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MELISSA BLOCK, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

MICHELE NORRIS, host:

And I'm Michele Norris.

Ogden Nash established his reputation with poetry, not biology, and yet we can't resist introducing this next scientific story with a few lines of Nash's verse.

Take it away, Melissa.

BLOCK: Here we go. I like the duck-billed platypus because it is anomalous. I like the way it raises its family, partly birdly, partly mammaly. I like its independent attitude; let no one call it a duck-billed platitude.

NORRIS: Actually, that's pretty accurate. Scientists have now completed a draft DNA sequence of the platypus genome.

And as NPR's Joe Palca reports, the platypus is, in fact, partly birdly, partly mammaly.

JOE PALCA: The platypus is definitely unusual - not just because it only lives in Australia. Richard Wilson directed the platypus genome project at Washington University in St. Louis.

Dr. RICHARD WILSON (Washington University School of Medicine): When the first specimens were brought back to England in the 18th century, most of the naturalists that looked at them thought that they were some sort of a fake - a taxidermist had gotten creative and sewn the bill of a duck on some sort of beaver-like creature and added webbed feet for grins.

PALCA: But the platypus is, of course, real, even if its genome is a bit strange. Most genes in the platypus make things typical of mammals - genes for fur, genes for making milk. But unlike most mammals, the platypus lays eggs. And as Wilson reports in this week's issue of the journal Nature, the platypus has the same genes birds have for making hard-shelled eggs. And that's not the only odd thing.

Dr. WILSON: They also have very bizarre sex chromosomes.

PALCA: Wilson says the sex chromosomes of platypuses - yes, I checked, it's platypuses - are more like the sex chromosomes of birds.

Dr. WILSON: A male platypus has five X chromosomes, no two alike, and five Y chromosomes.

PALCA: In addition to bird and mammal-like genes, the platypus also has reptile-like genes. Male platypuses make a kind of venom that they transmit using a spur on their hind legs.

Jerry Siegel is a neuroscientist at UCLA who's studied the platypus. He says you really want to steer clear of this venom.

Dr. JERRY SIEGEL (UCLA): It causes intractable pain which lasts for months. There's some thought that this is used primarily in mating rather than predation.

PALCA: Mating? That doesn't sound like any fun.

(Soundbite of laughter)

Dr. SIEGEL: It doesn't sound like fun for the matee, but they've been around for a long time, so it must work.

(Soundbite of laughter)

Dr. SIEGEL: Like a bad marriage, you know, they've worked out a relationship.

PALCA: Siegel says the platypus can tell us a lot about how animals evolved. That's because platypuses split off from the rest of the mammal line more than 150 million years ago. So if a platypus has a particular trait, you know it's an ancient mammalian trait, not something that showed up more recently. Take REM sleep. Siegel says some scientists argue that REM sleep only evolved after mammals got bigger brains. So Siegel studied sleeping platypuses.

Dr. SIEGEL: And what we saw is that in the platypus, the REM sleep is absolutely unequivocal.

PALCA: Meaning mammals had REM sleep long before our bulging brains evolved. The platypus is just one of many species that are currently having their genomes probed, but geneticist Richard Wilson says there are still plenty of gaps in the genetic record that scientists would like to fill.

Dr. WILSON: If we think about the platypus and the chicken, there's a couple hundred million years of evolution in between. So you'd like to sequence things in between.

PALCA: That's not a problem. Ogden Nash has written poems about lots of animals.

Joe Palca, NPR News, Washington.

NORRIS: And if you're curious, you can see a video of a platypus swimming and sleeping. That's at our Web site, npr.org.

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