Any parent of growing teenagers knows their kids' appetite (食欲) can be extremely large. Now, imagine having a young dinosaur checking the fridge. The out size appetites of growing dinosaurs reshaped food chains in their environment and crowded out other carnivores (食肉动物), according to a new study.

Most groups of animals have many small-sized species, somewhat fewer medium-sized species and the fewest large-sized species. However, the extinct dinosaurs—especially carnivores had plenty of species no bigger than modern-day chickens and also many extremely big species, but few medium-sized ones.

Scientists wondered whether teen dinosaurs crowded out medium-sized adults by taking advantage of the habitats and food sources those species might have taken. To test the idea, Katlin Schroeder, a Ph. D. student at the University of New Mexico (UNM), combed a global collection of data to determine the size of more than 550 dinosaur species in 43 ancient ecosystems.

"In most communities, plant-eating dinosaurs were in multifarious sizes. But carnivores were completely different," Schroeder says. Plant-eating dinosaurs came in a range of sizes, while carnivorous dinosaurs between 100 and 1, 000 kilograms were quite rare. "The size of the carnivorous dinosaur is surprising. It's as if you went to the savanna, a large flat area of grassy land, and saw nothing in size between a small fox and a lion," Schroeder says. "Patterns in all the dinosaur communities studied are very similar. We saw the gap in species' sizes for years, but never measured it."

Schroeder and paleontologists Eelisa Smith of UNM and Kathleen Lyons of the University of Nebraska, Lincoln, modeled the role that young carnivorous dinosaurs might have played in ecosystems. "If the teen dinosaurs are really using up this space, how many of them would you expect?" Smith says. Their study showed that "the teenagers fill the gap," she says. "If you fill them in, then you get a community that looks like what you'd expect."

"The effect maybe stronger in meat eaters because each carnivorous dinosaur species hatched (孵化) from small eggs; then they grew very quickly. They had to change diets and hunting methods to adapt to their new sizes and compete with a range of other species along the way," Erickson, another scientist, says.

"The study's stress on how animals' niches (生态位) can change as they grow offers fresh understandings," says Mike Benton, a scientist at the University of Bristol.