One hundred and fifty years ago Charles Darwin revolutionised the way we think about the natural world with the publication of On the Origin of the Species by the Means of Natural Selection. The ideas and theories he came up with in that book and subsequent books continue to stimulate biological research today, so it’s no surprise that he has been called the ‘father’ of biology. One area that fascinated Darwin and others (be they biologists or not) is sexual dimorphism, the physical differences between the sexes in traits other than the sexual organs. Sexual dimorphism is very common, be it the spectacular plumage found on many cock birds or simple differences in physical size and shape between the sexes such as the human hip.
Darwin was one of the first to propose that the origin of these differences may be explained by sexual selection, such as traits that have evolved through male to male combat. However, the way in which sexual dimorphism actually evolves from a genome that is almost entirely shared between the sexes remains puzzling. That is because male and female traits are controlled by the same set of genes, thus the influence of selection in one sex is expected to be countered by opposing selection in the other sex, a situation termed intralocus sexual conflict.
A classic example of sexual dimorphism, which may have evolved through such male-to-male combat, are the horns of sheep, which are significantly larger in males than in females. The bighorn sheep (Ovis canadensis) is a species found in North America and Siberia, aptly named for their massive, curved horns of males which can weigh up to 15kg (as much as the rest of the bones in the male body). Ewes also have horns but they are much shorter and have less curvature. Prior to the rutting season the rams create a hierarchy for access to the ewes for mating through contests settled by clashing horns. It’s clear that large horns play an important role in determining mating partners for males, however very few studies have been carried out on the considerably smaller horns of the ewes, which seem to have no known fitness benefit. Possibly the horns on ewes exist because of a genetic correlation with male horns, or perhaps they are a defence against predators.
Researchers in Canada, the United States and Scotland chose to look more closely at this phenomenon, and tested for sexual conflict over horn size and body mass in a wild population of bighorn. The study population inhabits Ram Mountain in Alberta, Canada. The 35 year study captured data on rams and ewes including measurements of body mass as well as horn length and circumference at its base to calculate horn volume.
The researchers used an animal model (a form of mixed model incorporating pedigree information, where the phenotype is modelled as a sum of its additive genetic value and other random and fixed effects) and restricted maximum likelihood in ASReml to determine additive genetic (co)variance for male and female traits. ASReml is well suited to this area of work having been developed by statisticians in animal breeding. Its fast and efficient algorithms enable analysis of large and complex data sets such as the one in this study.
The study tested for sexual conflict by estimating quantitative genetic parameters and sex-specific selection coefficients for two sexually dimorphic traits – body mass and horn volume. The traits showed significant additive genetic variance and were positively, but not perfectly, genetically correlated between the sexes. There was also no evidence of sexually antagonistic selection. The intralocus sexual conflict has therefore been resolved, as sex-biased genetic variation has allowed sexual dimorphism to evolve to a level that is currently somewhat satisfactory to both sexes.
The full paper was published in The Proceedings of the Royal Society of Biological Sciences, March 2008, 278, 623-628. Our thanks to the researchers, Dr. David Coltman (Professor) and his PhD student, Jocelyn Poissant for their help on this article. Dr Coltman also recognizes the work of his collaborator Professor Marco Festa-Bianchet.