Small horns, big …

Simmons and his students have been studying a variety of Onthophagus species in which the size and number of horns varies considerably. Some have head horns, others have horns projecting from the pronotum, the body segment behind the head.

They found a developmental tradeoff between horn size and number, and sperm production at maturity. By cauterising the proliferating cells that form the horns during embryonic development, they have shown that no-horned males divert their developmental resources into growing larger testes – revealing a tradeoff in resource allocation.

“The species with more exaggerated horns tend to have much greater canalisation of the development of the testes – they’re much less susceptible to environmental perturbation, so selection pressure has somehow broken down the connection between environmental influences and testes size.”

Simmons and his students use field crickets to explore the selective pressures that favour the evolution of multiple mating in females – the behaviour that promotes sperm competition among males. Sperm competition is more than a simple game of numbers: quality is also important, for both sexes.

“We’re looking at how the fitness of offspring varies between polygamous and monogamous females, and we’re finding some evidence that females do gain fitness benefits for their offspring by mating with multiple males.”

Male genetic variation seems to be a factor in high survival rates in offspring after fertilisation, as opposed to actual success in fertilisation.

“The offspring hatch two weeks after fertilisation, and there appears to be a critical period of embryonic development when survival is determined by protein components of the male’s seminal fluid,” Simmons says.

“By mating with multiple males, the female gets a variety of proteins to maximise the viability of her embryos.

“We can’t determine in advance what constitutes male quality, but females that mate with multiple males end up with more offspring.”

Singing may be a proxy for genetic quality in male crickets, according to Simmons. “Males chirp at night, and females assess the song from a distance, and seem to be more attracted to some songs than others.

“Some US studies have linked song attractiveness to the quality of a male’s seminal fluid, so the female is able to assess the male’s suitability as a mate from his song.

Simmons says some genetic mechanism appears to link the two traits, but how it works is unclear – genetically superior males may simply have the resources to invest simultaneously in high quality sperm and high quality songs, where lesser males struggle to do both.

Those superior resources may not be exclusively genetic – the male that keeps itself well fed, and out of harm’s way, is likely to have a superior phenotype.