You can call it dino daddy day-care.
Scientists who examined the fossilised remains of three types of medium-sized dinosaurs found with large clutches of eggs have concluded that the males rather than the females seem to have guarded the nests and brooded the eggs.
Writing in the journal Science, they said this behavior is seen in certain existing species of birds. Scientists believe birds evolved from small, feathered predatory dinosaurs more than 150 million years ago.
The three types of dinosaurs, Troodon, Oviraptor and Citipati, lived roughly 75 million years ago and were theropods -- the primarily meat-eating group that also includes monstrous beasts like Tyrannosaurus rex and Giganotosaurus.
"There are a lot of characteristics that we once thought were unique to birds that are turning out not to be -- that they first arose in their theropod ancestors," Montana State University paleontologist Frankie Jackson, one of the researchers, said in a telephone interview.
The scientists said the findings suggest that at least in these types of dinosaurs, the males may have mated with several females that laid eggs in one large clutch. When the females left, the males incubated and protected the eggs on their own.
Male-only care for eggs occurs among certain large flightless birds like emus and rheas and the South American tinamous, according to fellow Montana State University paleontologist David Varricchio.
In these cases, the dinosaurs were found with an unusually large number of eggs -- each nest containing from 22 to 30 eggs. They were found in Montana in the case of Troodon, and Mongolia in the case of Oviraptor and Citipati.
After a close examination of the fossils, the scientists concluded that the dinosaurs were males.
Florida State University paleobiologist Greg Erickson said there was no evidence of medullary bone -- the extra bone that breeding female birds and dinosaurs use to make eggs -- or evidence of another process by which female reptiles such as crocodiles acquire mineral salts to make eggs.
Males contribute to parental care in less than 5 percent of mammal and reptile species. Males and females contribute to parental care together in more than 90 percent of birds.
Among a group called Paleognathes -- an ancient lineage that branched off soon after birds evolved from dinosaurs and includes ostriches, emus and tinamous -- paternal care and polygamy are the rule, the scientists said.
The scientists are uncertain why these dinosaurs died suddenly while perching over the eggs. In the Mongolian ones, they may have perished in sandstorms or collapsing sand dunes.