Why Do Kangaroos Have Short Arms?

Kangaroo (Macropus giganteus), photo by Danielle Langlois, Wikimedia Commons

Put a kangaroo next to a person—two of the only mammals that stand on their hind legs—and a big difference will jump out at you. Ignore the fur and the snout and focus on the limbs. Kangaroos have really small forelimbs—they’re about 1/3 the length of their legs; our arms are more than 2/3 the length of our legs. 

It seems odd that an animal with such powerful legs has such dinky arms, but scientists believe they’ve figured out why that’s the case. According to a new study, Short arms are needed for the young to survive in the mother’s pouch, according to a study in the Australian Journal of Zoology that examined limb morphology in marsupial and placental mammals.

From the BBC:

Compared to placental mammals, marsupials such as kangaroos are born at an early stage of foetal development. Once born they immediately climb or crawl in the pouch to their mother's teat using unusually well developed forelimbs.

"It occurred to me that this type of birth strategy could have constrained the evolutionary diversification of their forelimb shapes," says Dr Jim Cooper of Syracuse University, New York in the US. "The idea is that since they need the forelimbs to climb across their mother's belly at birth, they end up 'stuck' with this forelimb shape in later life," he says.

The young that can’t make the traverse don’t survive, so having forelimbs that are good for climbing is more important than having ones that would be better suited to running.
And, of course, thy work just fine for boxing. (Just ask David Striegl, the unfortunate Australian football player who, while out for a jog in March, was knocked unconscious by a wild kangaroo and had the scrapes and bruises to prove it.)
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