When you look at a kangaroo or a wallaby, it's obvious the animal is well built for bouncing around the outback. What may be less obvious is that its arms are built for fighting—if it's male, that is. Males of these species have disproportionately long arm bones. And the more brawling a species does, the more exaggerated the difference between the beefy-armed males and their normal-limbed mates. To understand this evolutionary quirk, we'll need to review the rules of fighting in wallabies and kangaroos. (Together, these animals are called macropodids.) Males wrestle each other to show their dominance and to gain access to females. The first rule of macropodid fight club is not "don't talk about macropodid fight club," but "kick a lot." In smaller species, both males and females kick "opportunistically and savagely," write University of Western Australia physiologist Hazel Richards and her coauthors. Some males even have shields built into their bellies to protect themselves from being disemboweled by a kick. In larger species, though, grappling with the arms is more important than kicking. Males grab at one another's head, neck, and shoulders. They hit and shove. They may throw an opponent to the ground. The authors call these battles "deliberate, ritualized and complex." Macropodid males are big to begin with. Males can be up to 50 percent heavier than females of the same species. But the size difference isn't the same all over their bodies; males of some species have been observed to have extra-beefy arms. To learn more about this difference, the UWA researchers went into the collections of six Australian museums and took detailed measurements of kangaroo and wallaby skeletons. They chose 15 "promiscuous" species, since they knew these animals especially need to win mates. After examining the limbs of 470 skeletons, the scientists had some answers. Larger species, which do the most intense fighting, have the biggest difference between male and female arms. And the most exaggerated bone is the humerus, or upper arm bone. Kangaroos and wallabies are hardly the first male animals to evolve big, elaborate appendages for decoration or battle. There are antlers, rhinoceros beetle horns, peacock tails. But the macropodids are interesting because evolution has acted on a body part they need for everyday life. They use their arms to groom and feed themselves, and for walking slowly when they aren't hopping. Their enormous arm bones don't seem to impair the animals, though. The females don't mind them either. And if a male kangaroo with normal arms ever showed up and proved to be more attractive to the ladies, one of the other males could just deck him.
Image: by ilari lehtinen (via Flickr)
Richards, H., Grueter, C., & Milne, N. (2015). Strong arm tactics: sexual dimorphism in macropodid limb proportions Journal of Zoology DOI: 10.1111/jzo.12264