Free from competition from massive reptiles, mammals ate a lot and used their new size to stay warm and fend off predators, according to a global study released Thursday on how mammals grew after the extinction of the dinosaurs.
When dinosaurs roamed the Earth, the average size of mammals was rather small, ranging from the size of a baby rabbit to that of a beagle.
"When dinosaurs went extinct, maximum mammal size was between one and 10 kilograms (two to 22 pounds), in that size range," said researcher Jessica Theodor of the University of Calgary in Canada.
But it did not take long for mammals to start growing after the dinosaurs died out 65 million years ago, leaving loads of vegetation for others to eat, she explained.
"And 25 million years later we have mammals that are a thousand times bigger," she said, noting that they grew to up to a maximum of 17 tons.
Until then, quick-metabolizing mammals had to battle larger reptilian dinosaurs for food, and since there wasn't much to be had, the mammals never had a chance.
"Probably what was going on is that the things that they were eating were being eaten by the dinosaurs already so there was no way they could really break into that way of living."
Once the beasts died out, mammals surged. However, those gains leveled out about 40 million years ago and have not changed much since.
The research was funded by a US National Science Foundation grant, and brought together scientists from around the world to track how big mammals got across the globe and to analyze their patterns of growth as time passed.
Over the course of two years, they compiled a massive database of fossil records. Similar trends in mammal growth were seen on all continents.
"The database is powerful and unique," said John Gittlemen of the University of Georgia, who was part of the team. "It includes information on the size of all mammals, living and fossil, from around the world."
Having the wealth of data, much derived from bones and tooth fossils which gave solid indicators of overall size, allowed researchers to learn about the science of being big.
"There is strong selection pressure for herbivores to get really big," said Theodor. "Being big protects you from predation because very large herbivores do not get preyed on very much."
Also, living in a harshly cold climate is not such an issue of life and death once size begins to rise.
"Large mammals do not have to work so hard to stay warm, so cold climates tend to favor the evolution of large mammals," she added.
And more food is available to bigger mammals because their stomachs were larger and produced compounds that could break down tougher parts of trees and plants.
"There are some kinds of herbivory that are really only available to mammals if they are very large. The biggest herbivores tend to eat parts of plants that are not that nutritious. They use gut bacterial to break it down," said Theodor.
"That is a strategy that only works if you are relatively big. If you are small the food processes through your gut so fast there is not time to break it down."
Interestingly, scientists also found that he or she who was the "big dog" one day, or era, was not necessarily the biggest in the next.
"It was not the case that say, elephants get big and then elephants are always the biggest mammal on the planet," Theodor said.
"There are times when it is giant rhinos, there are times when in South America it is all kinds of weird things including - believe it or not -- camels," she said.
"So the role of who is biggest, who is playing that role, changes over time," she said.
"Which was really interesting to us because it implies that there is something fundamental about being a large mammal. It is a niche that exists apart from who is filling it. It is an open space into which somebody will move."
The study appears in the journal Science.