Combat traits usually get most of the attention from researchers, because they are thought to have arisen due to intense male-male competition for access to females. Large, elaborate weapons become the focus of sexual selection studies because the connection is obvious. Collared lizards (Crotaphytus collaris), for example, have bite force correlated with head width, and it positively predicts the number of offspring produced in a breeding season, suggesting sexual selection on head morphology and the role of attacking traits.

But maybe defense wins sex games too. Sexual selection in the Forked Fungus Beetle (Bolitotherus cornutus) favors larger body and horn size, and a new study investigates the relationship between these traits and the beetles' grip strength, which is crucial for the male to hold on to the female and shield her from other males in an elaborate courtship ritual.   But it also highlights defensive traits - by having a strong grip, males can keep females busy for hours.  And hours. And hours.  And that blocks out other males from getting involved during mating season.

The grip strength trial. (A) The male grips the dowel rod in a similar fashion to how males grip females during courtship and guarding in the wild. (B) The same male at maximum resistance just before releasing the dowel rod. doi:10.1371/journal.pone.0042738.g002

During the courtship ritual, male beetles grab onto the female and they hold on - that would seem to favor males with a stronger grip. The authors of a study tested the grip strength of 84 beetles, both male and female, by tying a piece of thread around the beetles' bodies, allowing them to wrap their legs around a small rod, and then pulling the beetle vertically until they released the rod. 

The larger attacking male uses his clypeal horns in an attempt to dislodge the smaller male. The smaller male was previously courting the female and during the combat grips her with all six legs. The video has been sped up to 5x actual speed. Credit: doi:10.1371/journal.pone.0042738

The researchers found that grip strength
grip strength was repeatable, consistent and differed between the sexes. It was very consistent for each individual, males overall have a stronger grip, and they believe that this sexual dimorphism appears to result from a complex relationship between body size and leg length. Their results, they write, suggest that there is a suite of traits that influence grip performance and therefore might affect sexual selection.

Citation: Benowitz KM, Brodie ED III, Formica VA, 'Morphological Correlates of a Combat Performance Trait in the Forked Fungus Beetle, Bolitotherus cornutus', PLoS ONE 7(8): e42738. doi:10.1371/journal.pone.0042738