The specific case of a penis with a hydrostatic structure, as well as an array of collagen fibres that allows both expansion and guards against aneurysms, has evolved in a strikingly convergent fashion in mammals and turtles.

Mammal and turtle penis form

Evolution of a male intromittent organ to effect impregnation of the female has appeared many times but the range of structures that have been employed is so wide that they can only be called convergent in the widest sense (some are near-miracles of engineering). However, the specific case of a penis with a hydrostatic structure (usually inflated with blood) that combines stiffness and non-bending, as well as an array of collagen fibres that allows both expansion and guards against aneurysms, has evolved in a strikingly convergent fashion in mammals and turtles. The former will probably require relatively little introduction, although humans are somewhat unusual in lacking the penis-bone (or baculum) that is also convergent. In the case of the turtle D.A. Kelly has written that “the similarity of the anatomical designs used by [mammals and turtles] to produce penile stiffness is astonishing” (2002, p.194).

The convergent evolution of the organ extends to other groups, although the similarities are somewhat less precise than those seen between mammals and turtles. Of note are a few birds (e.g. ducks) and some octopus species that have penis-like organs for copulation with or stimulation of the female.

Duck penis form

Whilst the majority of birds mate by pressing together the cloacas, a number of groups have evolved a penis. These too are erectile, and can show some intriguing features. The ducks are one of the main groups of birds to evolve a penis (arguably because copulation takes place on the water) but that of the Argentine Lake Duck achieves heroic dimensions of c.20 cm (half the body length) and bristles with spines (possibly to remove sperm of competitors and consistent with a promiscuous system of group mating). The bird penis differs, however, in a number of respects that include transport of the sperm along an external groove (or similar folds) and inflation by lymphatic fluids (as against blood). Given the awkwardness of typical bird copulation, with cloaca to cloaca contact, it seems surprising that the penis in birds has so rarely evolved (or if inherited from an ancestral form has been lost multiple times). There are several possibilities: the nature of the cloaca combines sexual products with waste so potentially increasing the risk of infection with either sexually transmitted diseases or pathogens. Alternatives are linked to risk from predators during copulation (keep it short!) or problems of balance.

Phalloid organs in weaver birds

In terms of the birds even more remarkable is what is effectively a false penis (phalloid organ) in the weaver bird Bubulornis. This robust structure is situated in front of the cloaca, but unlike a penis remains rigid and contains no duct to transfer sperm. Understandably its function has attracted some comment. Early suggestions included the plausible thought that if inserted into the female oviduct it could serve to remove a rival’s sperm (and hence be linked to sperm competition), but it is now evident that insertion does not occur. Rather, evidence exists that primarily it serves for male stimulation, leading in the final stages of copulation to an orgasm-like state that has no known counterpart in other birds and is evidently convergent with mammals. In addition to observations in captivity, both use of a taxidermist’s model bird equipped with an artificial cloaca and stimulation of the phalloid organ leading to ejaculation support this hypothesis that this organ is stimulatory.

Octopus copulatory arms

Given the many convergences in the octopus with the vertebrates it is less surprising, perhaps, to find that in some species the male has erectile tissue that is strikingly similar to the mammalian penis. In octopus the copulatory arm (heterocotylus) is specialized to transfer the spermatophore, but in this particular example the penile-like structure shows internal spaces for hydrostatic inflation (and an extensive system of blood vessels), as well as collagen fibrils to resist bending. The erectile tissue does not contain a duct, but the spermatophore is transferred along a groove which is reminiscent of what we see in the bird penis. Whilst not definite it seems unlikely the erectile structure is involved with actually intromission with the female.