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.
Culled from World-Science.com
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