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Now, let's talk about the sweet life of dinosaurs. Scientists say that they have evidence that among some dinosaurs, the stay-at-home parent was the father. You might wonder how anybody figured that out from a bunch of fossils dating back tens of millions of years, and so did NPR's Christopher Joyce. Here's what he found out.

CHRISTOPHER JOYCE: This parenting puzzle started with the discovery, about 140 years ago, of fossilized dinosaur eggs. Since then, dino eggs have turned up around the world, often in groups or clutches. Among the scientists fascinated with these eggs is David Varricchio from Montana State University.

Dr. DAVID J. VARRICCHIO (Paleontology, Montana State University): We kind of have been puzzling over the size of these clutches for some time.

JOYCE: The eggs are big, picture melons, but also they're often are lots of them in a single clutch. And sometimes next to these egg clutches, scientists have found the skeletons of adult dinosaurs. Occasionally, the skeleton has been on top of the eggs. Scientists studied everything they could about the eggs and the adult skeletons, such as, how much egg material was there compared to the size of the adult? And they noticed something unusual.

Dr. VARRICCHIO: When you compare the clutch size to the adult size, the clutches is rather large relative to the body size.

JOYCE: In other words, a lot of egg for one female to produce. So, Varricchio and a team of experts decided to see if they were modern animals that had especially big egg clutches, compared to the adults' body size. There were: birds, but only certain kinds of birds.

Dr. VARRICCHIO: The group that stands out are those birds with male-only care, and our dinosaurs basically align with that group.

JOYCE: That group includes ostriches, emus and kiwi birds, where the female lays the eggs, but then the male takes over. Sure enough, the dinosaur-egg-clutch-to-adult-size ratio was very close to that in ostriches, emus and kiwis. Now, that's not direct evidence of dinosaur Mr. Moms, but there was another clue: Inside the bones of the nesting dinosaurs, there was no medullary bone; that's a kind of bone that female egg-laying animals create for making eggshells. No medullary bone suggests that these nesting adults were males.

Writing in this week's issue of the journal Science, Varricchio speculates that among these kinds of dinosaurs - Oviraptor, Troodons and Citipati - laying all these eggs took a lot of time and energy. So, the male had to take over while the female went off to feed. Well, however it evolved, this male caretaking links the behavior of some modern birds to gigantic animals that once ruled the planet. That's what intrigues Richard Prum, an ornithologist at Yale University.

Dr. RICHARD O. PRUM (Ornithology, Ecology and Evolutionary Biology, Yale University): Well, you know, what I really love about it is the fact that, that these are representatives of the most ferocious and terrifying lineage of animals that ever was. And so, it's really ironic to me, to think that these macho, bipedal meat eaters were good dads.

JOYCE: In fact, the very name Oviraptor might need changing now. Prum points out that Oviraptor means egg-seizer, a name chosen by the man who discovered the first Oviraptor in 1924. It was found on top of an egg clutch. That, scientists said the dinosaur was, quote, "in the very act of robbing a dinosaur egg nest." But now, well, maybe it was just doting dad keeping the chicks warm. Christopher Joyce, NPR News.

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