Male kangaroos woo mates with bulging biceps

It's not just the men who flex their biceps to impress the opposite sex - kangaroos do it too!
Male kangaroos frequently adopt poses to show off their muscular arms to females, a new study has found.
Researchers said a male kangaroo's forearm size could be a sexually selected trait and help them find a mate.
The research by Murdoch University and Curtin University in Australia centred on data gained from dissecting 13 grey kangaroo males and 15 females.
Researchers found the arm and shoulder muscles of male kangaroos play a key role in attracting members of the opposite sex.

Dr Natalie Warburton and collaborators Dr Trish Fleming and Dr Bill Bateman 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.
"We found differences in muscle growth for those muscles more likely to be used in these wrestling matches - those involved in clutching and pulling toward the middle of the body," Warburton said.

She said that males at the top of the hierarchy had higher mating success, lending support to this theory.
"To take the point even further, dominant males frequently adopt poses which best display their muscularity and size," she said.
Researchers said that all of this indicates it may be time to revise our perceptions about traits that attract the opposite sex.
While bright feathers, horns or antlers may work for some species, in kangaroos, strong arms and shoulders could be the key to securing a mate.
Researchers also considered whether or not there were survival advantages to having larger forelimb musculature, which 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," she said.
The study was published in Biological Journal of the Linnean Society.