Genital evolution in birds: losing the penis and winning the battle

Most birds do not have a penis, and most ornithologists have not thought about bird penises much. However, evolutionarily, the loss of the penis in birds is an extremely significant event: why loose an organ that seems so handy to get sperm close to female eggs? The few birds that have kept their penis may offer some insights as to why this penis loss may have occurred in birds. There are multiple reductions and eliminations of the penis in the evolutionary history of birds, within the Tinamidae and the Megapodidae. Yet, the avian penis has the same erectile mechanism, and functional asymmetry in all birds that have been examined so far. Differences in penis morphology do not arise during embryonic development, but later, during the first year of reproduction. Developmental studies suggest that losing the penis is not as hard as one might imagine. Within birds that have kept their penis, waterfowl are remarkable in the variation of their genital morphology. In some waterfowl, males force copulations on females, and in these species males have longer and more elaborate penises. Male-male competition plays a critical role in influencing the morphology of the penis, surprisingly within the lifetime of an individual. In male biased group settings, males grow longer penises than males housed in pairs. Females suffer great direct and indirect costs from forced copulations and they have evolved complex vaginas that prevent the full eversion of the penis, and reassert female control over paternity. An evolutionary arms race is playing out in the complex genitalia of waterfowl, with some species having extremely exaggerated genitalia. One possible resolution of such conflict is disarmament, and Dr. Brennan will discuss how the avian penis loss may have been driven by female choice for increased sexual autonomy, giving females the upper hand in the reproductive battle.