Well I certainly could not have passed up this story by Sarah Webb from ScienceShot.
To fully grasp (sorry) the story, it’s helpful to understand that the great majority of male birds lack a penis altogether. Like the ovaries of mammals, the sex organs of male birds are contained within the body cavity, so there really isn’t anything “sticking out” to help identify a male of the species. Birds generally have a single urogenital opening, the cloaca. In most birds, copulation consists of a brief and fluttery “cloacal kiss” in which the mates align their cloacas and the male shoots his sperm inside. The efficiency with which the male fertilizes the eggs in his mate’s clutch will depend on how much sperm he’s able to transfer at any one time; small songbirds may increase the fertilization rate through repeated copulations of the mated pair.
Of course, anything that helps with sperm transmission is likely selected for, and even though those thousands of species lack a true penis, they do develop an engorged “cloacal protuberance” during the height of the breeding season that serves a similar purpose. The pronounced seasonality to breeding in most birds results in a great loss of size and development of both the cloacal protuberance and testes during the non-breeding season that alternates with growth and development of both during the breeding season.
Enter the ducks. Males of some species do, however, sport penises – right proper looking ones at that. Waterfowl are rather well known for their sometimes enormous and elaborately spiraled phalluses, as in the Argentine lake duck (a close relative of North America’s Ruddy Duck):
Why such enhancement? Well unlike swans, ducks aren’t really well known for their gentle, monogamous relationships. Males will often adopt a simple “rape strategy” to access females, with multiple males grappling at one time to fertilize the near drowned female. Thus, the males need a long organ just to reach the female sometimes. More likely, however, is that there has been a corresponding evolution in duck vaginas – to restrict those males to only those the female really does want to fertilize her eggs – in which the vagina is long and twisted and only those penises able to navigate a female’s individual curves will be any good at fertilizing her eggs. Of course just like other birds, the penis shrivels significantly during the non-breeding season and grows back to full size in preparation for the breeding season.
The new story here stems from a presentation delivered recently at the 47th annual meeting of the Animal Behavior Society, hosted this year at one of my alma maters, the College of William and Mary in Virginia. Research conducted by Yale University’s Patricia Brennan has determined that, in controlled conditions, the breeding season growth of a male duck’s penis will be influenced by the competition (ratio of males to females) he faces to succeed in acquiring a mate. Brennan compared penis growth in drake Ruddy Ducks and Lesser Scaup in which males were either kept in isolation with a single female (no male competition) or housed in groups with males outnumbering females (intense male competition).
In Ruddy Ducks with no competition, the males developed full-size penises that were maintained throughout the breeding season. In competitive situations, however, only some of the males developed and maintained their full-size penises for the duration of the breeding season. The penises of subordinate males began to shrivel soon after the hierarchy of competition was worked out and it became unlikely for such males to acquire mating opportunities. (Better luck next year, fellas!)
For the Lesser Scaup, males in the competitive situation actually grew larger penises than those in the monogamous situation: 15–25% larger. I suppose it’s only a matter of time before some clever marketer develops a “male enhancement product” based on the mating habits of our fascinating native ducks.