The prehistoric Earth was crawling with #1 Dads.
New analyses of fossilized dinosaur eggs and bones suggest that male dinosaurs likely sat on nests and cared for their young, similar to the parental division of labor seen in some modern birds. The results, published by paleontologist David Varricchio and colleagues online December 18 in Science, suggest that the primary paternal care system of some modern-day birds may have first evolved in the birds’ dinosaur ancestors.
“I think it’s more interesting than the authors know. This answers all sorts of questions about bird behavior,” comments Richard Prum, an ornithologist at Yale University.
Large birds called ratites, which are generally flightless and include emus, ostriches and rheas, are closely related to dinosaurs, particularly the type Varricchio’s team studied. Males of these avian groups shoulder the bulk of childcare responsibilities. Ratite fathers incubate eggs, defend the nest from predators and look out for the young hatchlings. What’s more, in an estimated 90 percent of bird species alive today, mom and dad share childrearing duties.
To get an idea about the origins of styles of parental care, researchers turned to some of the oldest known examples. Fossils from adult Troodon, Oviraptor and Citipati dinosaurs, all meat-eating, bipedal theropods, had been found on or near a clutch of eggs. Some adults were even caught in a brooding position (picture a hen sitting on her nest), raising suspicions that the adults died in the act of caring for the brood. But scientists didn’t know whether the adult sitting on the nest was the father or the mother.
Varricchio and his team found no female-specific markings on bones from two adult dinosaurs: a Citipati, fossilized in the brooding position on a clutch of eggs, and a Troodon, fossilized while at a nest site. Many female birds have medullary bone, a characteristically spongy, disordered layer lining the insides of long bones in many female birds, likely serving the female as a nutrient source while forming eggs. Medullary bone has also been found in Tyrannosaurus and Allosaurus.
The absence of this female-specific bone marker in the Troodon and Citipati adults led Varricchio, of Montana State University in Bozeman, to conclude that these two adults sitting on or near the eggs were most likely males. Varricchio points out that figuring out the sex of dinosaurs is a tricky business, and the absence of the medullary bone layer, while a good argument for maleness, is not conclusive proof. “There’s not a good, clear marker,” he says.
Another piece of evidence that dinosaur males cared for the young comes from studying the eggs.
The researchers compared the size and number of fossilized dinosaur eggs, which ranged from 22 to 30 large eggs per clutch, to the size and number of eggs in nests of modern-day birds. The unusually large size of the dinosaur clutches was most similar to the clutches of birds that exhibit paternal care, like emus and ostriches, even after body size of the adult bird was factored in. “Females are freed from other duties and have more energy,” which could have been used to make bigger and more eggs, he explains.
Both the large clutch size and the malelike bones of adult dinosaurs found near and on nests make it very likely that dinosaur dads looked after the kids, the team reports.