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Male kangaroos woo mates with bulging biceps

A male kangaroo’s forearm size could be a sexually selected trait and help them find a mate, a new study has found. In fact…

Male kangaroos often adopt poses designed to show off their guns. Terry Mercer

A male kangaroo’s forearm size could be a sexually selected trait and help them find a mate, a new study has found.

In fact, male kangaroos frequently adopt poses to show off their muscly arms to females, the authors have said.

The study, conducted by researchers from Murdoch University and Curtin University and published in Biological Journal of the Linnean Society, centred on data gained from dissecting 13 grey kangaroo males and 15 females.

Each forelimb was dissected and the weight relationships between the individual muscle mass and body mass were examined.

The body mass of each animal was calculated by femur (thigh bone) circumference, measuring the bone at the narrowest point. Forelimb muscle mass in males was found to be heavier than the equivalent muscles in females, suggesting different applications for the same limb between the sexes.

Study co-author, Associate Professor Trish Fleming, said male kangaroos use their muscles in wrestling matches with love rivals.

“We found a select group of muscles which are particularly used in grasping the other males and drawing them towards them, which were exaggerated in the larger males. The larger the males get, the more those individual muscles are worked up, so they are disproportionally larger than the rest of the body,” she said.

“The reason this study is unique is because it tells us a bit more about the functional morphology of their forelimbs. It tells us which muscles they could be using to gain a select advantage over other males.”

Sexy forelimbs?

Associate Professor Rod Wells, an Australian marsupial expert from Flinders University, said big arm muscles may offer males “additional advantage from either females finding big forelimbs sexy or alternatively the males which win the right to access the females are then strong enough to overpower any unwilling female.”

“Males are not simply getting bigger but their forelimbs are getting disproportionally bigger not only in bone length and diameter but also in muscle mass. The authors relate this to male-male competition for right to inseminate females,” he said.

However, Associate Professor Wells said that to demonstrate that a trait is a result of sexual selection, “there must be some evidence of differential mating success to those individuals carrying that trait.”

“In the larger kangaroos, females reach reproductive age at two years, maturity is later in males. Both species continue to grow in stature after maturity but males at a greater rate than females. So you get differences in body size of large male to female kangaroos 1.9:1 for greys and 2.5:1 for reds,” said Associate Professor Wells, who was not involved in the study.

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7 Comments sorted by

  1. Dale Bloom


    I am wondering the actual usefulness of this theory, and whether or not it was worth dissecting “13 grey kangaroo males and 15 females”

    I think it does naught for conservation or preservation of our kangaroos, and a much more useful way to spend taxpayer funding would be to find a way of reducing the killing of so many kangaroos on the roads.

    The road kill of kangaroos is awful, and the costs of damage to cars must be many millions each year, and it would be definitely worthwhile researching ways to reduce it.

    1. Peter Wilkin

      Australian Realist

      In reply to Dale Bloom

      The feeling that scientific research must be based on utility seems at first understandable from an ostensibly pragmatic and belligerently unsophisticated viewpoint, but I think maybe it's best to leave scientists alone to do their thing as they see fit after all without them we'd still be hunting and gathering and believing in every silly superstition and dying of the first non trivial disease that we catch.

      Besides, everything that scientists do helps to sharpen the focus on our view of the world and that is desirable in itself.

    2. Dale Bloom


      In reply to Peter Wilkin

      Drive along a busy outback road in the morning, and see how many fresh kangaroo carcases appear along the road from the previous night’s kill.

      If scientist don’t want to do something about that, then they must have no concerns for the kangaroo.

      At present, the only protection for kangaroos appears to be roo whistles, which apparently have about an 80% efficiency rate.

      That is, a driver will miss 8 kangaroos for every 2 they hit.

      Which obviously isn't good enough.

  2. Cienwen Hickey


    This research does absolutely nothing to aid understanding of Kangaroos or solve any of the myriad of perceived problems they are supposedly responsible for.
    Tax payer dollars could be better spent in finding a way to accurately count Kangaroos,to what extent do they compete with domestic stock for food and water,how successful are they at breeding, what are their mortality rates during drought and fire and Government 'burn offs',how many juveniles reach maturity, what diseases/contaminants does their meat have which can be passed onto humans, which grasses do they eat and which do they not eat, birth control, relocation. The list is endless because so little research of 'value' has been undertaken on Kangaroos. The majority of research is conducted by self serving science undertaken by those who are paid to say what the Governments want to hear, independent research about Kangaroos is almost nonexistent.
    'Scientists' shouldn't be held in 'Awe'

  3. ernest malley


    There is a small roo mob on my land, dam with joey at foot + one in pouch, last 2 year's adolescents and the buck who is easily 2mts tall with biceps like Arnie. Fortunately he flees before I get too close, unlike the others who continue to graze as I pass with touching distance - though joey is still a bit skittish.
    The odd thing is that there are two species of wallaby, rock & rufus, who never, ever graze the same area as the roos. They clearly disapprove of the great, grey gallumping things.

  4. Tim Benham

    Student of Statistics

    AFAIK the biceps are in the upper arm, not the forearm.

  5. James Hill

    Industrial Designer

    Meanwhile, around the shores of Lake Burley Griffin, female kangaroos are impressed by the ability of their potential mates to entice pesky, "entitled", suburban, middle class dogs into shallow water and then drown them, to the consternation of their alarmed and adoring owners.
    Do those bulging biceps help, or is it the stable platform of long legs and balancing tail that drowns the dodgy doggies?
    Or, do females get in on the act?
    Anyway, well done, marsupials.