By Jody Bourton
Earth News reporter
Limited reach, the kangaroo
Why do kangaroos have such small arms?
Scientists now think they know why, helping to explain why the unusually-shaped marsupials have tiny arms yet such long legs.
Kangaroos have small forelimbs because short arms are necessary to survive within their mother's pouch soon after they are born, a new analysis confirms.
This need to crawl at an early age constrained the evolution of marsupial body shapes, leading to the animals we see today, scientists say.
The research also provides intriguing clues as to why there are no marsupials with flippers or wings.
The study is published in the Australian Journal of Zoology.
Compared to placental mammals, marsupials such as kangaroos are born at an early stage of foetal development.
They end up 'stuck' with this forelimb shape in later life
Dr Jim Copper
Syracuse University, New York, US
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 idea is not new, being first proposed in the 1970s.
But the predictions of this so-called "constraint hypothesis" had never been tested.
So Dr Cooper worked with Professor Scott Steppan from Florida State University in Tallahassee, US, to design a study to do just that.
Using skeletons of an extensive range of mammals from various scientific and museum collections, the researchers measured and compared their different body shapes, mapping the diversity of limb proportions between marsupial and placental mammals.
That revealed that the evolution of marsupial forelimbs has indeed been constrained compared to those of placental mammals.
"Our results show tremendous support for the existence of a very powerful constraint on marsupial limb evolution," Dr Cooper says.
"We now know that marsupial forelimb shape has been evolving at a much slower rate in comparison to their sister group, the placental mammals."
"If they can't make the crawl, then they don't survive, so the importance of having 'good climbing forelimbs' trumps the importance of having good running forelimbs."
That also explains why kangaroos have such long hind limbs.
"We also know the rate of hind limb shape evolution has not been slower among the marsupials."
That is because the development of hind legs were not constrained by life in the pouch.
So they are able to grow long to enable kangaroos to hop fast, compensating for the lack of effective front legs that can be used for four-legged running.
The constraint hypothesis may also explain why marsupials have never taken to the air or water, as their forelimbs cannot evolve into structures capable of flying or swimming.
"This concept has profound implications for answering such questions as 'why are there no marsupial bats or whales?'" says Dr Cooper.
"You don't see any marsupials that swim with flippers, even though this useful adaptation has evolved three times in the placental mammals," he says.
"The stronger the support for this hypothesis, then the more certain we are that we can explain the root cause of this major evolutionary pattern."