Australasian Science: Australia's authority on science since 1938

Arms Maketh the Kangaroo

By Stephen Luntz

The secret to mating success is big forearms, at least if you are a male kangaroo, Western Australian researchers have announced in the Biological Journal of the Linnean Society.

Male kangaroos use their arms to drag opponents towards them so they can get close enough to kick them while balancing on their tails. Success in this endeavour leads to a dominant place in the hierarchy and access to females.

Dr Natalie Warburton of Murdoch University’s School of Veterinary and Life Sciences set out to see whether big arms had become a sexually selected trait. She compared the weight and size of arm muscles in 13 male grey kangaroos and 15 females.

“Forelimb measurements showed that whereas female musculature growth was proportional to body size, male musculature was overwhelmingly exaggerated,” Warburton says.

Males continue to grow after they reach sexual maturity, and larger males have an advantage in establishing dominance, but it seems large arm muscles matter as well. Warburton says the other uses the arms are put to, such as slow hopping, would be expected to see the muscles scale with weight.

“They also lick their arms when it is very hot to cool off,” she says. “But this might encourage longer arms rather than increased muscles in the upper arm as we found.

“Under conditions of extreme environmental stress there is evidence that male mortality is greater, suggesting that maintaining this additional musculature incurs a significant cost,” says Warburton, who explains that the research was done to study the ways sexual selection affects development.

There are anecdotal reports of male kangaroos posing in ways that show off their upper arms to females. Warburton says: “We didn’t test if females found big arms attractive”.

However, she says the simple model of the dominant male getting access to whichever females he chooses is wrong, and the females “are making a choice at some level on who gets to mate with them” so the arms may also have a display function.