SYDNEY: Female ducks have evolved “maze-like” genitals with many twists, pouches and dead ends, in a bid to prevent rape and retain control of who fathers their offspring – while male ducks have evolved equally convoluted penises to keep up.
Patricia Brennan, an American reproductive biologist at Yale University in Connecticut, found that some ducks and geese have co-evolved elaborate genitals in an “evolutionary arms race” between the sexes.
Ducks, especially mallards, are one of the few species of birds in which males will often rape females, in a violent act which can result in injuries or death by drowning. The females fight back by preventing successful fertilisation, said Brennan. In fact, of the 40 per cent of matings which are a result of forced copulations, only around four per cent are successful.
“Females are trying to remain in control of which male sires her offspring,” said Brennan, who is also based at Britain’s University of Sheffield. “Because they have … already established a pair bond when they arrive to the breeding grounds, they may be trying to avoid having their eggs fertilised by a male of unknown quality, that tries to pursue forced copulations.”
The unsolved mystery behind the research was why some male birds have penises at all. Most species of birds have lost their penis during evolution. Instead, both males and females having a single opening – the cloaca. This is used for waste deposit and sperm exchange.
Of the three per cent or birds that still have a phallus, the organ is highly variable in length, ranging from 1.25 cm to 40 cm in length, and is often twisted into extraordinarily elaborate spiral shapes.
“I originally started with the idea of looking at the male phallus, but as soon as I looked at the first specimen and saw how extraordinary this organ was, I became intrigued at what the female would look like.”
It may seem like an obvious question, but it turns out no-one else had thought to look, said Brennan.
What she found amazed her: oviducts with many folds, pouches and dead ends, which were spiralled in the totally opposite direction to the male phallus. She details her findings this week in the Public library of Science journal PLoS One.
“It’s probably because previous researchers had assumed that the duck vagina would be just like other avian vaginas: a simple tube folded upon itself. [But] in the case of ducks the oviduct can be quite complex, and the complexity goes hand in hand with the male anatomy.”
Brennan’s research showed a close correlation between the shape of the genitals of many different duck species – to the point where her team could successfully predict the shape of the oviduct of the female of that species based on the penis, and vice versa.
The findings suggest because there’s such a poor fit between the male and female genitals, that female ducks have to mostly be relaxed and willing for mating to succeed.
The study “provides an insight into the complexity of sexual selection,” said Leigh Simmons, an evolutionary biologist at the University of Western Australia in Perth. The study has important implications for our understanding of reproductive biology, he said.
“It sounds kind of counterintuitive, but if a female can resist insemination by a forced copulation, the end product is good quality offspring,” said Simmons. This is because males that can successfully circumvent the female defence mechanisms are more likely to sire offspring, and pass that advantage on to their own young.