Not so cocky: Roosters lose penises to please hens
The revelation came from research published in the journal Current Biology. It said male chickens kill their own penises in the embyro to enhance their chance of mating.
The research team, from the University of Florida, set out to find why chickens do not have a penis – 97% of all avian species have little in the way of a penis.
Chickens have an all-in-one opening, which they use for excreting waste and for copulating. To impregnate the female, the two press their openings together in a move called a “cloacal kiss”. The male then pumps his sperm into the female.
This might have evolved to ensure that mating was mutual – instead of the male chasing the woman into submission and “forced copulation”, it said. Females would have chosen males with smaller penises, giving them the evolutionary advantage.
To find out why chickens do not have a penis, the team compared the foetal growth of chickens and ducks. To see inside, they cut miniscule windows in the eggs to see how the penis were developing.
The two grew at similar speeds until the ninth day, then the chicken’s penis stopped and started shrinking. The team isolated one protein – Bmp4 – at the tip of the penis. This promotes death in cells. When they neutralised this protein, the penises grew 6.5 times longer than they had before.
In contrast to the chicken, male ducks have penises half the length of their bodies. These have to be coiled up during normal working hours.