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From mouse to mammoth, foot posture may explain why some mammals got so massive

Our very earliest, rodentlike ancestors likely trod the earth some 200 million years ago on tiny, flat feet. As the posture of those feet evolved, so, too, did the size of mammals, according to a new study, giving rise to everything from the tip-toed African lion to the relatively flat-footed human.

“It’s an excellent paper,” says Christofer Clemente, an expert on the evolution of locomotion at the University of the Sunshine Coast in Australia, who wasn’t involved in the work. “It’s a really clever idea to link these things.”

To conduct the study, Manabu Sakamoto, an evolutionary biologist at the University of Reading in the United Kingdom; Tai Kubo, a paleontologist at the University of Tokyo; and colleagues assembled a data set of 880 living mammal species and their foot posture. The team divided the postures into three major categories: plantigrade (flat-footed), digitigrade (tiptoed), or unguligrade (hooved).

Most animals were relatively easy to place: Humans and mice are flat-footed, dogs and cats are tiptoed, horses and bison are hooved. Others presented challenges. Elephants and rhinoceroses, for example, have hooves, but don’t distribute their weight like horses do. Elephants bear their great weight on their heels, earning them a place in the flat-footed category, whereas rhinos actually walk on the balls of their feet, putting them in the tiptoed category.

Next, the researchers plotted these species on a family tree that traces the branching evolutionary history of all known mammalian life. They filled in the likely foot postures of extinct animals based on their relationships with the living species from their data set. Their model suggests the earliest ancestral mammal was probably flat-footed, they report today in the Proceedings of the National Academy of Sciences.

But size was the biggest surprise: On average, tiptoed mammals tend to be about twice as massive as flat-footed mammals, and hooved animals are typically even bigger—a whopping 57 times more massive than tiptoed ones.

There are of course outliers: Flat-footed elephants are the most massive terrestrial mammals on Earth, for example, and tiptoed dogs and cats are smaller than flat-footed humans. But the overall trend held. When the scientists mapped these changes to branch points on their phylogenetic tree—say, when the ancestor that gave rise to warthogs, elk, and giraffes evolved to walk on hooves instead of tiptoes—they found there was virtually always a rapid increase in body size.

“Throughout evolution, increases in body size tend to come in bursts at these transition points,” Sakamoto says. “It’s not a gradual thing.”

That suggests one of two possibilities: Foot posture allowed mammals to move into and flourish in new ecological niches in which being big was an advantage. Or foot posture evolved as a result of animals becoming more massive in the first place. Which came first—the hoof or the heft—remains an open question, Sakamoto says. He believes adding fossil mammals to their model in future studies could help clear up the direction of the evolutionary arrow.

“These conclusions all fit together for me, and make a lot of sense overall,” says John Hutchinson, who studies biomechanics at the Royal Veterinary College in London.

The answer could shed light on other evolutionary mysteries, Clemente adds. “If we can better understand the link between body size and posture we can go back in time … and answer questions like, ‘How did the dinosaurs get so big?’ which would be really exciting.”