TERRY GROSS, host:
Underneath the cow patties in the pasture and the monkey dung in the jungle, there's a miniature world of sex and violence. Dung beetles with fierce-looking horns are battling over female beetles. And my guest, Douglas Emlen, is studying them. He's an expert on the evolution and development of bizarre or extreme shapes in insects. He's particular interested in insect weaponry. Dung beetles have what he's looking for. Lots of his work is in the lab, but he's also had some wild adventures collecting different families of dung beetles from around the world. Emlen is a professor of biology at the University of Montana. If you want to follow along, on our Web site there's a dung-beetle slide show and a video of two beetles battling. That's at freshair.npr.org.
Doug Emlen, welcome to FRESH AIR. Now I think it's fair to say that dung beetles wouldn't be most people's first choice of an animal to study. So what is unique about the dung beetle's armor that makes you so interested in studying them?
Mr. DOUGLAS EMLEN (Professor of Biology, University of Montana): There are a variety of things, and I guess I should first of all qualify that by saying they weren't my first choice of organism, either.
(Soundbite of laughter)
Mr. EMLEN: It's a hard thing to wake up in the morning and decide you're suddenly go out there and study dung beetles. I started out trying to study some of the big rhinoceros beetles as a graduate student, and the project failed pretty spectacularly. And it was a biologist named Bill Everhart that took me under his wing after that failed project and said no, no, no. You're looking at the wrong beetles. And he opened up this box, and it was full of all these little, tiny beetles that are the size of your pinky or even the size of an eraser on a new pencil. And he opens up the box, and it smells like horse manure, and I looked him, and I said those are dung beetles. There's no way I'm going to work on those things. And he laughed at me at the time and told me I was a fool, and basically explained that for all the kinds of biology that I wanted to understand, genetics and behavior and diversity, these things were perfect.
And since that time, to come back full circle to your question, the dung beetles have turned out to be an amazing system to study, and they've taught us so much about biology in many ways.
But you mentioned the weapons. The thing that first drew me to them is the spectacular diversity in their morphology. They have these incredible shapes, and most of the diversity in those shapes involves these weapons. And so for lack of a better analogy, these tiny, little insects, that again, as I said, are often the size of an eraser on a pencil, on a new pencil, these things are the insect equivalent of a bull elk or a male deer, and they have incredible weapons coming off of their bodies.
And you look across the species, and there's variation among the species. Sometimes the horns comes from the back of the head, sometimes from the front of the head, sometimes from the thorax, which is equivalent to sort of between your shoulder blades. Sometimes they're branched, forked, curved, straight. There's just an incredible variety of forms.
And I was interested as a biologist in how a little, tiny animal like that could strut around with these enormous weapons sticking off the side of their body and survive.
GROSS: You know what? It's kind of amazing, and I mean, not only do they have these weapons, like, some of them are like antlers. Some of them are like a rhinoceros horn. Some of them are like lobster pincers. But they also have, like, hair.
(Soundbite of laughter)
Mr. EMLEN: Many of the horns - you mean the beetles or the horns?
GROSS: Well, the beetles, and yeah.
Mr. EMLEN: They do, and some of the horns in some of the species tend to have hair. We see that more often in the rhinoceros beetles than in the dung beetles, per se, but very often the sides of these horns are adorned with thick rows of hairs, and it's really sort of embarrassingly unknown what those things do.
There's a good chance that they are sensory structures so that when a male locks with another male, fighting, they can tell where the horns of the opponent male are because they're distending or pushing down all these fine hairs on the surface of the horn.
But some people think that the hairs might actually have chemo-detectors. They may be good for smell or for releasing signals. We don't know.
GROSS: I can tell you one thing for sure. They're not fashionable.
(Soundbite of laughter)
GROSS: They are really creepy looking.
Mr. EMLEN: That's a matter of opinion. I disagree. I think they're fantastic.
GROSS: Well, they are fantastic in a creepy kind of way. Let me ask you to describe one or two of the types of dung beetles that you find most fascinating in terms of their form and shape.
Mr. EMLEN: Sure. So one of the species of dung beetle that we spent quite a bit of time studying is - has the species name Taurus, as in the bull, and it actually has a pair of long, curved horns that extend upwards and backwards from the head of male beetles.
