When dinosaurs died out, some mammals became huge, according to a new Science paper.
The research helps to explain mammal giants such as Indricotherium transouralicum, an 18-foot-tall, 17-ton hornless rhinoceros-like herbivore that lived in Eurasia 34 million years ago.
This towering plant eater, along with Deinotherium -- a member of the order elephants belong to -- were the largest mammals to ever walk the earth. They were a far cry from the often mouse-sized mammals that scurried around dinosaurs.
"Mammals evolved around 210 million years ago and stayed small for their first 140 million years," lead author Felisa Smith told Discovery News. "This is probably because of competition from reptiles and dinosaurs, who dominated the ecosystem."
"Although mammals were also severely influenced by the K/T extinction (65.5 million years ago), they had a number of adaptations that helped after the devastation that followed the impact," explained Smith, an associate professor of biology at the University of New Mexico. "Many of the survivors were small, burrowers, and ate just about anything."
Such scrappy ways permitted mammals to fill ecological niche voids that opened up when dinosaurs went extinct.
Smith and her team made that determination after compiling fossil data indicating the body size of land mammals belonging to each taxonomic order, on each continent, throughout their evolutionary history.
The scientists found that the maximum size of mammals began to increase sharply about 65 million years ago. It peaked in the Oligocene Epoch around 34 million years ago in Eurasia, and again in the Miocene Epoch about 10 million years ago in Eurasia and Africa.
Some mammals stayed small or evolved to occupy niches previously occupied by tiny to medium-sized animals. For the mammals that evolved to become larger, their pattern of growth was consistent across time and across various groups, even if those groups had different diets and were descended from different ancestors.
The researchers now believe that global temperature and the amount of land available for an animal's range are two primary ecological factors that appear to correlate with the evolution of maximum body size.
Despite the impressive size of mammals like Indricotherium, dinosaurs such as Argentinosaurus were up to 130 feet long, weighing 110 tons. Even the largest mammals today could never become that big, Smith believes, because they are endotherms that regulate their body temperature. Around 90 percent of the energy we take in goes to this activity, constraining our growth.
"It's interesting that the biggest dinosaurs were just about ten times as big as the biggest mammals, just in line with differences in energy requirements," she said.
The lack of humongous mammals now is also an artifact of the Pleistocene extinction event, which Smith said, "was probably caused by human hunting and other activities."
"If you go back to 13,400 years ago, we did have mammals (mammoths) of 10-12 tons around," she added.
Mammoths then must have been like giant walking steaks to our hungry human ancestors. Today, however, times are especially tough for large predators that have to work so hard to find their next meal, according to another new study in this week's Royal Society journal Biology Letters.
"We found that the largest species exhibited a five to six fold greater decrease in relative abundance in response to a decrease in their prey," Phillip Stephens, who worked on this second study and is a Durham University researcher, said.
"It's hard work being a large predator roaming and hunting across extensive areas to find food," he added. "The apparent vulnerability of tigers and polar bears to reductions in the availability of prey may be linked to the energetic costs of being a large carnivore."
Given these pressures, and additional ones affecting other animals, it's doubtful that most mammal species will experience much of an overall growth spurt anytime soon.
© 2012 Discovery Channel