FreeLists / tn-bird / [TN-Bird] New Findings In Waterfowl Anatomy

["Charles P. Nicholson" <cpnichol@xxxxxxxxxxxxx>]

As someone who is a student and sometime teacher of avian biology, as well
as someone who has attempted to determine the sex of thousands of wild birds
captured in nets and traps, including a few dozen waterfowl, I found the
following article from yesterday's New York Times fascinating.  I think some
of you will agree.
 

Chuck Nicholson

Norris, TN

 

New York Times

http://www.nytimes.com/2007/05/01/science/01duck.html

 

May 1, 2007


In Ducks, War of the Sexes Plays Out in the Evolution of Genitalia 


By CARL ZIMMER

LITCHFIELD, Conn. - "This guy's the champion," said Patricia Brennan, a
behavioral ecologist, leaning over the nether regions of a duck - a Meller's
duck from Madagascar, to be specific - and carefully coaxing out his
phallus. 

The duck was quietly resting upside-down against the stomach of Ian Gereg,
an aviculturist here at the Livingston Ripley Waterfowl Sanctuary. Dr.
Brennan, a post-doctoral researcher at Yale University and the University of
Sheffield, visits the sanctuary every two weeks to measure the phalluses of
six species of ducks. 

When she first visited in January, the phalluses were the size of rice
grains. Now many of them are growing rapidly. The champion phallus from this
Meller's duck is a long, spiraling tentacle. Some ducks grow phalluses as
long as their entire body. In the fall, the genitalia will disappear, only
to reappear next spring.

The anatomy of ducks is especially bizarre considering that 97 percent of
all bird species have no phallus at all. Most male birds just deliver their
sperm through an opening. Dr. Brennan is investigating how this sexual
wonder of the world came to be.

Part of the answer, she has discovered, has gone overlooked for decades.
Male ducks may have such extreme genitals because the females do too. The
birds are locked in an evolutionary struggle for reproductive success.

Dr. Brennan was oblivious to bird phalluses until 1999. While working in a
Costa Rican forest, she observed a pair of birds called tinamous mating.
"They became unattached, and I saw this huge thing hanging off of him," she
said. "I could not believe it. It became one of those questions I wrote
down: why do these males have this huge phallus?"

A bird phallus is similar - but not identical - to a mammalian penis. Most
of the time it remains invisible, curled up inside a bird's body. During
mating, however, it fills with lymphatic fluid and expands into a long,
corkscrew shape. The bird's sperm travels on the outside of the phallus,
along a spiral-shaped groove, into the female bird.

To learn about this peculiar organ, Dr. Brennan decided she would have to
make careful dissections of male tinamous. In 2005 she traveled to the
University of Sheffield to learn the art of bird dissection from Tim
Birkhead, an evolutionary biologist. Dr. Birkhead had her practice on some
male ducks from a local farm. 

Gazing at the enormous organs, she asked herself a question that apparently
no one had asked before.

"So what does the female look like?" she said. "Obviously you can't have
something like that without some place to put it in. You need a garage to
park the car."

The lower oviduct (the equivalent of the vagina in birds) is typically a
simple tube. But when Dr. Brennan dissected some female ducks, she
discovered they had a radically different anatomy. "There were all these
weird structures, these pockets and spirals," she said.

Somehow, generations of biologists had never noticed this anatomy before.
Pondering it, Dr. Brennan came to doubt the conventional explanation for how
duck phalluses evolved. 

In some species of ducks, a female bonds for a season with a male. But she
is also harassed by other males that force her to mate. "It's nasty
business. Females are often killed or injured," Dr. Brennan said. 

Species with more forced mating tend to have longer phalluses. That link led
some scientists to argue that the duck phallus was the result of males'
competing with one another to fertilize eggs. 

"Basically, you get a bigger phallus to put your sperm in farther than the
other males," Dr. Brennan said.

Dr. Brennan realized that scientists had made this argument without looking
at the female birds. Perhaps, she wondered, the two sexes were coevolving,
with elaborate lower oviducts driving the evolution of long phalluses. 

To test this idea, Dr. Brennan traveled to Alaska. Many species of waterfowl
breed there, with a wide range of mating systems. Working with Kevin
McCracken of the University of Alaska and his colleagues, she caught and
dissected 16 species of ducks and geese, comparing the male and female
anatomy.

If a male bird had a long phallus, the female tended to have a more
elaborate lower oviduct. And if the male had a small phallus, the female
tended to have a simple oviduct. "The correlation was incredibly tight," Dr.
Brennan said. "When you dissected one of the birds, it was really easy to
predict what the other sex was going to look like." 

Dr. Brennan and her colleagues are publishing their study today in the
journal PLOS One.

Dr. McCracken, who discovered the longest known bird phallus on an Argentine
duck in 2001, is struck by the fact that it was a woman who discovered the
complexity of female birds. "Maybe it's the male bias we all have," he said.
"It's just been out there, waiting to be discovered."

Dr. Brennan argues that elaborate female duck anatomy evolves as a
countermeasure against aggressive males. "Once they choose a male, they're
making the best possible choice, and that's the male they want siring their
offspring," she said. "They don't want the guy flying in from who knows
where. It makes sense that they would develop a defense."

Female ducks seem to be equipped to block the sperm of unwanted males. Their
lower oviduct is spiraled like the male phallus, for example, but it turns
in the opposite direction. Dr. Brennan suspects that the female ducks can
force sperm into one of the pockets and then expel it. "It only makes sense
as a barrier," she said.

To support her argument, Dr. Brennan notes studies on some species that have
found that forced matings make up about a third of all matings. Yet only 3
percent of the offspring are the result of forced matings. "To me, it means
these females are successful with this strategy," she said.

Dr. Brennan suspects that when the females of a species evolved better
defenses, they drove the evolution of male phalluses. "The males have to
step up to produce a longer or more flexible phallus," she said.

Other scientists have documented a similar coevolution of genitals in flies
and other invertebrates. But Dr. Brennan's study is the clearest example of
this arms race in vertebrates.

"It's rare to find something so blatantly obvious in the female anatomy,"
Dr. Brennan said. "I'm sure it's going on in other vertebrates, but it's
probably going in ways that are more subtle and harder to figure out."

To test her hypothesis, Dr. Brennan plans to team up with a biomechanics
expert to build a transparent model of a female duck. She wants to see
exactly what a duck phallus does during mating.

Dr. Brennan also hopes to find more clues by studying phalluses on living
ducks. At the waterfowl sanctuary in Litchfield, she is spending the year
tracking the growth and disappearance of phalluses in ducks and geese.
Hardly anything is known about how the phallus waxes and wanes - not to
mention why.

"It may be easier to regrow it than to keep it healthy," Dr. Brennan said.
"But those are some of the things I may be able to find out. When you're
doing something that so little is known about, you can't really predict
what's going to happen."

 



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