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The Science of Things
JULY 2005
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Photo illustration by Cary Wolinsky and Jen Christiansen
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PALEONTOLOGY
Dino-size
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 Why were dinosaurs so humongous?
One of the most cherished features of dinosaurs—their gigantism—is also one of the most mysterious. Why in the world were these animals so huge?
Any explanation has to start with the arms-race theory: Some species of dinosaurs may have evolved to a larger stature as a way of escaping predators or gaining a competitive advantage. When you're a predator, you usually don't like to mess with a creature that can stomp you like a bug. Studies of growth rings in dinosaur bones by Greg Erickson of Florida State University and others show that baby dinosaurs grew to maturity at dazzling speed. They practically exploded from their eggs. A bouncy little apatosaur could become a 30-ton beast in just 20 years.
Tyrannosaurs, the dominant meat-eaters of the late Cretaceous, grew steadily until they were teenagers and then went through a growth spurt, becoming five times larger in just seven years. That meant that one species came in two lethal sizes—medium and large—and could effectively dominate two ecological niches. The juvenile T. rex devoured the little animal nuggets and Big Daddy T. rex handled the supersize meals. But there's a problem with the simple bigger-is-better explanation: Most dinosaurs weren't giants. Small bones don't preserve as well, so the fossils (and museum displays) tend to overrepresent the behemoths. There were plenty of dog-size and even chicken-size dinosaurs, and one, known as Microraptor, no bigger than a pigeon. That guy was a real terror.
Gigantism doesn't go on forever: Many of the biggest dinosaurs found their numbers thinning at some point, their niches largely replaced by creatures only half their size. It's as though evolving toward larger size is rewarding for a while—and then the bill comes due.
Sara Decherd, a doctoral candidate at North Carolina State, has studied the plants available during the age of dinosaurs and suggests that the rise of flowers may be connected to a decline of the largest dinosaurs. You'd think that nutritious plants would lead to bigger dinosaurs, but it's the other way around. During the Jurassic, vegetation tended to consist of fibrous, low-nutrient plants such as gymnosperms. Decherd says that gigantism may have arisen in part to give dinosaurs big stomachs that could act like fermenting vats. With the dramatic arrival of high-nutrient angiosperms, or flowering plants, the largest dinosaurs began to vanish.
Matt Carrano, dinosaur curator at the Smithsonian's National Museum of Natural History, says that many lines of dinosaurs may have evolved toward gigantism simply because they could. "Given enough time," says Carrano, "you'll explore all the different sizes you can potentially be."
Perhaps the real question should be, Why aren't mammals bigger? Reproductive strategies surely hold the answer. Mammals gestate their young, and the biggest mammals have the longest gestation periods. This is a slow process that results in relatively few offspring, and the big mammals can't easily adapt to an environmental crisis. Carrano says, "There are benefits to being big, but at some point the benefits are outweighed by the risks."
In concluding that bigger may not be better after all, keep in mind that only one line of rather small dinosaurs survived the end-of-Cretaceous mass extinction. They're called birds.
—Joel Achenbach
Washington Post staff writer
Related Links
More Articles by Joel Achenbach
www.washingtonpost.com/wp-dyn/style/columns/achenbach
Read some of writer Joel Achenbach's columns for the Washington Post.
Greg Erickson's Dinosaur Growth Page
bio.fsu.edu/~gerick/dinogrowth.htm
Visit this website to see excerpts and images from "Dinosaurian Growth Patterns and Rapid Avian Growth Rates," Nature (July 2001), 429-33.
Science Museum of Minnesota
www.smm.org/research/Paleontology/histology
Learn how scientists get important clues about dinosaur growth by studying their bones from the inside out.
"Dino" Don's Dinosaur World
www.dinosaurdon.com/dinodon
They're big! They're scary! So why do kids like dinosaurs so much? Visit "Dino" Don's website to get his explanation—and to have fun learning about dinosaurs.
Jurassic Park Institute
www.jpinstitute.com/index.jsp
Get up-to-date information about dinosaur research, check out the "Dino of the Day," and create your own dinosaur greeting cards at this multimedia resource.
Bibliography
Achenbach, Joel. "Dinosaurs: Cracking the Mystery of How They Lived." National Geographic (March 2003), 2-33.
Barrett, Paul. National Geographic Dinosaurs. National Geographic Society, 2001.
Erickson, G. M., and others. "Gigantism and Comparative Life-history Parameters of Tyrannosaurid Dinosaurs," Nature (August 12, 2004), 772-5. Abstract available online at dx.doi.org/10.1038/nature02699.
Rogers, Kristina Curry. "Growth Rates Among the Dinosaurs." In The Scientific American Book of Dinosaurs, ed. Gregory S. Paul. St. Martin's Press, 2000.
Scotchmoor, Judith G., and others. Dinosaurs: The Science Behind the Stories. American Geological Institute, 2002.
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