Sunday, May 17, 2009

Size Matters, But So Does Maturity

Starting in the 1940’s, Angelo Siciliano pioneered the cultural meme of “brawn over brains” and actively encouraged the males of one mammalian species - Homo sapiens - to pack on muscle in order to win over rivals for the affection of potential mates. However, lost to Charles Atlas was the possibility that the iconic “97-pound weakling,” which he vowed to aid through a rigorous program of dynamic-tension, may have been capable of achieving success on his own, independently of an increased body size. Key to such success would be to avoid the gauntlet of bullying sand-kickers altogether thereby reducing the necessity for brute force, and rather than investing in mail order fitness regimens, to instead turn the “chump to a champ” through allotting resources to two key areas - growing-up fast and getting to the female first!

Thanks to the vector of modern media, Angelo’s meme is still infectious today (among populations of H. sapiens) and has probably met with some limited success; however there are several species that have found competitive advantage in strategies other that those associated with an increased body size – one such animal is the redback spider of Australia.

The redback, Latrodectus hasselti, is a polyandrous species in which intrasexual selection relies less on choosey females and more on males competing for access to un-mated –“virgin” - females. Virgins are preferred by the male redbacks because, as with many other spiders, the males introduce a post copulatory plug to the female following sex, this has the effect of decreasing the likelihood of secondary male suitors successfully transferring genetic material – to have the best probability of forwarding their germ line to future generations, male redbacks seek out virgins. The virgin status of redbacks is identified by the males through a pheromone signal emitted by the female; within hours of mating, these chemo-signals cease being produced and the female becomes less attractive as a potential mate.

Typically, sexually mature males of the species, having converged on the virginal pheromone, will find themselves geographically amassed at the female’s web with as many as five other males vying for mating access. When this occurs, the males combatively engage each other until a single victor remains. To the victor goes the mate.

When it comes down to brute force, generally the male with the best weapons, most strength or greatest mass has the advantage in combat. This polyandrous scenario is familiar to most, but generally images of big horn sheep ramming each other, or gladiatorial deer with interlocked antlers are called to mind; the spiders are little different… Little different - except for an alternative intrasexual strategy to which some males find themselves better adapted.

By way of sexual and natural selection, some male redbacks have decided that because of the dynamic between pheromone signaling, male-to-male competition and copulatory plugs, that it may be better to focus more on being the first to achieve sexual maturity via rapid development than to spend extra effort and time in growing to a larger size. In this way they would have first access to females, avoid web-sparring with males and because of the pheromones, even discourage rivals post copulation. Even Charles Atlas could respect that!

KASUMOVIC, M., & ANDRADE, M. (2009). A change in competitive context reverses sexual selection on male size Journal of Evolutionary Biology, 22 (2), 324-333 DOI: 10.1111/j.1420-9101.2008.01648.x

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