Male kangaroos and wallabies, like a lot of seemingly quiet grazing animals, get into knock-down drag-out fights over females. They obviously don't have antlers or horns to spar with, but they're perfectly willing to grapple rivals with their forelimbs and kick the crap out of each other with their big hind feet.
Those fights have influenced the evolution of kangaroo limbs. According to research by Hazel Richards and her colleagues at the University of Western Australia, released today in the Journal of Zoology, males from the largest promiscuous kangaroo species have forearms that are much larger and more muscular than female arms, even when the difference in their sizes is taken into account.
Male forelimbs are weapons. Where males from smaller species of kangaroo and wallaby use an acrobatic wrestling style, with many shifts between grabbing and kicking, the males in large species spend a lot more time in a clinch. They use their arms to grab heads, shoulders, and necks -- manoeuvring a rival into a good position to take a heavy two-legged kick or simply throw him to the ground.
Winning these fights -- and access to receptive females -- depends on maintaining a good grip.
And here are some videos of large kangaroos fighting.
Picture: Lucy Takakura via Flickr