Male ducks are infamous for their bizarre penises, which are corkscrew-shaped, exceptionally long and develop anew each year.
Researchers have now found that the size of a male duck’s penis changes when he hangs out with rivals.
Ducks facing more competition grow bigger penises to compensate, while some speed up when they enter sexual maturity to avoid syncing with bigger rivals.
Male ducks are infamous for their bizarre penises (pictured), which are corkscrew-shaped, exceptionally long and unfurl explosively when it’s time to mate. Researchers have now found that the size of a male duck’s penis changes with his social environment
This gives the birds a chance of successfully mating before they are driven off by competitors.
Most birds lack penises, but male ducks sport spiralling phalluses, which have evolved through an ongoing cat-and-mouse game with females.
Around a third of all duck matings involve forced penetration, driving females to adapt long, twisted reproductive tracts to defend themselves.
While a male’s spiralling penis curls clockwise, the females’ tract or ‘oviduct’ spins counterclockwise, meaning most forced matings don’t lead to fertilisation.
Some female ducks have also evolved specialised pouches along their oviducts to capture unwelcome sperm.
To test how far this genital arms race goes, researchers at Mount Holyoke College, Massachusetts, studied how male-on-male competition affects penis size in two species of duck.
The males from one species studied, the ruddy duck, are very promiscuous, and as a result have relatively long penises.
Males from the second group, the lesser scaup, form seasonal partnerships, and so have relatively short penises.
The team studied captive ducks of both species over two breeding seasons at the Livingston Ripley Waterfowl Conservancy in Litchfield, Connecticut.
Some male ducks were kept with other males during mating season, while others were kept with a single female.
The research found that the Lesser Scaup ducks had longer penises on average when kept with other males.
This suggests that competition among males directly changes the way their genitalia grow.
For ruddy ducks, the effect was more complicated, with only some males experiencing growth and reproductive delays when housed together.
Many males failed to reach sexual maturity until the second year of the experiment.
Researchers have found that the lesser scaup duck (A/B) grows its penis longer when it is around other males during mating season. The ruddy duck (C/D/E) can grow its penis longer and speed up when it enters sexual maturity to avoid bigger competitors
Male lesser scaups (file photo) form seasonal partnerships, and so have relatively short penises. They have been found to grow their penises larger when surrounded by other males to give themselves a competitive edge during mating season
When they did, the smaller ruddy duck males housed in groups grew their penises faster than males housed in pairs, but grew out of sync with each other and stayed in reproductive condition for only short periods of time.
The team suggest that small ruddy duck males faced with intense competition may strategically offset their development from other males.
This strategy gives them more of a chance to mate because not all males were sexually mature at the same time.
The study shows that male competition can have a significant effect on duck penis size.
Male ruddy ducks (file photo) are very promiscuous, and as a result have relatively long penises. The birds have been found to change when they enter sexual maturity to avoid mating at the same time as bigger rivals
Researchers must now determine whether these changes give ducks a competitive edge.
Professor Bob Montgomerie, an expert on reproductive strategies at Queen’s University, Belfast, who was not involved in the study, said: ‘This is an excellent experimental study of penis morphology, looking at the effects of social environment on penis size in two duck species that have different mating systems.
‘The question now is whether the observed increase in penis size in Lesser Scaup under the threat of sperm competition actually gives males a competitive advantage.’