Social Competition Can Cause Duck Penises to Grow Longer, Faster

A male ruddy duck

Chemical signals released by the individuals around us can have a profound effect on our physical states. For example, many women have noticed a still-mysterious phenomenon coined “menstrual synchrony,” whereby those who have been living or working together in close proximity have their periods at around the same time. 

Now, new research on ducks finds that social environment can even impact penis size. While further research is needed to pinpoint the specific factors that affect penis size in men, the findings — published in The Auk: Ornithological Advances — could lead to future treatments for erectile dysfunction.

As for why scientists are investigating ducks in particular, project leader Patricia Brennan of Mount Holyoke College explained to Seeker that ducks are among the few birds that have a penis.

“I wanted to find out the potential role of sperm competition on shaping the morphology of the penis,” she said. “So I decided to set up experiments to test whether male-male competition could influence male genital morphology.”

Brennan and her colleagues partnered with the cushy Livingstone Ripley Waterfowl Conservancy in Litchfield, Connecticut.

“The ducks were in beautiful, outdoor naturalistic enclosures, and they got used to our manipulation very quickly,” she said, explaining that the “manipulation” meant picking up the male ducks every so often for penis measuring.

A male ruddy duck

The ducks consisted of two species housed separately in either pairs or groups: ruddy ducks, which are very promiscuous, do not form pair bonds, and have relatively long penises and lesser scaups, which form seasonal pair bonds and have relatively short penises.

Over a two-year period, the researchers found that lesser scaup ducks had longer penises, on average, when they were housed in groups with other males. This may also happen to male ducks, like various pekin species, raised for egg, meat, and feather production.

“Anecdotal data suggests that pekin ducks in captivity have longer penises than their wild mallard counterparts, but this may be the result of selection for better breeders in general,” Brennan said.

For the ruddy ducks in the study, the effects were more complicated.

Many ruddy duck males did not reach sexual maturity until the second year of the experiment. When they did reach this life stage, smaller ruddy duck males housed in groups grew their penises faster than males housed in pairs, but they grew out of sync with each other and remained in a reproductive state for only short periods of time.

Collectively, the determinations show that the level of inherent competition that individual male ducks experience just by living together can have a major impact on their genital development.

As for menstrual synchronicity, the probable underlying biochemical factors have to do with hormones. In this case, testosterone and di-hydrotestosterone (DHT) probably underlie the observed duck penis growth differences, outside of factors related to genetics and overall health.

Stress hormones could come into play as well. “In ruddy ducks, the stress from male aggression may be a contributing mechanism for why small males in the groups can’t grow a long penis,” Brennan said.

Before this study, ruddy duck penises had already captured the attention of scientists. These male genitals grow dramatically in the spring — up to about 10 inches long for the ruddy duck, or more than half of a typical male duck’s body length — before shrinking in size just as dramatically in the summer. Additionally, the ruddy duck penis, like that of the pseudo-penis in mallard ducks, is shaped like a corkscrew.

In prior research, Brennan and her team found that this unusual penis shape likely co-evolved with vaginal morphology. Female ruddy ducks have long, corkscrew-shaped vaginas that spiral in the opposite direction of the male’s member. Because insertion is a bit of a challenge, females can ward off some forceful males. The males, in turn, have a better chance of out-competing other would-be suitors, if they manage to penetrate the female first.

Having such a long penis comes with major drawbacks, though.

“I have seen females and other males pecking at the penis immediately after a copulation, and I would imagine this organ is quite vulnerable,” Brennan explained.

She added that male ruddy ducks otherwise keep their penis “inside out, inside a pocket” in the male’s body.

The ancestors of birds all used to have a penis, researchers believe, but most birds evolved other reproductive systems over time, losing their penises in the process. Brennan suspects that dinosaurs had penises, but no one has yet been able to validate this theory, since these are soft tissues that do not fossilize well.

“Part of my research is figuring out the penis loss in birds,” Brennan said. “We don’t have enough evidence to know for sure, but female choice for less aggressive males may have played a role in favoring males with smaller genitalia.”

She would like to perform experiments on female ducks that would parallel those conducted on the ruddy duck and lesser scaup males.

“However,” she said, “to do this study, we would have to sacrifice the females to dissect their vaginas, whereas with the males, we can evert the penis, measure it, and then the males go on happily.”

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