24 June 2009

Crickets fly away from bats, but do they run away, too?

ResearchBlogging.orgTo get a sense of how crickets might feel about bats, you’d probably have to sit next to a lion cage at the zoo while the lions are roaring. You’re faced with an animal hugely larger than you that can kill you in a flash.

Not surprisingly, crickets try to escape from that situation. Escape responses are a favourite behaviour for neuroethologists to tackle, because the behaviour is usually simple and the neurons are usually large. In brief, a series of paper from several labs have shown that flying crickets will turn away from ultrasound, which is the sort of sound that bats use to echolocate and pick off prey.

Although the nature shows usually depict bats catching insects while both are in flight, some bats specialize in picking insects off while they’re on a stationary surface. This is called gleaning. Gleaning bats also use ultrasound when they’re feeding, which we know crickets can detect.

But does an escape response to ultrasound when crickets are flying imply that they also have an escape response triggered by ultrasound when they are sitting?

Surprisingly, the answer seems to be no.

Nictophilus geoffroyiter Hofstede and colleagues tested male and female crickets’ (Teleogryllus oceanicus) responses separately. The males sing, so the authors presented ultrasound from a bat species (Nictophilus geoffroyi, pictured) found in the cricket’s home range, while males were singing. As I’ve written about recently, singing for male crickets is great for attracting mates, but it can also be used by bats and parasitoids to find crickets. So it would be logical to think that if you hear sounds made by an approaching predator, you’d have a nice tall glass of, “Shut the heck up.”

None of the males stopped singing, even when the sound was quite loud (82 decibels sound pressure level; roughly alarm clock or busy city street loud). The males did stop singing to a loud ultrasound dog whistle, however.

The authors also tested to see if the females would stop walking when they were presented with a bat sound. The analysis here is tricky, because females tend to pause when walking anyway, but the females didn’t seem to stop more when bat sounds were presented.

After they tested each cricket for behaviour, they also recorded from one of the key neurons that responds to bat ultrasound, with the undramatic name of ascending neuron 2 (AN2). They found that the AN2 neuron would indeed generate action potentials in response to the bat ultrasound.

So if the crickets can detect the sound of a predator, and take evasive action when flying why do they bumble along doing whatever they’re doing on the ground? The authors suggest that gleaning bats may not be a significant enough predator to have caused escape responses to be selected in the wild.

Another possibility, however, is that AN2 is multi-functional. Although it fires in response to ultrasound, it also spikes in response to lower frequencies, like cricket calling song. The authors raise the possibility that when crickets are on the ground, AN2 is “preoccupied” by listening for other crickets, and can’t trigger any sort of evasive response. To test this hypothesis, someone will have to go back and do some experiments where AN2 is recorded an actively behaving animal (which can be done), rather than in recording when the animal is restrained and partially dissected.


ter Hofstede H, Killow J, Fullard J. 2009. Gleaning bat echolocation calls do not elicit antipredator behaviour in the Pacific field cricket, Teleogryllus oceanicus (Orthoptera: Gryllidae). Journal of Comparative Physiology A. DOI: 10.1007/s00359-009-0454-3

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