with constant frequency, substrate-gleaning species generate short calls that sweep from low to high frequencies (FM echolocation). Less clear, however, is how effective the latter is at distinguishing the prey item from the substrate when the substrate contains clutter. Under such conditions, one would expect the background objects¿leaf litter on the forest floor, for example¿to produce their own echoes, which could mask those of the bat's intended target. Now new research shows that bats have a third strategy for just this kind of tricky circumstance: they turn down the sonar and wait for the insect to reveal itself. The findings appear today in the journal Nature.
Rapha¿l Arlettaz of the University of Bern and colleagues studied the mouse-eared bat's ability to obtain live and dead insects on clean and cluttered surfaces. As it turns out, the animals scored well when it came to capturing moving prey on both substrates and still prey on a smooth surface, but they labored to locate still prey on a complex surface. Additionally, the researchers found that bats attempting to pinpoint prey in the air or on smooth surfaces emitted so-called feeding buzzes. Those searching among the rubble, in contrast, emitted only faint calls or no calls at all for more than a second just before detecting prey. The scientists thus suggest that the bats listen for prey-generated sounds during this moment of silence, which would explain why they struggled to locate the dead prey in the leaf litter. "The low-intensity calls emitted during prey approach," the team writes, "may detect the immediate surroundings so the bats avoid colliding with obstacles or the ground."
According to the researchers, the study results indicate that echolocation does not provide a detailed picture of objects to substrate-gleaning bats. Indeed, when hunting among clutter, echolocation appears to render the bats "acoustically blind." This, they conclude, "suggests that FM echolocation is mainly adapted to orientation and capture of prey either in the open space or from simple backgrounds.