In a tunnel beneath a freshly deposited cowpat, two male Onthophagus dung beetles lock their elaborate horns for the right to mate with a female in a nearby nest chamber. But the winner of their physical contest is not assured of exclusive mating privileges.
In a side chamber lurks a third contender – a female look-alike, lacking the elaborate horns of his rivals. His strategy is very different: wait until they are strenuously engaged, then sneak in and mate with the receptive female.
But it’s the fourth player in the mating game – the female – who ultimately determines who wins the paternity stakes, according to Professor Leigh Simmons, Federation Fellow and director of the Centre for Evolutionary Biology at the University of Western Australia.
Simmons and his colleagues are investigating how different Onthophagus species and grylloid field crickets play the mating game, and how female choice drives the evolution of male morphology, and mating strategies.
Beetles and crickets are convenient, potentially informative models for other, harder-to-study species, including our own. Simmons uses Onthophagus beetles as a model because, under laboratory conditions, they can cram six generations into 12 months, they’re easy to manipulate experimentally, and their enormous diversity offers many permutations of the mating game.
Female choice imposes selection pressures that can cause males of very different species to converge on similar mating strategies – and even similar physical traits.
Simmons’ hornless Onthophagus males seem to have independently evolved their own version of a mating gambit observed in red deer on the island of Rum, off the west coast of Scotland, indecorously dubbed the ‘sneaky f…er strategy’.
A dominant stag must be constantly vigilant and ready to fight off testosterone-charged raiders intent on mating an unattended doe in his personal harem of does. Zoologists initially wondered why such contests don’t result in runaway sexual selection, creating huge, super-aggressive stags with massive antlers.
Their answer came when they observed timid stags with small racks preserving the status quo, by dashing in for a sneaky quickie with any compliant female while the dominant stag is contending with pretenders to his title.
Darwin recognised that sexual selection is a more potent force in shaping species than natural selection, but the elaboration of a male characteristic like antlers or tail features through quirky female preference is not an open-ended process. At some point, the exaggerated trait becomes a liability to the male, and natural selection reins it in.
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