So it's a big, curled set of horns, and when the head is tucked back against the rest of the body, the horns sort of wrap in snugly around the equivalent of the shoulders or the thorax. But when these males are fighting, they pull their heads forward, and they've got quite an impressive pair of horns.
And we've looked at these beetles in a variety of ways and looked at what they do with the horns and who has the horns and who doesn't have the horns and how the horns develop, and all kinds of questions coming from that basic oddity of their morphology - the fact that they've got these huge weapons.
GROSS: You want to describe another one?
Mr. EMLEN: Sure. So one of my favorite species, unfortunately it's not one that we've been able to study in numbers, but it's one of the most spectacular of these beetles, is a species that actually was illustrated in Darwin's book, his treatise in which he initially described the whole theoretical concept of sexual selection, the process of biology that myself and so many other biologists study.
And in this beetle, the beetle is a brilliant, iridescent green color, turning sort of shimmering from a green to a yellow to a blue and in some angles even purple. So it has this beautiful sheen to it. And the males have a long, curved pair of horns that come up from the head that are much longer than the ones in the species I just described to you. And they have a branch in the middle of them with a tine that comes off and points inwards. And then at the very tips of the horns, it swells outwards into almost a little plate, so beautiful branched, curved horns coming off of this species.
GROSS: So the purpose of these weapons, the horns and the pincers, are basically to fight off other guys for girls.
Mr. EMLEN: Yup.
GROSS: So, like, the male dung beetles fight each other over the available female beetles.
Mr. EMLEN: They do, they do.
GROSS: So does that mean - have you watched them fight?
Mr. EMLEN: Oh yes, yes, many times.
GROSS: Oh, describe what a fight looks like.
Mr. EMLEN: It actually looks like pandemonium. It was not what we expected at all. So these are dung beetles, the ones that we're talking about right now, and so what happens in these cases are these beetles are very good at smelling their food source, and they follow these odor cues into the food. And when they find dung of the particular species that they feed on, they often have to deal with hundreds, and in some cases thousands of other individuals that are coming in and piling into that same place and competing to carve up and use that food resource.
So what happens is the beetles go instantly underground. They go right beneath the food source, the dung pile, so to speak, and the females actually excavate these burrows in the ground, and the males guard the entrances to these burrows. And they have big teeth-like spines on their legs, and they lock themselves, they brace themselves against the tunnel walls and try to prevent rival males from getting into the tunnels where the females are.
The females - these are amazing insects because they have elaborate parental-care behaviors. The females will take pieces of the dung down into the burrows and chew them up with their mouth parts and process them and pack them together into these sort of balls, these little sausages of buried dung underground. And then she'll excavate a chamber at the end of that and place an egg in there. And that's essentially the full food resource, like an allocate of food for each one of her offspring. And she'll do that, lay an egg and then close in the tunnel and then build another one of these balls and then close the tunnel. And we've watched these beetles do this.
The females will make 50, 100 different trips down into the ground for every single one of these eggs that she lays. And during this time, the male is guarding that tunnel, keeping everybody else away and mating with the female absolutely as often as he possibly can. And other males come and challenge.
And the fights take place inside these tunnels. And they scramble, and they push, and they pry, and they twist. And we tried forever to figure out how it is, exactly, that they use these horns because we wanted to know why would one species have a straight horn and another species have a curved horn and another species have a branched horn.
And we were assuming that we'd be able to pick up intricacies in the nature of these fights that would tell us why one species had one shape or another, and we failed. The fights are just chaos. We filmed hundreds of them, and they're amazing things to watch, but there's nothing predictable or repeatable about them except the outcome.
Somehow or other, bracing and twisting and prying and pushing and pulling and trying to get the other beetle out of the tunnel, somehow, at the end of the day, the male that has the longest horns and/or the largest body sizes typically is the one who wins. And the smaller males get pushed out of the tunnels and leave and go find another tunnel and try to push their way into that.
GROSS: So they just use these horns to bash away at their competition?
Mr. EMLEN: Yeah, I wouldn't call it bash. I would say they more use the horns to block the tunnel. You've got essentially a round or an egg-shaped animal blocking a tunnel, and they're very strong. So you have a rival pushing itself past, and it seems that the horns, regardless of what precise shape they take, seem to function like bars of a jail cell. They sort of block - they make it easier for a male inside the tunnel to block it so that another male can't push past.
