When the sun sets, male mole crickets settle down in their carefully constructed calling burrows and begin serenading the females passing overhead. Cricket veteran Henry Bennet-Clark wondered if male trills are amplified by the burrow's horn-shaped opening, which looks rather like an old-fashioned hearing aid. Do burrows really improve sound production efficiency? To find out, Ken Prestwich developed a unique method to measure the singing efficiency of two mole cricket species ().
As Prestwich explains, the efficiency of sound production is the ratio of acoustic power (output) to metabolic power (input). But the traditional method to measure metabolic rate – placing a creature in a small enclosed space to record its O2 consumption and CO2 production –makes it difficult to measure acoustic power at the same time. Since mole crickets only sing inside their burrows, Prestwich came up with an unusual set-up to measure acoustic and metabolic power at the same time; he decided to use the burrows as masks to measure the breathing of singing mole crickets.
To try his novel idea, Prestwich rounded up some mole crickets. Roaming around Florida at dusk, he listened for mole cricket calls and dug the males out of their burrows. Back in the lab, the crickets industriously dug burrows in sand-filled buckets. Rigging up an artificial sunset, Prestwich was relieved to find that the males were happy to call from their new homes. To measure the insects' metabolic rate as they sang, he pushed a tube through the sand into the bulb, a dead-end in the burrow in front of the males' faces,turning the bulb into a mask. He then set up a stream of air past the insects into the bulb and up into CO2 and O2 analysers. But there was a problem. `My own breathing was messing up the gas measurements,'Prestwich says. The solution was simple; whenever he was in the lab, he breathed into a large plastic bag. At the same time, he determined mole crickets' acoustic power by placing a wire hemisphere (the shape of mole crickets' sound fields) over the burrow opening and detecting sound pressure levels with a microphone at various points on the frame.
The new approach really paid off; Prestwich now has the first concurrent measurements of the metabolic cost of calling and acoustic power. But when he calculated mole crickets' song production efficiency, he was in for a surprise. The two species sang with 0.23% and 0.03% efficiency, `which is remarkably low,' says Prestwich. Maybe the burrows don't improve efficiency after all?
But there may be another explanation for the lab crickets' poor efficiency. Field workers have long suspected that air between sand grains in dry burrows dissipates sound and damps crickets' songs. To see if this was true for his crickets, Prestwich sprinkled water over the lab burrows. Sure enough, their songs grew louder, but the crickets' metabolic rates didn't change; the wet burrow was enhancing the sound, not the crickets pumping up their metabolic power. Which means that damp lodgings near ponds and lakes are hot property as far as mole crickets are concerned.
