The mating allure of kangaroo arms

They may not be pumped from the gym, but researchers say the arm and shoulder muscles of male kangaroos play a key role in attracting members of the opposite sex.

Dr Natalie Warburton from Murdoch University’s school of veterinary and life sciences said she and collaborators used a number of criteria to confirm their conclusion that forelimb musculature was a sexually selected trait.

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

“This could be linked to the fact that male kangaroos establish and maintain their dominance hierarchy through sparring contests that involve grasping their opponent and using their back legs to box them.”

The researchers found differences in muscle growth for those muscles more likely to be used in wrestling matches – those involved in clutching and pulling another male towards the middle of the body.

They found that males at the top of the hierarchy had higher mating success, lending support to the theory, and dominant males frequently adopted poses that best displayed their muscularity and size.

The researchers considered whether there were survival advantages to having larger forelimb musculature but found this was not the case for kangaroos. In fact, the extra bulk could be a disadvantage.

“Under conditions of extreme environmental stress, there is evidence that male mortality is greater, suggesting that maintaining this additional musculature incurs a significant cost,” Warburton said. “This is consistent with sexually selected traits in other species.”

The researchers said the findings indicate that it might be time to revise biologists’ perceptions about traits that attract the opposite sex. While bright feathers, horns or antlers may work for some species, strong arms and shoulders could be the key to securing a mate.

A report on the research was published in the Linnean Society’s Biological Journal.