Lesser scaup
The lesser scaup grows a larger penis when there are many males around to compete with. Ken & Nyetta

Being raised around a lot of other males makes the lesser scaup duck's penis grow larger, scientists have found.

The duck is unusual among birds in having a penis. The organ itself is unique, with a long spiralling shape that can be about as long as the duck's whole body. Most male birds simply have an opening which as to be aligned carefully with the female to transfer sperm.

Bu the lesser scaup has a sizeable penis, which appears to be sensitive to the social environment males are raised in, finds a study in The Auk: Ornithological Advances. The more males around as the juvenile develops, the longer his penis.

Lesser scaups form pair bonds that last the breeding season. It's thought that a larger penis could help the male be more successful with females.

But the penis of another species, the ruddy duck – which are more promiscuous than the lesser scaup – didn't respond to social cues in the same way, the study authors found. However, the timing of when they reached maturity and became reproductively active differed when males were housed together.

This could be to avoid unnecessary competition and conflict, the researchers suggest. The response by altering the timing of fertility rather than penis size could be because the ruddy duck is one of the most well-endowed of all birds. Their penises can reach 9.8 inches long – more than half the animal's body size.

"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," said Bob Montgomerie of Queen's University, who researches reproductive strategies and was not involved in the study.

"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. Like all good studies, this one will undoubtedly stimulate more research, as it provides both methodologies and a clear focus on interesting questions."

Ruddy duck
A slightly excited ruddy duck. P. Brennan