Ever wished you had a spare leg to help you get around? Some Australian mammals had that thought and, well, went out and evolved one. And now we know what drove them to do such a thing.
Kangaroos were recently confirmed to use their tail as a fifth leg. While most ground-dwelling mammals simply use their tail for balance, kangaroos can firmly place theirs down on the ground and lift their body up and away, allowing them to swing their back legs forward while they support their weight on their front legs and tail.
When the kangaroos’ fifth leg was discovered, it was thought similar animals – like wallabies and pademelons – wouldn’t use their tail in the same way because they were too small to need a fifth leg. But now Rebekah Dawson from the University of Western Australia in Perth has documented the behaviour in both large and medium sized wallabies, and shown the behaviour is driven not just by size, but rather the animal’s habitat and overall body plan.
Dawson’s work also, for the first time, conclusively establishes that smaller macropods – those Australian animals that look like different sized kangaroos – don’t have a fifth leg. This includes rock wallabies, pademelons and the internet’s favourite animal, the quokka.
Dawson and her colleagues from Murdoch University in Australia looked at 16 species of macropod and examined the way each part of their body moved as they walked on four (or five) legs. They then looked at the relative size of different parts of each animal’s body, their overall sizes, and their habitats.
It turned out that animal size wasn’t the main predictor of whether the tail acted as a fifth leg or not. Instead, the best predictor was the relative size of the back legs compared to the rest of their body. When kangaroos and their cousins move quickly, they hop on two legs. But when they move more slowly, they lean over and use their arms too in a kind of jumpy-crawl.
Now, to hop efficiently at high speeds, especially if you’re a big kangaroo, you need very big back legs, says Dawson. But it seems those big back legs make for awkward crawling. So those macropods that grew big back legs evolved to use their tail to help them crawl on four legs – creating a fifth leg.
But that didn’t explain all the associations – some wallabies with relatively long hind legs didn’t make use of their fifth leg. It turned out habitat explained the rest. All of the species that use five legs live in open forests and grasslands. Those with the standard four legs are found in denser forests and areas of complex topography.
That’s a neat discovery and makes sense, says Terry Dawson from the University of New South Wales (no relation to Rebekah Dawson). “In open country fast hopping is required for predator avoidance,” he says – so macropods living there developed very long back legs to increase their hopping speeds. They then needed to begin using their tail as a fifth leg to help lift their awkwardly long back legs of the ground when moving at slower speeds.
But in densely vegetated or very rocky habitats, predator avoidance is more about manoeuvrability than moving at high speed. Although macropods in these environments still have relatively long back legs, they are short enough to allow the animals to get around on four legs without their tail. The work “nicely resolves a considerable puzzle”, says Terry Dawson.
Australian Journal of Zoology, DOI: 10.1071/ZO15007
https://www.newscientist.com/ New Scientist
https://www.newscientist.com/article/dn28043-mystery-of-australias-five-legged-animals-cracked/ Original web page at New Scientist