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Biological hydrostats

Wrapping the armadillo's penis

Nature volume 388, pages 826–827 (1997)Cite this article

A multitude of soft-bodied organisms (such as worms) and parts of organisms (such as the tube feet of echinoderms, the trunks of elephants, and even our own tongues) are biological hydrostats. They lack solid skeletons, yet hold their shape because of the internal fluid pressure, which is resisted by tension in the sheath that surrounds the fluid space. These enwrapping sheaths are reinforced with fibres that classically spiral up and down the long axis of the structure, forming a crossed helical array.

The mechanical properties of biological hydrostats are greatly affected by the angle of the supporting fibres within the sheath2. When the fibres cross the long axis of the structure at about 55° — which they typically do — the structure can easily bend without buckling, while it retains a constant volume. This is fine for worms and tongues, but what about hydrostats that need the opposite physical properties? Suppose one has a biological structure that must be able to increase its volume enormously, and then resist any bending or long-axis compression. Such a structure is the mammalian penis.

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Figure 1

JEFF FOOTT/BBC NATURAL HISTORY

Figure 2: Biological hydrostats hold their shape because the pressure of the fluid inside is resisted by tension in the surrounding sheath.

References

  1. Kelly, D. A. J. Morphol. 233, 249–255 (1997).

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  2. Vogel, S. Life's Devices: The Physical World of Animals and Plants (Princeton Univ. Press, Princeton, NJ, 1988).

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  3. Koga, S.et al. Br. J. Urol. 72, 228–229 (1993).

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  1. Department of Anatomy and Neurobiology, Dalhousie University, Sir Charles Tupper Medical Building, Halifax, B3H 4H7, Nova Scotia, Canada

    Richard Wassersug

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  1. Richard Wassersug
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Wassersug, R. Wrapping the armadillo's penis. Nature 388, 826–827 (1997). https://doi.org/10.1038/42130

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