The movie Jurassic Park gave advanced, birdlike dinosaurs a fearsome reputation--swift, intelligent, and deadly. Now it turns out that they had a softer side. Researchers report that males in three species were stay-at-home dads that incubated the eggs in their nests.
There's plenty of evidence to suggest that dinos baby-sat their offspring. A predatory dinosaur called Oviraptor, for example, was discovered in the Gobi Desert in 1993, its fossilized remains protecting a brood of eggs. In addition, dinosaurs' closest living relatives, birds and crocodiles, display nesting behavior: Female crocs guard the eggs, whereas in birds the gender of the stay-at-home-parent depends on the species. But whether male or female dinos had nest duty has remained a mystery.
For the new research, paleontologist David Varricchio of Montana State University in Bozeman compared three species of birdlike dinosaurs--Oviraptor, Citipati, and Troodon--with birds and crocodiles. All three types of dinosaurs were found on nests, and those nests contain large clutches of eggs, as many as 30 each. Varricchio and his colleagues investigated whether they could discern the nesting behavior from the relationship of the clutch size and the animal's body size. Measurements in 433 living birds and crocodiles revealed that, for a given body size, species in which males took care of the nest tended to have the largest clutches. The next-largest clutches were cared for by mothers. Mom-dad partnerships had the smallest clutches. Extrapolated to dinosaurs, the data revealed a pattern of paternal care in the ancient beasts.
Another line of evidence in the paper comes from Gregory Erickson, a biologist at Florida State University in Tallahassee. His studies of dinosaur bone tissue showed that none of the seven specimens associated with nests showed signs of changes associated with egg laying, such as medullary tissue (ScienceNOW, 2 June 2005). That's not sure-fire proof the nest-caretakers were male, he notes, but it's consistent with the hypothesis. The findings are reported in tomorrow's issue of Science.
Richard Prum, an ornithologist at Yale University, says he never expected paternal care in dinosaurs. But with their dinosaur ancestors showing more and more traits once thought to be exclusive to birds, such as feathers, he says the finding makes sense. It also points to a question on the reproductive frontier: Did dinosaurs practice polygamy?