The males stand out from the crowd, arms flexed in the sunlight, muscles bulging, surveying their surroundings as they look for a likely partner.

It's a scene repeated at the gym and in bushland the nation over.

Now, a Perth university research team may have unlocked the key to the mating success of the western grey kangaroo.

The team, led by Dr Natalie Warburton from Murdoch University's School of Veterinary and Life Sciences, has been studying several roo mobs in Perth and Dunsborough.

They identified several possible criteria for sexual attractiveness and found the size of a male kangaroo's forearms was a key factor in their ability to find and keep mates.

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

Much like elsewhere in the animal kingdom, it is the habits of youthful play that contribute to the kangaroo's ability to find a role in the mob as an adult.

Dr Warburton says males engage in wrestling and other strength-based play as juveniles, skills that take on a more aggressive component as they age.

"Male kangaroos establish and maintain their dominance hierarchy through sparring contests that involve grasping their opponent and using their back legs to box them," she said.

She says the stronger the males get, the more wrestling contests they win which means they stand out and appear more attractive to potential partners.

'Body builder' roos spend time posturing

Team member Dr Trish Fleming says the dominant males are also able to ward off some challenges through physical display.

"You'll usually have a couple of really large individuals and they'll be very bulked up," she said.

"If you look at them from front-on, they look like they're body builders and they'll spend quite a bit of time posturing and displaying to females, but also to other males.

"Obviously, that's part of their competitive success."

Dr Fleming says the dominant male's strict control of his females makes it dangerous for smaller males looking to secure a mate.

"There's the possibility of males getting a sneaky mating in but they would have to be very brave," she said.

"They'd have to be able to get in and distract the dominant male and it would be quite unlikely.

"When the female is in oestrus [in heat] she'll be hounded quite relentlessly by the dominant male."

Larger muscles come at cost of own survival

But evolution has always been a matter of trade-offs and the kangaroo is no exception.

While larger muscles may lead to more opportunity for securing its lineage, the extra bulk may actually lead to shorter life spans.

"We think that could be due to them having higher body mass and bulk that they've got to maintain," Dr Fleming said.

"So, they get hit by drought and lack of food much more readily than the females do.

"That is a really telling part of the trait.

"It might be advantageous to be very muscly for getting a mate, but it comes at a cost of your own survival."