No way out
Insectivorous bats can sense and react to last-ditch evasive action taken by their prey. In fact, their reaction times in this case lie under 0.1 seconds, as demonstrated by LMU researchers in a new study.
Bats give targeted insects no chance to escape. They can react to the efforts of their prey to avoid capture up to less than one-tenth of a second before capture. In other words, bats, which can orient in the dark with the help of their echolocation system, can alter course faster than the human eye can shift its gaze. The findings are reported in a study carried out by researcher groups led by Professor Lutz Wiegrebe of LMU’s Department of Neurobiology and Professor Annemarie Surlykke at Odense University in Denmark. The paper appears in the “Proceedings of the National Academy of Sciences” (PNAS).
Lightning-fast response times
Bats, like toothed whales, orient themselves in space with the aid of echoes reflected from their environment. The animals emit high-frequency sound signals that are imperceptible to the human ear, and analyze the echoes they pick up. This allows nocturnally active bats to register the presence of insect prey and to pursue them as they move through the dark. The researchers set out to determine the characteristic response times of the bat’s echolocation system by suddenly removing prey at different times before capture. Just before the insectivorous bat Myosotis daubentonii finally closes in on its prey, it begins to emit short bursts of ultrasound impulses every 6 milliseconds. This is the so-called final buzz. If you and I could hear these signals, they would sound rather like a rapid-fire salvo from a machine gun. “Up to now, it was not clear why the bats do this. It had even been proposed that it represents a stereotyped behavior pattern, which is not designed to evoke a specific reaction,” says Lutz Wiegrebe.
The LMU team and their Danish colleagues have now, for the first time, determined how fast the bats can process the information encoded in the incoming echoes, and their results clearly refute the idea that the final buzz is a non-adaptive behavior: “The bats actively analyze the information present in the echoes excited by the final buzz, and they use the results to determine whether the insect in their “sights” has suddenly changed its flight-path. In fact the bats are capable of reacting to a change in the position of the prey executed less than 100 milliseconds before capture,” Wiegrebe explains. The experimentally measured response times were in the range of 50 to 100 milliseconds.
To establish how fast the bats could react to the incoming signals received by the echolocation system, the LMU researchers carried out their experiments on the river Würm in the Munich suburb of Pasing. They set up a fishing-rod baited with a mealworm that was suspended 40 cm above the surface of the water, and trained an array of high-speed cameras on the scene. As soon as the bats began to home in on the mealworm, the tension in the fishing line was automatically reduced, rapidly altering the position of the target. Using this set-up, the team could snatch the prey out of the line of attack at various times during the bat’s approach. They found that the bats could respond to such a change in position up to 100 milliseconds before capture, at which time the distance between hunter and prey is only about 30 cm.
Bats outperform electronic devices
“Bat echolocation systems therefore exhibit shorter response times than does the human visual orientation system,” Wiegrebe remarks. Reorientation of the human retina takes between 200 and 300 milliseconds. So we take two to three times longer to react when someone snatches food from our plates than bats do. Even modern electronic echolocators cannot surpass the response rates displayed by the bats. “None of the sonar systems on the market performs as well,” says Wigrebe.
The researchers now plan to turn to the question of how bats choose which aspect of the echo to focus on. They would like to know, for instance, how bats select between different types of stimuli encoded in the echo, and what features of the signal lead them to concentrate their attention on the prey they are hunting.