Aug. 6, 2008 -- Unlike many dinosaurs that had natural armor, body spikes, huge teeth and other means of defense, the duck-billed hadrosaur was a seemingly vulnerable, soft-bodied plant eater. But it did evolve one very effective way of beating predators: It outgrew them.
The hadrosaur Hypacrosaurus, which lived 67 to 80 million years ago, grew three to five times faster than any of its predators, including Tyrannosaurus rex, according to a study published in this week's Proceedings of the Royal Society of London B: Biological Sciences.
Adults of the duck-billed giant measured around 9 feet at the tips, grew to over 30 feet in length and could weigh as much as 4.4 tons.
Lead author Lisa Cooper admitted that such big game is often desirable, "but hunting requires a lot of energy and is dangerous."
"By reaching its full body size quickly, Hypacrosaurus probably obtained a size refuge, or a body size that made it more difficult to be hunted, compared to smaller dinosaurs," said Cooper, who is a doctoral student at Kent State University and a researcher with the Northeastern Ohio Universities College of Medicine.
Cooper and colleagues Andrew Lee, Mark Taper and John Horner obtained growth rate data from a Museum of the Rockies' Hypacrosaurus, which means "near the highest lizard," in reference to the dinosaur's impressive height. To collect the data, Cooper explained that she and her team looked at cross sections of the dino's limb bones by "grinding them down until we could see light through them."
The scientists found that, during the fossilization process, the shape of bone cells and, "more importantly, growth rings" are preserved. Similar to calculating the age of a tree, the researchers then counted the growth rings to determine the dinosaur's growth rate.
They next compared that rate to those of three duck-billed predators: T. rex, Albertosaurus and Troodon, a small, Velociraptor-like dinosaur that possessed sharp, serrated teeth.
In addition to growing up to five times faster than the tyrannosaurs, the duck-billed dinosaur grew at least twice as fast as the relatively tiny, 6.5-foot-long Troodon. Since growth in the hadrosaur slowed after about 2.5 years, the scientists believe it reached sexual maturity at only two to three years of age.
"As long as the dinosaur was able to successfully reproduce, in an evolutionary sense, it was able to withstand predation," Cooper explained. "The whole point is to reproduce as quickly as possible -- that's the stuff of evolution."
Prior research determined that modern fish, insects and animals employ the fast maturation strategy as well to out-produce and outgrow predators. These quick growers include killifish, wildebeest, butterflies, toads, salamanders, guppies and some birds, such as penguins.
Kevin Padian, a professor of integrative biology at the University of California at Berkeley and curator of the university's Museum of Paleontology, told Discovery News that the new findings make sense.
"This growth game has been played by many prey animals for millions of years," he said, mentioning that other dinosaurs, such as ceratopsians, sauropods, and even large, carnivorous dinosaurs, like Allosaurus, appeared to evolve faster maturation and speedier growth rates to both avoid, and survive, predators.
Explaining why certain toothy meat eaters would need to evolve faster growth, Padian said, "There are so many carnivores that, no matter who you are, there's somebody out there that is ready to kill you."
In the future, Lee hopes to gather and compile specific growth rate data for other dinosaurs, in hope of learning more about dinosaur evolution and the fast growth survival strategy.
Related Links:
Discovery News blog: Born Animal
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