Watching a cricket stop dead in mid-air and plummet to the ground might not look like a terribly well-adapted behavior to the uninitiated, but new research published in the journal Philosophical Transactions of the Royal Society B has uncovered the clever motivation for this strange behavior. Sword-tailed crickets can eavesdrop on predatory bats' vocalizations, triggering them to fall from the sky when an attack is imminent and dive-bombing out of harm’s way.
The highly efficient strategy is employed by sword-tailed crickets of Barro Colorado Island in Panama. While several species are able to eavesdrop on bat vocalizations, these crickets are the first flying insects known to science to respond to danger by simply ceasing to move. The degree of stop-and-drop is determined by the amplitude of the bat call, with higher amplitudes triggering the crickets to cease flight for longer and fall further.
Life in the nocturnal jungle is a noisy affair and identifying specific threats from within the deafening chorus of nightlife requires sophisticated adaptations. This becomes particularly complex in the context of katydid (bush-cricket) calls, which form 98 percent of high frequency calls and are easily confused with bat vocalizations. The team of researchers from Bristol’s School of Biological Sciences and the Graz’s Institute of Zoology in Austria wanted to know how sword-crickets were able to identify and respond to bat calls so well despite the ongoing din of the nocturnal jungle.
Their answer: Sword-tailed crickets only respond to ultrasonic calls above a high-amplitude threshold. In doing so, the crickets save themselves from plummeting unnecessarily in response to harmless katydid calls. They also only respond to the loudest bat calls, which, while giving the animal less information on their environment, means they only react to bats within around 7 meters (23 feet) of themselves. Seven meters is the distance at which bats can detect the echo of the crickets (whose physical mass rebounds some of the bats' vocalizations), so the adaptation is effective in alerting the crickets to the presence of hunting bats who are aware of their position.
This rare avoidance strategy is seldom seen in nature as most prey animals rely on quieter environments to allow them to distinguish between high amplitude and ultrasonic sounds.
"The beauty of this simple avoidance rule is how the crickets respond at call amplitudes that exactly match the distance over which bats would detect them anyway,” said Dr Marc Holderied, a senior author on the study from Bristol's School of Biological Sciences, in a statement. “In their noisy world it pays to only respond when it really counts."