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First published online October 5, 2007
Journal of Experimental Biology 210, ii (2007)
Copyright © 2007 The Company of Biologists Limited
doi: 10.1242/jeb.012526
Inside JEB |
STICK-SLIP ACOUSTICS
laura{at}biologists.com
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California spiny lobster (Panulirus interruptus), and their relatives have been tormenting fishermen for centuries with their anti-predator rasp; according to Sheila Patek from the University of California, Berkeley, it's an `abrasive, obnoxious, squawking noise'. Despite this, Patek and her colleague Joe Baio have characterised for the first time the unusual mechanism – stick-slip friction – that spiny lobsters use to make sounds (p. 3538).
Other noisy creatures such as crickets generate sounds by rubbing a cuticular `scraper' over a toothed `file'. In katydids, the scraper can get stuck behind the teeth in the file and suddenly release, moving over many teeth at once and generating higher-frequency sounds. Lobsters, however, rely on the same mechanisms that underlie the earthquakes and the excitation of bowed stringed instruments: stick-slip friction. In a stick-slip system, the friction and elastic energy storage between the two surfaces opposing each other means that one surface becomes `stuck' to the other, before the pressure builds up and it slips across the other surface, creating a sound.
The team's microscopy images showed that lobsters have a soft-tissue ridged `plectrum', a small extension at the base of their antennae, which they rub against a hard `file' covered in knobbly `shingles' under each eye. The shingles, around 12.3 µm long and 7.3 µm wide, oppose the movement of the plectrum ridges, which are around 2.3 mm long and 0.1 mm wide. As the plectrum sticks and slips over the file, it rubs over thousands of shingles.
To test whether plectrum movements correlate to sound production, Patek and Baio filmed rasping lobsters at 3000 frames per second while simultaneously recording the generated sounds. Comparing the timing of the plectrum movements with the sounds, they found that the physical movements, lasting around 3.6 ms, were shorter than the recorded sounds at 7.9 ms. This difference was caused by the microphones picking up acoustic reverberations in the tank. They also found that as plectrum speed increased, the rate of pulses in each rasp and the volume of the rasps also increased.
Their results led Patek and Baio to expect that by moving the plectrum at different speeds, they would be able to make louder or quieter noises. So to investigate further they extracted the plectrums and files from a group of lobsters and glued them to separate force transducers that measured the frictional forces generated as they moved the plectrum over the file. What they found was unexpected. Despite moving the plectrum at a constant 10 cm s–1, the actual plectrum speed during slip varied between 7 and 76 cm s–1. This suggested that the lobsters don't have much control over rasp volume. The coefficient of friction – the frictional force between the plectrum and the file relative to the downwards forces between the two surfaces – decreased greatly when the plectrum started to move. On the whole, the faster the plectrum moved, the bigger the change in the frictional forces, and the louder the sound.
Patek suspects that the lobsters should be able to control their rasping, and thinks that they could do this by pushing the plectrum and file together harder, so that more force is needed to push the plectrum across the file. This is like someone pushing a pencil eraser into a table, making it much harder to push it forwards than if it was just resting on the table. Working out how much control the lobsters do have, though, will keep the researchers busy for a while yet.
References
Patek, S. N. and Baio, J. E. (2007). The
acoustic mechanics of stick–slip friction in the California spiny
lobster (Panulirus interruptus). J. Exp.
Biol. 210,3538
-3546.
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