Dinosaur Dads Played "Mr. Mom"?

Matt Kaplan
for National Geographic News
December 18, 2008
The paternal care common among birds may have its origins among dinosaurs closely related to Velociraptor, reports a new study.

Researchers studying the evolution of reproduction in the swift and carnivorous creatures, which are believed to have evolved into birds, found that one species, Troodon, frequently laid large clutches of eggs.

"By volume, these dinosaurs were laying clutches that were two to three times larger than what would be expected for their adult body size, and we wanted to know why," said study author David Varricchio, a paleontologist at Montana State University, whose research appears in tomorrow's issue of the journal Science.

(Test your dinosaur knowledge.)

Eggstraordinary Care

In birds, the dads often shoulder some or all of the responsibility for their young. Many fathers incubate eggs, feed babies, and guard nests. This behavior is rare in other animals.

Paleontologists have known for some time that many theropods—dinos who walked on two feet and sported stumpy arms—had some form of parental care, because at nest sites, adult skeletons are often found lying on top of eggs.

What gender these parents were is often a mystery, however.

In some cases, the adult skeletons had their legs folded, suggesting that they were sitting, as if warming the eggs with their abdomens.

Researchers have also known that many birds with large clutches showed paternal care. But until now, nobody had put all the data together to find out if dinosaur fathers gave their young the kind of care that modern avian fathers frequently give theirs.

The team compared the sizes of Troodon egg clutches to those of other egg-laying animals' clutches.

Troodon and several other theropods had relatively large clutches, similar to those of a number of modern bird species like emus and rheas.

Males emus and rheas essentially raise their hatchlings alone.

Bone to Be Dad

Compared to the sizes of the adult skeletons, the clutches were very large, suggesting that multiple females contributed eggs to each clutch, which would have been guarded by a single male.

This led the researchers to question which theropod parents were doing what. To test the idea that these dinosaurs had male-only care, the team took a closer look at the brooding adults' fossils.

Varricchio and his colleagues microscopically studied the bones of eight brooding adults to identify gender.

(Related: "He Rex or She Rex? Experts Find Way to Tell Dino Gender" [June 2, 2005].)

Female birds in preparation for egg-laying generate extra bone tissue inside long bones like femurs. The mothers draw on these stored minerals during egg production.

Traces of this extra bone can often be seen when avian leg bones are studied carefully. Past research on Tyrannosaurus and Allosaurus leg bones has proven that such traces can also be seen in dinosaur females.

Yet, of the brooding theropods Varricchio and his colleagues analyzed, none possessed this special bone.

"The absence of this bone does not definitively prove that they are males, but it certainly suggests that these were males caring for the eggs," Varricchio said.

"It's fantastic to see that the wealth of data on egg and clutch size in living birds and crocodiles is being used to so effectively shed light on parental behaviors in dinosaurs," said Darla Zelenitsky, a paleontologist at the University of Calgary who was not involved in the study.

"This represents another significant piece of the puzzle for understanding the evolution of birds from their theropod dinosaur ancestors," she added.

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