Why are there no living animals as large as the dinosaurs?
--Steve Tidwell, Orlando, Florida
Matthew Carrano, a vertebrate paleontologist at the Smithsonian Institution in Washington, D.C., responds:
There are no obvious structural reasons why terrestrial mammals could not be as large as the giant dinosaurs, so other factors must be at work. Some scientists have suggested that plants were more productive in the Mesozoic, providing enough food to sustain huge herbivores.
More likely, mammals are limited to smaller sizes because they gestate their young internally. Large mammals tend to have long gestation times—nearly two years for elephants—in part because most feed on plants that have relatively low nutritional value.
A lengthy gestation period has two important side effects: The mothers almost always have single births, and the loss of an individual baby is tremendously costly. It is especially difficult for the largest mammals to recover from population disturbances such ...
