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They were fierce hunters with sharp claws, but two types of dinosaurs related to Tyrannosaurus rex were also attentive fathers that hovered over their eggs to keep them warm.

That's the conclusion of a team of American researchers who have published a paper on dinosaur daddies in today's edition of the journal Science.

Their work highlights the softer side of dinosaurs, and adds to the growing evidence that they were nurturing parents, more like modern birds than reptiles when it came to looking after their young. It also suggests where birds - modern descendants of dinosaurs - got their parenting skills.

"The old view was that dinosaurs were reptiles and that most reptiles sort of bury their eggs and leave them, like turtles on a beach," says David Varricchio, with the department of earth sciences at Montana State University and lead author of the Science paper.

The theory that dinosaurs were more involved parents has been building for more than two decades, bolstered by the discovery of a number of fossils of adults that died while incubating nests packed with eggs.

The nesting dinosaurs were troodons and oviraptors - theropod dinosaurs related to T. rex, but much smaller.

It was widely assumed they were females, but Dr. Varricchio and his colleagues are arguing that they were dads, not moms. They followed a hunch that any species of animal that attempted to raise 30 offspring at a time would need to have the fathers making a substantial contribution.

They compared the large numbers of eggs found in these nests - between 22 and 30 - to the number produced by 400 species of living reptiles and birds.

When the body size of the dinosaurs was taken into account, the number of eggs they produced closely matched the number laid by flightless birds such as emus and rheas. In those species, the fathers are single parents who incubate and raise the young.

As with modern emus, rheas and to a lesser extent ostriches, there is no way that laying so many eggs would be a successful reproductive strategy unless the males pitched in, Dr. Varricchio says.

In 90 per cent of bird species, the males help look after their young. But in only 5 per cent of mammal species do males make a contribution to parental care, and that number is even lower among reptiles.

The theory that modern birds descended from dinosaurs is now widely accepted by experts, and is based on similarities in their skeletons. This work suggests that birds inherited their approach to parenting from their dinosaurian ancestors.

Dinosaurs had different nesting styles. Oviraptors plunked themselves in the middle of a ring of eggs, and draped their arms over them. "The clutch has a doughnut shape," Dr. Varricchio says. Birds, however, keep the eggs warm with their bellies, he says.

One of the best examples of an oviraptor on a nest was discovered in China and described by the University of Alberta's Philip Currie in 1996.

But he says in that case, it was almost certainly the mother - not the father - that died with the eggs, because the nest was not complete.

Dr. Currie's not convinced Dr. Varricchio and his colleagues have made the case that dinosaurs were as attentive to their offspring as birds, but says they take an interesting approach to the question.

The name troodon means "wounding tooth" in Greek. Oviraptors had more beak-like mouths, and the first one was discovered in 1924 with a bunch of eggs and named "egg seizer" because it was assumed it was a thief. Both were about the size of a person. "I wouldn't want to meet a hungry one," Dr. Varricchio says.

*Troodon dinosaurs were about the size of a human and had sharp teeth.

*The females laid between 22 to 24 eggs, and planted them on end in the ground.

*Each egg was about half a litre in volume, and fit snugly with others.

*A team of American dinosaur experts says there is evidence males incubated the eggs.