In a study by the University of Exeter and the University of Okayama, researchers studied wide-horned meal beetles and concluded that natural selection opposes sexual selection.
The authors found that male beetles with the largest mandibles win more fights and mate with more females — this is an example of sexual selection where certain characteristics (such as the tail of a male peacock) improve mating success.
However, having large lower jaws requires a masculinized body (large head and neck) and a small abdomen, which for females limits the number of eggs they can lay. The masculine body is not suitable for females.
However, experimentally enhanced natural selection through predation targets the same males that successfully undergo sexual selection, and this leads to the evolution of less masculinized bodies.
In the course of the study, wide-horned meal beetles were exposed to the predator killer beetle, which ate males with the largest mandibles.
As the number of beetles with large mandibles declined, the benefits of sexual selection diminished, which means that natural selection affects beetles more strongly.
As a result of the experiment, eight generations of beetles produced about 20% more offspring during their entire life, compared with a control group of beetles, where large-horned males were not eaten.