Biophysics

Swimming in Sand

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Science  31 Oct 2008:
Vol. 322, Issue 5902, pp. 653
DOI: 10.1126/science.322.5902.653a

Several species of lizard are capable of traveling for long distances beneath the surface of desert sands. Most have very reduced, or even absent, limbs and adopt a serpentine motion akin to the swimming of water snakes. In contrast, the sandfish lizard (Scincus scincus) of North Africa and the Arabian peninsula has well-developed limbs that it was assumed were held tightly against its body when moving through sand.

Baumgartner et al. used nuclear magnetic resonance imaging to observe sandfish movement directly and found that they actually propel themselves with their limbs. Unlike a swimming snake, which drives its near-stationary head forward with sinusoidal movements of its body that increase in amplitude toward its tail, the whole body of the sandfish underwent sinusoidal oscillations with a frequency of 3 Hz and an amplitude of around half its body length. The oscillations of the lizard's body act to fluidize the surrounding sand, an effect well known to engineers dealing with granular media. Using a vibrating metal rod of similar dimensions to a sandfish, the authors confirmed that the resistance to motion through dry sand dropped dramatically when horizontal oscillations were faster than 2.5 Hz. Within this localized volume of fluidized sand the sandfish swims by paddling its fore and hind limbs in synchrony with the flexing of its body. — CS*

PLoS ONE 3, e3309 (2008).

  • * Helen Pickersgill and Chris Surridge are locum editors in Science's editorial department.

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