National Geographic : 2005 Jul
THE SCIENCE OF THINGS Who Knew? PALEONTOLOGY Dino-size 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 ecological niches. The juvenile T rex devoured the lit tle 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 crea tures only half their size. It's as though evolving toward larger size is rewarding for a while and then the bill comes due. SDecherd says that gigantism may have arisen in part to give dinosaurs big stom achs that could act like fermenting vats. With the dramatic arrival of high-nutrient angio sperms, or flowering plants, the largest dinosaurs began to vanish. Matt Carrano, dinosaur cura tor 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 big gest mammals have the longest gestation periods. This is a slow process that results in relatively few offspring, and the big mam mals can't easily adapt to an envi ronmental crisis. Carrano says, "There are benefits to being big, but at some point the benefits are outweighed by the risks."