Duck phalluses
The New York Times
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."
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."