Male dinosaurs may not have had a caring side after all.

Five years ago, a study of theropod dinosaurs concluded that it was male dinosaurs that incubated the eggs of their offspring. Now a new analysis of the same data is challenging that finding.

It is notoriously difficult to work out how long-extinct animals behaved, but a few fossils found in recent decades show clearly that some Mesozoic theropods, a bipedal group of carnivorous dinosaurs, made — and sat on — nests, apparently in the same way that birds do today.

In 2008, David Varricchio at Montana State University in Bozeman and his colleagues set about learning more about dinosaur parenting. Their strategy was to combine data from those fossils with what we know about how their descendants behave today.

They deduced the adult body mass of the nesting dinosaurs and counted the maximum number of fossil eggs in the nests attributed to each species. They then compared their figures to similar data from studies of birds and crocodiles.

This revealed that nesting theropod dinosaurs produced unusually large clutches for their body mass, a pattern often seen in birds in which the male alone cares for the eggs. In these species, female birds can plow more resources into bigger clutches, because the female is free to leave the nest and replenish her energy reserves after laying eggs. Varricchio's team concluded that among theropods, the males were also the egg incubators.

But recently a group of British researchers reanalyzed the data and came to a different conclusion.

Led by Charles Deeming at the University of Lincoln, the researchers say the 2008 analysis didn't consider a few key points. For instance, some birds today deliberately lay their eggs in another bird's nest to avoid having to care for them. This distorts the size of some clutches, making them seem unusually large. Theropod dinosaurs may have behaved in the same way.

To try to eliminate this effect, Deeming's team counted the eggs in all known fossil nests and worked out an average clutch size for each theropod species, instead of simply taking the largest clutch size for each species, as Varricchio had done.

When they compared these average figures with the adult body mass, they found that the theropod dinosaurs no longer fall into the group of male-only brooders.

Varricchio says the new analysis looks solid, but “regardless of what this paper or our paper says, we are really operating with only a few pieces of the puzzle,” he says. “To address the (parental) care in these dinosaurs, one needs to consider their other relatives and not just birds.” For instance, crocodiles, which share a common ancestor with all dinosaurs, might be one source of clues to brooding behavior.

Deeming agrees. “If you look at the eggs in those dinosaur nests, they're structure is similar to crocodile eggs,” he says. Crocodiles must bury their eggs to prevent them from drying out, and Deeming thinks the dinosaurs buried their eggs, too.

“Crocodiles don't incubate their eggs; they just sit on the buried eggs to protect them from predators,” Deeming says. “I think that's probably what was going on in the dinosaurs, too.”