Felisa Smith was a graduate student at UC Irvine in the 1980s when she became obsessed with mammal size. "I worked on a num­ber of islands off the coast of Baja, Cal­i­for­nia where rodents had evolved into gigan­tic body sizes. I've been inter­ested in size ever since."

Her latest study is three years in the making and a collaboration with a team of pale­on­tol­o­gists, evo­lu­tion­ary biol­o­gists and macro­e­col­o­gists from uni­ver­si­ties around the world.  The team studied the growth of mammal size after the dinosaurs went extinct around 65 million years ago and published their results last week in the journal Science.

Smith and her colleagues found that mam­mals grew from a max­i­mum of about 10 kilo­grams (22 pounds) when they shared the Earth with dinosaurs to a max­i­mum of 17 tons after­wards. They found that this pat­tern was sur­pris­ingly con­sis­tent glob­ally and across time and trophic groups and lin­eages—that is, ani­mals with dif­fer­ing diets and/or descended from dif­fer­ent ances­tors.

From the Observations blog in Scientific American:

But across all of the major continents, during the first 25 million years after the dinosaurs were wiped out, mammals underwent an explosive growth spurt. By 42 million years ago, however, the researchers found, the intense growth had leveled off.

What was the peak of land mammal growth? Indri­cotherium tran­souralicum was the largest mam­mal that ever walked the Earth. It was a horn­less rhinoceros-like her­bi­vore that weighed approx­i­mately 17 tons, stood about 18 feet high at the shoul­der and lived almost 34 mil­lion years ago.

The overall results give clues as to what sets the lim­its on max­i­mum body size on land -- the amount of space avail­able to each ani­mal and the cli­mate they live in. The colder the cli­mate, the big­ger the mam­mals seem to get, as big­ger ani­mals con­serve heat bet­ter.  The results also show that no one group of mam­mals dom­i­nates the largest size class.

Size does matter, says Smith, "Under­stand­ing the con­straints oper­at­ing on size is cru­cial to under­stand­ing how ecosys­tems work."

Image: Alison Boyer/Yale University

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