When I was a kid, we'd all look forward to Friday evenings - because Dad & Grandma would come back from town with the weekly supply of comics. The ads in the back were almost as good as the cartoons, although we were very disappointed to find out that sea monkeys were definitely not as advertised! I also remember regular ads featuring a poor weedy guy who, having had sand kicked in his face by various over-muscled bullies, followed the instructions of various manly authorities and ended up developing his own set of biceps, triceps, washboard abs & all the rest: all the better to impress the girls at the beach.
I was reminded of all this earlier this week when I noticed a Facebook report on bicep-flexing in male kangaroos, based on this 2013 paper in the Biological Journal of the Linnaean Society. (It's behind a paywall, alas, but you should be able to read the abstract.)
As Darwin recognised, at least some of the physical differences between males and females may be the result of competition: think showy male peacocks and their relatively dowdy mates, for example. This competition may be directly between males, as they try to gain access to females - an example of this would be elephant seals, where the males battle each other to become 'beach-masters', and the winners gain access to females coming ashore on their beaches. In this case, selection favours male strength. In other cases females select males to mate with on the basis of some attribute - perhaps the brightest plumage, or the loudest song.
As Warburton & her colleagues point out, these different forms of sexual selection are not mutually exclusive, & it's sometimes hard to tease out which has the greatest impact on a species' characteristics. They decided to investigate sexual selection in a large species of marsupial, the western grey kangaroo (Macropus fuliginosa), a polygynous species where males tend to have greater development of the chest & arm muscles. It's fairly clear that in large kangaroos & wallabies, male reproductive success is linked to body size, which in turn is linked to social status: "the large males... gain an exclusive consort relationship with oestrous females". Larger males have larger home ranges and so may have more chance to find and mate with females, and in addition genetic data suggest that
alpha males may be able to outcompete and exclude smaller males from access to females.
But how to tell if the larger arm & chest musculature of males is down to sexual selection, or simply a factor of differences in overall body size? You measure some kangaroos (in this case, purchased from pet food manufacturers. Poor Skippy!) This allowed the research team to look at the slope of trait size (eg bicep size) against body size:
When the slope of a trait size against body size is 'isometric', the relative trait size is constant across a range of body sizes. Where the trait size decreases with body size, the slope shows 'negative allometry' and where the trait size increases with body size the slope shows positive allometry. A trait that is positively allometric is therefore relatively larger, in proportion to body size, in larger individuals. If this relationship differs between the sexes, then it can be interpreted that there is differential selective pressure acting on males and females as they grow larger.
While the numbers involved were fairly low (13 males and 15 females), the team ensured they had a range of body sizes. (I'm guessing the fact that that body mass was estimated from measurements of the thigh bone rather than directly, by weighing, reflects a lack of scales big enough to plonk an entire kangaroo corpse onto.)
The results? Male kangaroos had larger ("more exaggerated") forelimbs, and more variability in muscle mass, than females. Both of these suggest that sexual selection might be acting on forelimb size, and it's likely to be through male-male competition. This is because male kangaroos use their forelimbs to push or wrestle with their opponents as they fight to determine their place in the social hierarchy. There's also a suggestion that the larger male muscles
may also be an important aspect of visual signalling (presumably of potential fighting ability) and dominant males will frequently adopt poses which best display their muscularity and size.
So perhaps there's also potentially an element of female choice: those exaggerated biceps may attract admiring female glances (or at least, allow them to appraise a male's genetic quality), rather than being simply a part of the roo equivalent of sand-kicking.
N.M.Warburton, P.W.Bateman & P.A.Fleming (2013) Sexual selection on forelimb muscles of western grey kangaroos (Skippy was clearly a female). Biological Journal of the Linnean Society 109(4): 923-931