The extinction of the dinosaurs, 65 million years ago, allowed mammals to rapidly evolve a range of body sizes, according to a November 2010 study. But even so, no land-dwelling mammals were able to get as large as the largest dinosaurs. Biologist Felisa Smith, an author of the study, explained.

Felisa Smith: Mammals are what are called endotherms. They regulate their own body temperature. A mammal of a given size uses ten times more energy than does a reptile or a dinosaur of the same size.

In other words, mammals can’t evolve bodies as large as the largest dinosaurs because they need to use so much of their physical energy – provided by the food they eat – towards keeping their bodies warm. For example, we humans need to maintain a temperature of 98.6 degrees Fahrenheit, or 37 degrees Celsius, in order to stay alive. But dinosaurs, like today’s reptiles, did not regulate their body temperature, and the extra energy allowed them to grow larger, Smith said.

Felisa Smith: I think it’s really intriguing that the largest dinosaurs are just about ten times larger than the largest mammal.

Smith said that size difference is in line with the theory that energy capped the maximum body size of mammals, and energy constraints might have also limited the size of the dinosaurs.

Felisa Smith: It’s important to consider how much the evolution of any given organism is influenced by the environment, and by the presence of other organisms on the planet. Mammals evolved 210 million years ago. But for first 140 million years of our existence, we didn’t do much. We stayed small, didn’t diversify in any huge capacity. As soon as dinosaurs were removed from the scene, we diversified rapidly to a huge size of body size niches, and also ecological niches.

Smith said that before dinosaurs went extinct, mammals were no larger than football-sized, weighing about 10 – 100 grams. Many mammals went extinct along with the dinosaurs, but the species that survived expanded dramatically.

Felisa Smith: When the Earth reset, if you will, here was enormous ecosystem that had no dominant animals in it. Mammals diversified incredibly quickly, and occupied all ranges of body sizes we see today, plus some that no longer exist.

But 42 million years after the dinosaurs went extinct, mammals hit a plateau in their body sizes. Smith said this was a phenomenon that occurred across all continents, shown in the fossil record. In the study, Smith and her colleagues found two things that explain the constraints on mammal size.

Felisa Smith: Our study suggests that the constraints acting on maximum body size of mammals were twofold. Temperatures constrains mammal evolution. Colder temps permit larger body sizes. That may mean that it has to do with heat loading. Maybe, when you get too big, you can’t get rid of excess heat. The second one is land area, which is probably a proxy for energy that’s available to support populations of really large animals.