The rhinoceros beetle is well-known for its massive pitchfork-shaped horn, but how did it get its wonderful weapon?
When a rhinoceros beetle male wants to reproduce, he first has to acquire a feeding site (sap leaks on trees). The problem is these sites are very in-demand, and he will usually have to fight other males for them. If he has a large horn, he’s likely to win – if he doesn’t, he stands a poor chance of passing his genes on.
Horn size is highly variable between males, ranging from minuscule to two-thirds the size of their body. Previous research found that a male beetle’s fitness can be seen from his horn (horn cells are extremely sensitive to nutrition signals). A healthy beetle has a larger horn. What’s more, this is an “honest signal” – a male with poor health can’t fake it and grow a big horn.
The benefits of such a weapon are obvious, but what about the costs? It seems carrying a huge object two-thirds the size of your body on your head would create some serious flight problems. Sexually-selected characteristics can become costly to their bearer – for stalk-eyed flies, having huge eye stalks is attractive, but males with larger stalks actually experience flight problems.
Not the rhinoceros beetle though. The horns are actually very lightweight, and as the beetles fly at slow speeds with a high body angle there’s little effect on flying. Even the largest-horned males only have to increase the force needed by 3%. Horns also don’t appear costly in resource allocation, survival or immune costs, meaning they are less constrained by natural selection and can diversify without impeding the beetle’s survival.
Photo credit: W. Freihofer.