Paleontologists conclude that many adult male dinosaurs spent considerable time with their offspring
By David Brown
The Washington Post
Did oviraptor daddies look forward to trips to the park?
Alas, that's a question the fossil record can't answer. But it does appear that many dinosaur fathers spent an awful lot of time around the nest watching the kids.
Using statistical comparisons with birds and an analysis of leg bones found atop nests of unhatched eggs, a team of paleontologists has concluded that at least three types of dinosaur males did the brooding and incubating.
The study, being published Friday in the journal Science, continues the extreme makeover of dinosaurs from cold-blooded, pea-brain tyrants to warm-blooded, empathetic helpmates.
The finding also "sheds light on the origins of parental care systems in birds," said Frankie D. Jackson, a paleontologist at Montana State University, one of the authors.
Males protect or support offspring in more than 90 percent of bird species — a distinctly rare attribute in the animal world. In mammals, males provide parental care in only 5 percent of species, and it's even rarer in reptiles.
If the hypothesis holds up — the evidence, like that for much of dinosaur behavior, is sketchy and indirect — it suggests that life in the Jurassic and Cretaceous periods was more New Age than anyone had imagined.
The researchers, led by David J. Varricchio of Montana State, examined three genuses of meat-eating dinosaurs — Troodon, Oviraptor, and Citipati. All had distinctly bird-like reproductive habits.
They laid their eggs sequentially over time, rather than in large batches as turtles do. The eggs were asymmetrical rather than round, and the shells were multilayered, like those of birds. The three types of dinosaur also produced unusually large clutches of eggs, up to 30 in a nest.
The scientists looked at the relationship between the mass of the adult dinosaurs' bodies and mass of the eggs in a clutch. They compared that body-to-clutch ratio in the dinosaurs to the body-to-clutch ratio in more than 400 modern species of birds, as well as numerous species of alligators and crocodiles, which are birds' closest living relatives.
The dinosaur ratios most closely matched that of a group of primitive birds — ostriches, rheas and emus — that all have a "paternal model" of care of the eggs and young. The ratios were least similar to songbirds, in which both parents tend the offspring.
In between are the body-to-clutch ratios seen in other modern bird and crocodilian species in which mothers alone nurture the young.
The scientists then examined the fossilized leg bones of Troodon and Citipati adults that had been found on nests, (in one case still in the brooding position). They had features characteristic of male, not female, animals.
From those two lines of evidence the researchers deduced that it was the males who guarded the eggs as the females built the clutch over days or possibly weeks, and subsequently incubated them.
What happened after they hatched is uncertain.
There is good evidence in other species of dinosaurs that adults provided food to juveniles at least until they were strong enough to leave the nest. It's unclear whether that's true for Troodon, Oviraptor and Citipati. However, Jackson said, the nesting grounds in central Montana where some of the eggs were found are littered with the bones of a small, plant-eating dinosaur that may have been prey carried back for food for the adults or young.
The evolutionary advantage of stay-at-home fathers is also unknown, although Jackson has a theory.
Females needed to consume large amounts of food — and especially food high in calcium — in order to produce the number of eggs seen in the Oviraptor and Troodon nests. That required lots of long-distance foraging and lots of time away from home.
"So the male tends the eggs, leaving her free to look after her own nutrition and possibly to mate with other males," she said.
Other paleontologists were generally supportive of the idea that the age of the dinosaurs might also have seen the emergence of Mr. Mom.
"Is it convincing? I think it is," said Jack Horner, curator of paleontology at the Museum of the Rockies, in Bozeman. "I think it's a very interesting way that they've gone about trying to figure this out."
Horner discovered the first dinosaur eggs in the Western Hemisphere and provided the original evidence that dinosaurs cared for their young. He has also raised emus in order to draw inferences about how fast dinosaurs might have grown and how they might have moved.
Mary Higby Schweitzer is a paleontologist at North Carolina State University who has studied the rare soft-tissue remains of dinosaurs and found molecular similarities between material from Tyranosaurs and chickens. She called the new work "an intriguing hypothesis" that needs substantially more evidence to support it.
"I think a little more data or a little more caution might be used before in trying to apply this to a whole clade (group) of dinosaurs," she said.