Female duck anatomy evolves to block "rape"
Female duck anatomy evolves to block "rape"
Biologists have found anatomical details about the female reproductive tract in waterfowl that they say indicate male and female anatomy have co-evolved in a “sexual arms race.”
Bird copulation mostly consists of a simple, and rather chaste, “cloacal kiss” in which two openings come together. But a few ancient bird lineages, including waterfowl, retain the grooved phallus of their reptilian ancestors. Waterfowl are also distinct in having great diversity among species in the length and ornamentation of the phallus.
Scientists previously attributed this diversity to sperm competition. They speculated that sperm from males with a longer phallus had a competitive edge over sperm from those less well-endowed. The new report in the online research journal PLoS One finds more to the story.
“As part of a research program on the evolution of the avian phallus, I was curious to know if there were consequences to female ducks of the tremendous anatomical variation found in the male phallus,” said lead author Patricia Brennan of Yale University in New Haven, Conn. and the University of Sheffield, U.K.
Her study is a complementary exploration of the anatomy of the female reproductive tract, called the oviduct or vagina, which is usually very simple and similar among birds. Brennan found two unexpectedly complex and new structures that she said seem designed for one purpose — to selectively exclude the phallus.
In most birds, the oviduct is a simple tube, but in some waterfowl, the tube has unique sacs and spirals. The sacs are outpocketings in the sides of the tube that are just inside the oviduct opening. “They appear to function as ‘dead-ends,’ or false passages,” said Brennan. “If the phallus were to enter one of these sacs, it would not progress further into the oviduct where it would deposit sperm more effectively.”
The second novelty is a series of tight, clock-wise spirals in the tubular oviduct. “Interestingly, the male phallus is also a spiral, but it twists in the opposite, counter-clockwise, direction,” said Yale ornithologist and co-author Richard Prum. “So the twists in the oviduct appear designed to exclude the opposing twists of the male phallus.”
The number of sacs and spirals in the reproductive tract of various female waterfowl correlates strongly with the length of the male phallus, the scientists wrote.
Comparing the phallus size and oviduct shape in 14 different species of ducks and geese, the authors concluded that the genitalia of males and females have dynamically co-evolved: in various separate duck lineages, females developed more elaborate oviducts as males evolved longer phalluses. In other lineages females lost oviduct complexity as the phallus evolved toward smaller size.
Brennan hypothesizes that the female waterfowl have evolved these anatomical features to block male attempts at reproductive control. “Despite the fact that most waterfowl form monogamous pairs, forced copulations by other males — the avian equivalent of rape — are common in many waterfowl,” said Prum. “The length of the phallus of a species is strongly correlated with the frequency of forced copulations.”
“In response to male attempts to force their paternity on females, female waterfowl may be able to assert their own behavioral and anatomical means of controlling who fathers their offspring,” Brennan said.
The authors propose that ornate phalluses and female oviducts have co-evolved in response to one another. More elaborate phalluses have selected for improved means of excluding them, and vice versa.
What happens when a female duck wants to mate with its chosen partner? The authors speculate that these physical barriers are easily overcome when females cooperate, and that they only function to exclude unwanted advances. Brennan is pursuing the findings with further exploration of the development and evolution of bird genitalia. “I am sure there are more surprises out there,” she said.
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