Mammals are able to maintain stiff erections because their penises have a rare form of internal reinforcement. Now, a discovery inspired by playing with a garden hose means we finally understand the extent of the benefits this offers.
鈥淚t was a pandemic lockdown issue,鈥� says Peter Palffy-Muhoray, a physicist at Kent State University in Ohio who normally studies liquid crystals. 鈥淲hile out in the back yard, we noticed the funny undulations of a hose, and wondered about it. Then we stumbled on the amazing biological parallel. Of course, how can one resist a puzzle involving penises?鈥�
As many gardeners will know, a hose stretched across a lawn will sometimes form a wavy pattern once the water is turned off. Palffy-Muhoray and his colleagues created a mathematical model of a hose and calculated that this happens because most hoses are reinforced by a helical mesh.
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With the water on, and the pressure high, a helical mesh forces the hose to get fatter and shorter like a spring contracting. Once the pressure is off, the hose springs back out again, but friction from the grass causes it to buckle along its length, forming ripples.
Having created this mathematical model, the team looked for other things it could describe. They found that helical reinforcement is common in nature, found everywhere from sunflower stems to shark skins. One rare exception is in mammalian penises, which are reinforced by parallel and perpendicular collagen fibres.
Diane Kelly, a biologist at the University of Massachusetts, Amherst, discovered this in 1997, and performed experiments on dissected armadillo penises to show that it results in enhanced stiffness, but why this was better than helical reinforcement was unclear.
By adjusting the parameters of their model, Palffy-Muhoray and his colleagues were able to quantify the improvement, finding that this arrangement of collagen fibres results in between three and five times greater stiffness than helical reinforcement.
鈥淚t鈥檚 really interesting to see someone take a more theory-driven, mathematical approach to the system,鈥� says Kelly.
Reference: American Journal of Physics, in press
