Male Beetles Mount Other Males to Establish Dominance, Study Finds

Male beetles tussling in their flour-floored gladiator arena. Matthew Silk
Male beetles tussling in their flour-floored gladiator arena. Matthew Silk / Male beetles tussling in their flour-floored gladiator arena. Matthew Silk

We believe it was the great philosophical collective Sublime who first postulated that “...[mating] and fighting; it’s all the same.” They were right—at least when it comes to flour beetles. Scientists say male broad-horned flour beetles intentionally mount other males in order to dominate them. Their findings were published in the journal Animal Behaviour.

Same-sex behavior (SSB) is incredibly common throughout the animal kingdom, but not all animals do it for the same reasons. For instance, female bonobos engage in all sorts of SSB with astonishing frequency—up to once every two hours—but primatologists say they’re not just doing it for pleasure. The bonobos have more sex when they’re stressed or social bonds are strained, which suggests that they use SSB as a tool for keeping the peace. And dog behavior experts say dogs of either sex may hump others because they feel their place in the hierarchy is threatened. 

The SSB of male broad-horned flour beetles (Gnatocerus cornutus) may be driven by a combination of these motivations, the researchers found. G. cornutus males have big jaws that they use to bite, push, and flip over their opponents during fights. After the battle, the loser typically retreats to recover, while the winner goes on to find and mate with nearby females.

The same beetles also go in for SSB, which for G. cornutus means one male beetle mounting another and drumming on his shell (the same thing they do when mating with females). Scientists suspected that the two activities served the same purpose: enforcing the pecking order. 

To find out, they set up little arenas in the lab and brought in 311 pairs of male beetles. The researchers left each pair in the arena for 20 minutes, noting both “courtship attempts” and aggression, and which beetle initiated. At the end of the round, after a 5-minute breather, they pulled out one male and brought in a female. Then, they watched for another 20 minutes, recording any courtship attempts and successful mating.

They found that 70 percent of the time, paired male beetles agreed on which beetle would be the mounter and which would be the mount-ee. In these cases, SSB was far less likely to be followed by a fight. But when the bottom beetle didn’t want to be the bottom, or tried to take a turn on top, aggression often followed. Interestingly, pairs that engaged in SSB, even if only one beetle was into it, were far less aggressive than pairs with no SSB at all.

The data also showed that mounter male beetles were more confident with female beetles, approaching them more often and having better success than mountee males.

"Our findings provide the first empirical support for the hypothesis that same-sex sexual behaviour is an extension of male-male competition,” lead author Sarah Lane said in a press release. “They also suggest that SSB may act as a non-injurious display, allowing males to resolve dominance hierarchies without escalating into an injurious fight."