Turtle and mammal penis designs are anatomically convergent.

Males in many modern amniote taxa have a hydraulic penis that inflates for copulation. Hydraulic skeletons are typically reinforced with inextensible fibres; the specific arrangement of the fibres within the skeleton determines whether it is flexible or resists bending. I show that the hydraulic skeleton in the turtle penis is reinforced by an axial orthogonal array of collagen fibres. This microanatomy is evolutionarily convergent with that of mammalian penises, and implies that there is a limited number of mechanical designs for an inflatable structure with high bending stiffness.