Battle of the Beetles

The study of Japanese Rhinoceros Beetles explores their complex sexual selection.


For male Japanese rhinoceros beetles, trying to woo a lady means they must endure a beetle battle royale.

Opposing males step into the arena, and a fierce jousting match begins.

The male that successfully ejects the weaker competitor is the victor, but there's a twist in this science story of love and war.

Two biologists at the University of Montana are now studying these Japanese rhinoceros beetles to get to the bottom of their complex sexual selection.

Our partner, 'Science Friday,' has the story.

Sometimes the difference in winners and losers has to do with who succeeds in reproducing, and that can be a vicious landscape of competition and battle.

Whenever a rival male comes up, they'll kind of size each other up.

They'll kind of touch horns.

They're ready to joust.

They're ready to push him away, pry him away, anything.

The winning male finally gets the female.

He doesn't just mate with her.

He sings to her first.

And they won't mate with the male.

They'll reject the male.

They'll resist that male.

And, you know, to the winner go the spoils, supposedly, except not so much.

My name is Jillian del Sol, and I study the Japanese rhinoceros beetle.

And I'm Doug Emlan, and I study the evolution of extreme weapons in animals.

The beetle that we focus on right now is called the Japanese rhinoceros beetle.

It's very hard to look at an animal the size of a chicken egg with an enormous horn sticking off it and not be curious.

An example I like to use is that's like walking around with a coffee table fused to the top of your head, and so we spent a lot of time looking at how the horns grow, learning about the genes and the physiological, hormonal pathways that help cause a weapon to be a weapon and said, 'Okay, now, knowing what we know about how these things grow, you could get in there and start looking at the behavior and see if you can explain, from a perspective of natural selection or sexual selection, what's happening in the wild with these animals in this different locations.'

If you're ever seen an animal where the males look way different than the females, you can bet that there's some sort of sexual selection going on.

We're going all the way back to Darwin here, right?

So sexual selection happens when the members of one sex must compete with one another or woo a member of the opposite sex to pass on their genes by gaining mates.

If you have weapons, you're going to be able to more effectively keep another male away from females that come in.

So in terms of the rhinoceros beetle, the males will guard these very kind of small, discrete sap wounds on the sides of trees.

So the females fly to these feeding sites, and they feed, and then they go off and lay their eggs.

If a male can successfully guard that territory, he will gain opportunities to mate with every female that shows up, and so that's a big deal, especially if you only have 2 months to live, 2 months to find a mate.

Now, standing there is not easy.

They have to fight rival after rival after rival after rival because there aren't very many of these feeding sites.

They don't just lunge into battle.

That's a really good way to get killed.

They sort of hang back, and they try to assess who's bigger.

And then if that's not enough, they'll push a little bit.

They'll spar, but they can get dangerous.

Not only is a male trying to pry the other male, but it's also clinging on to the tree for dear life, trying not to get pried up, so these battles are very energy-intensive.

And all the time that they're fighting, they're not feeding.

That's the irony.

They're standing over the food source.

That has led, in our system, to intense selection for ability to fight.

And because, in these fights, the males with the bigger weapons win, it's worth it to them to invest in a really big weapon.

The weapons, they're exquisitely sensitive to nutrition, but even given that, they grow relatively longer in some populations than they do in others.

A male in Japan that's the same size as a male in Taiwan is going to have a way bigger horn.

So our main hypothesis for why these beetles differ in horn size is that in a place like Taiwan, beetles will carve their own sap sites, and they do so in the thinnest-barked trees, including ash trees, and so where beetles can make as many territories as they want in a night, the horns suddenly don't become as important.

In contrast, in Japan, beetles cannot carve the really thick bark of the oak trees, so he had to work really hard to get that territory and to keep it, so the horn, all of a sudden, becomes very important.

But if everybody can make their own territory, then, yeah, you can fight all night long to guard that feeding site, but there's 27 other feeding sites, and females might not bother to visit you at all.

Why should they?

But it's only one part of the story, so here's a twist where Jill's work sort of pulled the carpet out from under everything that we thought we knew about these beetles.

When the winning male finally gets the female, we noticed that the males will spend up to a few hours singing to females.

What I actually mean is they squeak a lot.

They have multiple phrases in their songs, but they have a little grinding noise that they make as well.

These males who have already put everything into the weapons are now also having to sing.

He's there, and they don't accept matings from the male, so that's not supposed to happen.

The males have these big horns.

Why not pay attention to the horn?

They don't seem to care at all, so then you have a system where they're paying attention to something else, which, of course, gets us all excited.

So to find out what's going on, we are going to record a lot of these songs in the lab.

We have about 150 beetles in the lab that we are going to put them in our little sound recording studio and record as many male songs as possible to try and figure out how complicated these songs really are and what information they might be conveying.

The other possibility is that these animals live in a world of chemicals.

Their antennae pick up specific types of chemical signals.

So we're taking swabs of beetles now to see if the big males and the small males are chemically different in a way that might be informative.

We don't know.

We're going to look at everything, measure it all and sort of statistically parse it and see, is there something, when we look across individuals, that is sufficiently correlated with condition, quality, body size, that it could work as a signal?

I'm always surprised by these beetles.

And we thought we knew so much about them, and then we get to the field, and they're doing all these things that, by rights, they shouldn't have to do.

That gives us the chance to ask so many more questions about what might be going on.

Every question we try to answer leads us to another question and then another question.

I've been sort of sliding down that rabbit hole for 20 straight years, and I'm not tired yet.

They keep us guessing.