The genetics of primary and secondary sexual character trade-offs in a horned beetle

When structures compete for shared resources, this may lead to acquisition and allocation trade-offs so that the enlargement of one structure occurs at the expense of another. Among the studies of morphological trade-offs, their importance has been demonstrated primarily through experimental manipulations and comparative analyses. Relatively, a few studies have investigated the underlying genetic basis of phenotypic patterns. Here, we use a half-sibling breeding design to determine the genetic underpinnings of the phenotypic trade-off between head horns and the male copulatory organ or aedeagus that has been found in the dung beetle Onthophagus taurus. Instead of the predicted negative genetic covariance among characters that trade-off, we find positive genetic covariance between absolute horn and aedeagus length and zero genetic covariance between relative horn and aedeagus length. Therefore, although the genetic covariance between absolute horn and aedeagus length would constrain the independent evolution of primary and secondary sexual characters in this population, there was no evidence of a trade-off. We discuss alternative hypotheses for the observed patterns of genetic correlation between traits that compete for resources and the implications that these have for selection and the evolution of such traits.