Did egg-laying lead to the dinosaurs' demise?

Dinosaurs' egg-laying may have left them vulnerable to extinction when disaster struck.

The bigger the egg, the thicker the shell, but growing embryos need access to outside air. Therefore, egg size is necessarily limited.

Because of this fixed egg size, huge adult dinos began their lives as tiny babies. For example, Titanosaur newborns were 2,500 times smaller than their parents who at full-size weighed-in at 4 tons. In comparison, elephant calves are only 25 times smaller than their parents.

A new study attempted to explain this size discrepancy and its repercussions. Through modeling, a team of researchers lead by Daryl Codron, a zoologist at the University of Zurich in Switzerland determined that there weren't many small or mid-sized adult dinosaurs because they would have been forced to compete with growing youngsters for food. In other words, competition forced them to grow big.

The paper suggests that dinosaurs, except for birds, were too large to survive the asteroid that hit Earth about 65 million years ago. On the other hand, birds and mammals survived because they did not have the same size limitations. Their young were not born with as much growing left to do. In addition, mammals did not need to compete with other species for food at birth, relying instead on their mothers' milk for sustenance.

“I love the idea that egg-laying led dinosaurs to engage in such different niche occupation from mammals and that this played a part in their eventual extinction,” John Hutchinson, an evolutionary biologist at the Royal Veterinary College in Hatfield, United Kingdom, told Nature.

But egg-laying wasn't all bad. Dinosaurs' reproductive habits, including egg-laying, may have contributed to their success in the first place. They grew relatively rapidly, began mating before they were full-grown and laid several eggs per clutch.

Scientific discussions on this topic will undoubtedly continue. The new study is based on virtual ecosystems because the real ones existed millions of years ago and are of course impossible to study directly.

Hutchinson told Nature, “we are going to see a lot of debate over how accurate these models really are.”

The study appears online in Biology Letters today.