Male and female dinosaurs may have shared the responsibility of incubating their offspring, but how to determine which parent was involved remains a mystery, according to a new study that re-examines the idea that the brooding behavior of modern birds may predict similar behavior in their dinosaur ancestors.

Modern birds are thought to have evolved from theropods, a group of carnivorous dinosaurs that include such recognizable predators as the Velociraptor and Tyrannosaurus rex.

In research published in the journal Science in 2009, scientists examined the way existing birds incubate their eggs, claiming that only male theropods took part in incubation. But the study, which compared the size of male and female birds with the size and number of eggs that were laid, omits some important factors, said Geoff Birchard, a professor in the department of environmental science and policy at George Mason University in Fairfax, Va., and co-author of the new study. [Image Gallery: Dinosaur Daycare]

"They looked at the number of eggs and how big they were, and said they could figure out whether mommy incubated, daddy incubated, or both did," Birchard told LiveScience. "The problem is, the biology behind it is a little bit off."

Birchard and his colleagues repeated the 2009 study using more data from living bird species. They determined that comparing the size of the birds with the clutch size — which is determined by multiplying the number of eggs laid in a nest by the volume or mass of the eggs — could not effectively determine whether it was the male or female guarding the eggs.

"Our analysis of the relationship between female body mass and clutch mass was interesting in its own right, but also showed that it was not possible to conclude anything about incubation in extinct distant relatives of the birds," study co-author Charles Deeming, a researcher at the University of Lincoln in the United Kingdom, said in a statement.

Part of the problem is that birds do not all exhibit the same brooding behavior.

"There's a huge amount of variation with birds," Birchard said. "With certain bird types, two parents are always involved, but with some bigger birds, only the daddy is incubating the eggs. With dinosaurs, overall, there's a huge amount of variety, too."

And whether the actions of modern birds can be used to predict the behavior of dinosaurs is also a source of debate.

"There are great differences of opinion about it," Birchard said. "There's a long time gap between dinosaurs and the origin of birds, so it's an awful long time for us to say what's being done with birds was also being done with dinosaurs. We use this kind of inference sometimes, but birds are also a very unique group."

The findings of the new study were published online Tuesday (May 14) in the journal Biology Letters.

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Denise Chow is the Sci-Tech Editor at Live Science. Before joining the Live Science team in 2013, she spent two years as a staff writer for Space.com, where she wrote about rocket launches and covered NASA's final three space shuttle missions. A Canadian transplant, Denise has a bachelor's degree from the University of Toronto, and a master's degree in journalism from New York University.