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2013

The Sex Life of Birds, and Why It’s Important
New York Times, May 2013
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For a strange sexual history, it’s hard to beat birds. In some lineages, bird penises have evolved to spectacular lengths. Ducks, for example, have corkscrew-shaped penises that can grow as long as their entire body. They use their baroque genitalia to deliver sperm to female reproductive tracts that are also corkscrew-shaped — but twisted in the opposite direction.

In other lineages of birds, however, the penis simply vanished. Of the 10,000 species of birds on Earth, 97 percent reproduce without using the organ. “That’s shocking, when you think about it,” says Martin Cohn, a biologist at the University of Florida.

Research on the sex life of birds has come under fire from critics who claim that it’s unimportant and a waste of federal money, particularly in times of lean spending. In April the criticism from Fox News and conservative pundits became so intense that Patricia Brennan, an expert on bird genitalia at the University of Massachusetts, wrote an essay for Slate defending the value of her research.

The mystery of the vanishing bird penis is actually an important question — not just for understanding the evolution of our feathered friends, but for clues it may offer to little-understood human genetic disorders.

Male birds that lack a penis have an opening known as a cloaca. To mate, a male bird presses his cloaca against a female’s, so that his sperm can flow into her body. Scientists have a poetic name for this act: the cloacal kiss.

Dr. Cohn and his colleagues have figured out how the bird penis performed this vanishing act. They started with detailed anatomical studies of the embryos of two species without penises, chickens and quails, and three species with them — ducks, geese and emus.

At first, the male embryos of all five species developed identically. Within the first few days of growth, they developed a bump, called the genital tubercle. In the ducks, geese and emus, the tubercle continued to grow until it became a full-fledged penis.

In chickens and quails, on the other hand, the tubercle stopped growing after a couple of days. It then shrank almost entirely away. When Dr. Cohn and his colleagues looked closely at the vanishing tissue, they observed that the cells were dying off.

The scientists then identified the genes that produced these different results. When the tubercle started to grow, the same set of genes switched on in all the bird species. But in chickens and quail, the cells at the tip of the tubercle started making a protein called Bmp.

To see if Bmp was the cause of the penises’ disappearance, the scientists loaded beads with Bmp proteins and implanted them in the genital tubercles of ducks. Instead of growing normally, their penises ended up as withered vestiges.

The scientists then performed the opposite experiment. They loaded beads with a protein called Noggin, which blocks Bmp proteins. When they inserted the Noggin-laced beads into the tubercles of roosters, the cells stopped dying. Instead, the tubercle continued to grow. After tens of millions of years, the scientists had resurrected the bird penis, if only briefly.

In their report, published on Thursday in the journal Current Biology, Dr. Cohn and his colleagues suggest that bird penises may have started to shrink as a side effect of some other evolutionary change. Scientists know that Bmp proteins aren’t important only for building penises. They help build the skeleton and other tissues. Scientists have found that changes to Bmp proteins have been pivotal in the evolution of other parts of birds, like feathers and beaks. As Bmp proteins took on a new role elsewhere in the bodies of birds, they may have also stunted the penis.

For some reason, male birds with smaller penises had more offspring than other birds. Why is still a matter of debate. Some scientists have suggested that males with smaller penises were less likely to acquire sexually transmitted diseases. Others have proposed that smaller penises were lighter, and thus made flying easier. Or perhaps females preferred males with smaller penises because they could gain more control over which males sired their offspring.

“Evolution has likely come up with more than one way to lose the penis,” wrote Dr. Brennan, the University of Massachusetts bird genitalia expert, in an accompanying commentary.

Dr. Cohn dismisses criticism of this type of research, arguing that it’s valuable for several reasons. “Understanding how evolution works is part of understanding our natural world,” he said. “One shouldn’t apologize for that.”

Studying bird penises may also provide some insights into human medicine. Doctors have found that malformed penises are among the most common birth defects in humans, but scientists still have only a crude understanding of how they occur. Birds and mammals use many of the same genes to assemble penises, and evolution has tinkered with their interactions for millions of years, producing all sorts of strange results — including penises that self-destruct.

Copyright 2013 The New York Times Company. Reproduced with permission.
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