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Our next story is about some peculiar chickens - really peculiar. These chickens look like they're split right down the middle, with one half male and the other half female. And if you don't believe me, there's a picture of it on our Web site.

Scientists are pondering how these birds got that way and what it might mean for other species.

NPR's Joe Palca has more.

JOE PALCA: At the University of Edinburgh in Scotland, Michael Clinton studies chickens. In particular, he's interested in the molecular signals that tell a chicken whether it will be male or female. One day he heard about a man whose job included inspecting chicken farms. This man was seeing some very strange chickens - chickens that looked half male and half female.

Dr. MICHAEL CLINTON (University of Edinburgh): And he kept seeing these things. So he got in touch with us and said, are you interested? I said, you know, okay.

PALCA: The reason Clinton wasn't particularly interested is he didn't think that these odd-looking birds would tell him anything surprising. You see, like humans, chickens have male and female sex chromosomes. And usually, these chromosomes tell a chicken what sex to be.

Dr. CLINTON: The sex chromosomes determine whatever gonad forms.

PALCA: So male sex chromosomes tell the gonads to be testes, and female sex chromosomes tell them to be ovaries.

Dr. CLINTON: And then the hormones produced by the gonad define what the individual looks like.

PALCA: So Clinton figured these half-and-half birds would have been some weird chromosomal abnormality, so the gonads would send out a scrambled signal. But that turned out to be wrong. The chickens were a mixture of normal male and female cells. And it was the cells, not the hormones, that seemed to be calling the shots.

Dr. CLINTON: Well, it's a really unscientific way of putting it, but it seems that these cells know whether they are male or female.

PALCA: Normally, chickens only have male or female cells, but not both. Clinton says chickens with this mix of cells that gives them this mixed appearance are rare, but maybe not as rare as people think.

Dr. CLINTON: If they are the same color, for example, you know, you might think, well, that's a funny-looking chicken. But it wouldn't be obvious that it was half-male and half-female.

PALCA: Clinton reports his study in today's issue of the journal Nature. Arthur Arnold of the University of California, Los Angeles says Clinton's work raises a fundamental question about how animals and the cells that make up the animals know what sex to be when they grow up.

Professor ARTHUR ARNOLD (UCLA): Is that sex imposed on it by hormonal signals coming from the outside of the cell, or from signals coming from within the cell, from its own genome? And the answer is it's both.

PALCA: Up till now, scientists who study sexual development assumed that in birds and mammals, hormones were by far the more important signal. But these chickens are telling a different story.

Dr. BLANCHE CAPEL (Duke University): Maybe this is something that's specific to chickens or to vertebrates other than mammals. It's not clear yet.

PALCA: Blanche Capel is a developmental biologist at Duke University. Capel says it shouldn't be all that surprising that signals other than hormones may be important. She studies turtles, and they use temperature to determine which sex to be.

Dr. CAPEL: There are many, many variations on the mechanisms that actually control sex determination, and many different animals do it in completely different ways.

PALCA: So who knew sex was so complicated?

Dr. CAPEL: Well, we knew.

(Soundbite of laughter)

Dr. CAPEL: Sex is very complicated.

(Soundbite of laughter)

PALCA: And just by the way, there's a name for animals that are half male and half female. They're called gynandromorphs - not a word you usually hear bandied about in polite conversation.

Joe Palca, NPR News, Washington.

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