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The big picture

Ecology dean, postdoctoral researcher contribute to paper about evolution of world's largest mammals

The largest mammal that ever walked the Earth –Indricotherium transouralicum, a hornless rhinoceros-like herbivore that weighed approximately 17 tons and stood about 18 feet high at the shoulder-lived in Eurasia almost 34 million years ago.

In a paper published in the journal Science, an international team of researchers, including John Gittleman, dean of UGA’s Odum School of Ecology, and postdoctoral researcher Patrick Stephens, has compiled and analyzed an enormous database of information about the largest mammals across time and around the globe, revealing striking patterns in their evolution.

The research, funded by a National Science Foundation Research Coordination Network grant, was led by scientists at the University of New Mexico, who brought together paleontologists, evolutionary biologists and macroecologists from universities around the world. 

The goal of the research was to revisit key questions about size, specifically in mammals.

“Size impacts everything, from reproduction to extinction,” said Gittleman, whose research focuses on large-scale ecological and evolutionary problems, from disease to extinction to organism characteristics. “And mammals are a good test case. There is so much variation-everything from mice to elephants-and there also is far more data available about mammals than other taxonomic groups.”

“There is a much better fossil record for mammals than for many other groups,” said Stephens. “That’s partly because mammals’ teeth preserve really well. And as it happens, tooth size correlates well with overall body size.”

The researchers spent two years assembling data on the size of all mammals, living and fossil, from around the world. The group was able to test a hypothesis about the evolution of mammal size.

“During the Mesozoic, mammals were small,” said Gittleman. “Once dinosaurs went extinct, mammals evolved to be much larger as they diversified to fill ecological niches that became available. This phenomenon has been well documented for North America. We wanted to know if the same thing happened all over the world.”

The researchers found the pattern was indeed consistent, not only globally but across time and across trophic groups and lineages-that is, animals with differing diets and descended from different ancestors-as well. The maximum size of mammals began to increase sharply approximately 65 million years ago, peaking about 34 million years ago in Eurasia and again about 10 million years ago in Eurasia and Africa.

Global temperature and the amount of land available as an animal’s range are two ecological factors that appear to correlate with the evolution of maximum body size, but Gittleman warned against assigning cause and effect.

“A big part of science is seeing patterns, and then producing new hypotheses and testing them,” he said. “We now have identified this pattern very rigorously.”