Rhino beetle horns come cheap

If you don’t like bugs* or other creepy crawlies, rhinoceros beetles are not for you. But these insects are marvelous critters, remarkable for both their large size and the interesting shapes of their bodies.

The Japanese rhinoceros beetle, for instance, has a long forked horn that looks as if someone had glued a miniature antler onto its nose. Kids in Japan collect these beetles (in some places there are even beetle vending machines), and others bet on beetle battles.

The beetle fights are normal behavior. Two males will duke it out, usually in the hopes of gaining access to a female. And that kind of thing is always interesting to evolutionary biologists, going as far back as Charles Darwin, because it belongs to a subset of natural selection called sexual selection. Sexual selection is often the reason behind things like why male birds have elaborate plumage and why male fiddler crabs have one enormous claw.

Evolutionary theory about sexual selection says that weapons and ornaments most often evolve when they reliably signal a male’s genetic quality. The cost of developing those traits is the subject of two competing theories: One side says that weapons and ornaments should be costly to an animal because there would be no reason to pay attention to such a trait if it wasn’t an indicator that the male has the resources or genetics to back up the signal. The other side, though, says that these signals needn’t necessarily be costly to create as long as there’s a penalty for cheating. For weaponry in particular, investing in something big and showy isn’t helpful if the male can’t back that up and win a fight.

A pair of researchers at the University of Montana in Missoula recently tested out these competing theories by studying the Japanese rhinoceros beetle. Their results appear in Animal Behaviour.

In the study, the researchers examined how beetles allocated resources to horns, wings, eyes, forelegs and genitalia and also looked at several measures of immune response. They also tried to determine whether any trade-offs in these allocations affected beetle survival. If there was some kind of cost to having big horns, then one of these traits should be affected.

But the team found no evidence of any kind of cost associated with horn size. Big-horned beetles tended to be big overall, and there was no effect on immune function. A previous study had looked at the effect of horn size on flying ability and likewise found no impairment. Plus, big horns and large body size didn’t affect survival.

If big, elaborate horns don’t come at a big price, the researchers say, it’s more than just a bit of evidence to back up one theory — it might explain the diversity of structures found among these beetles. “If our results…are typical for rhinoceros beetles, and horns are indeed cheap to produce and carry,” they write, “then horns may be free to diverge in size and form.” In other words, if horns or other traits such as a crab’s big claw, are cheap, then evolution can become more creative.

* Yes, I know, rhinoceros beetles are not true bugs and I am not implying that. They belong to the order of beetles, not bugs (hence, the name).