Dr Natalie Warburton from Murdoch University’s School of Veterinary and Life Sciences said she and collaborators Dr Trish Fleming and Dr Bill Bateman (Curtin University) 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,” Dr 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.”
Dr Warburton 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 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,” Dr Warburton said.
“This is consistent with sexually-selected traits in other species.”
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.