Size isn't everything. Reptiles and mammals share a penis design optimized for strength.

The Penis Plan

When it came to phallus design, Mother Nature stuck with a good thing. Although mammals and reptiles apparently evolved penises independently of each other, they share an underlying design optimized for stiffness and form, according to new research.

A penis is essentially a "big, reinforced water balloon," says functional morphologist Diane Kelly of the University of Massachusetts, Amherst. Kelly's previous work suggested that mammals had a swell design. She found that mammalian penises--from those of rats to armadillos to elephants--have two layers of collagen fibers. One set of fibers runs along the long axis while another wraps around perpendicularly. When pumped full of blood, such an arrangement is extremely stiff--it can't bend because the fibers are stretched to their maximum length. The perpendicular fibers also help the penis hold its shape, she says. Kelly wondered whether distantly related animals such as reptiles had come to the same evolutionary solution.

To get the story straight, she examined flaccid specimens from two turtle species and from the workhorse of mammal penis research, the armadillo. Shining polarized light on the tissues revealed a similar pattern of collagen fibers, she reports in the 25 February Biology Letters. Because reptilian and mammalian penises develop from different embryonic tissues, Kelly says the two groups must have reached the same design independently.

There were differences between the species, however. While the armadillo, like other mammals, had only two perpendicular collagen layers, the turtles had multiple layers, like plywood. That means a turtle penis could withstand more bending forces than could a mammalian one, she says. Why turtles need a stronger penis, however, remains one of Mother Nature's secrets.

The study shows that evolution finds similar solutions for problems across species, says comparative physiologist Jim O'Reilly of the University of Miami. He points out that penises with perpendicularly arranged fibers are more prone to buckling than other conceivable designs, but for most species, a dependable shape may be worth that risk.