Size Does Matter in Sexual Selection

Posted Oct 26, 2012 by Tim Sandle
It is an age old debate: does size matter in sexual selection? There is no new light shed in relation to the human male. However, there has been some interesting new research been published about beetles.
Mating seed beetles
Mating seed beetles
Fleur Champion de Crespigny
A new project among researchers from Uppsala University in Sweden and the University of Cincinnati has demonstrated experimentally that the evolution of male genitals, in relation to size, is related to mating success, focusing on a species of seed beetle.
As outlines in the research brief, the scientists studied a species of seed beetle known as Callosobruchus maculatus. Mating among these beetles involves several males engaging in copulation with individual females. As part of this ritual, the males compete with each other over the fertilization of the female’s eggs.
For the study, the researchers bred male beetles with very long genital spines. They also bred male beetles with very small genital spines. The experiments demonstrated that the males with long spines produced substantially more offspring. This was through experiments which focused on the competitive mating process.
Eureka notes that, according to the research findings, that the function of the male copulatory organ determines which of the males fertilizes most of the female beetle’s eggs. Here the morphology of the male genitalia affects his fertilization success in these beetles: the larger the organ, the more chance of fertilization.
The scientists are now researching to see why ‘size does matter’. It may be that longer spines help introduce bio-active molecules which may create effects within the female that help with fertility. Or it could be that longer genital spines simply exert greater mechanical effects within the female by stimulating her reproductive tract during copulation, thereby inducing the female to uptake and use more of the sperm.
No link has been made to humans from the work. However, readers may draw their own conclusions.
The research was led by the Polak Lab and the findings have been published in the scientific journal Current Biology.