But it's not as simple as that because they don't sit still. They twist and push and pry. But they don't stab with them. They aren't able to puncture the armor of the rival males. So they don't actually get injured in this process, and they don't really bash each other either, but they definitely spar and twist, sort of strength contests, I suppose.
But there's a twist to this story, too.
Mr. EMLEN: Which is that the biggest males have the biggest weapons, and these are the ones that are usually the most successful at guarding that tunnel and at mating with the female inside the tunnel, but there are small males in these populations, too, and they don't produce the horns.
And so these small males look much more similar to a female. They don't have the big antlers or the big weapons, and they don't fight over the tunnels, either. They have a sneak tactic, a satellite tactic. What'll happen is they'll work their way into a tunnel and try to stay there, but they get kicked out right away by bigger males. And instead of going from tunnel to tunnel to tunnel to keep trying, they stay after they get kicked out, and they dig their own tunnel right next to the main tunnel. And after they get sort of a centimeter or so down below the ground, they cut horizontally, and they can intercept the guarded tunnel beneath the guarding male. And sometimes they're able to sneak in, zoom down to the female, mate with the female and get out again before the guarding male has essentially figured out what's going on.
So you've got big males with weapons fighting, and little, tiny sneak males without the weapons sneaking into these tunnels on the sly.
GROSS: So the less macho dung beetles still find a way.
(Soundbite of laughter)
Mr. EMLEN: They try. I don't think that they're as successful as the larger, dominant males, but yes, they definitely find a way.
GROSS: Can you see the armor and the weapons with your naked eye?
Mr. EMLEN: Yes, but I've had some practice. I think the first thing - if somebody were to go out and look for this in their backyard, which is the other fun thing about this system is that these beetles live in so many different habitats and so many places that you'd be pretty hard-pressed to live anywhere where you couldn't go into a pasture or into the backyard and turn over a cow-manure pile and look underneath it and find some of these beetles. They're that abundant and that widespread.
So if you were to go out and look for these things yourself and tip over a cow pile and look at the dirt, maybe dig with a trowel into the first couple inches of the soil and turn it around, you'd find these beetles. They're often, in most habitats, certainly around most places in the U.S. - they would be on the order of half-a-centimeter to a centimeter long, and they're squat.
The beetles that I'm talking about with the weapons walk like little tortoises. They sort of jerk their way along. And you can pick them up and see the horns with the naked eye.
We often look at them under microscopes to take more precise measurements of these things, and we photograph them either using really good camera lenses, or sometimes scanning electron microscopes will take really good pictures of these things, as well. But you can definitely, for a lot of these beetles, pick them up out of the ground and look at them with a hand lens or with your naked eye and see these really cool weapons.
GROSS: My guest is Doug Emlen, a biology professor at the University of Montana. We'll talk more about his work studying dung beetles after a break. This is FRESH AIR.
(Soundbite of music)
GROSS: If you're just joining us, my guest is biologist Doug Emlen, and we're talking about his work studying dung beetles. And he studies them because he's interesting in insects who have armor, and a lot of dung beetles have really fascinating armor, horns and antlers and so on.
Now you've described these, like, you know, antler-like and branch-like weapons and armor and weapons that the dung beetles have, and they sound, like, really big compared to the size of the actual beetles. Aren't they kind of cumbersome to tote around? I mean, I know they need them to fight and to bar off the tunnels and prevent other males from getting in, but they also must really get in the way.
Mr. EMLEN: They do, and so to try to give you a sense for how much they get in the way, the analogy I like to use is that for some of these beetles, it would equivalent to you producing another leg and wearing it around on your head for your entire adult life.
Mr. EMLEN: That's the best analogy I can give you because when you - if you try to estimate how big these things are, in some of these cases the horn, the weapons, can be 10, 15, 20 percent of the body weight of the animal. That's a fifth of the total weight of the animal is allocated to this one thing.
And so it really is analogous to an extra leg on your head for your entire adult life, and it has to be costly. So we've used a variety of types of experiments to try to measure how costly it is, and we found, for example, if a male beetle puts a ton of resources into making a horn, it means that he has to stunt something else.
There's not enough resources left, in a sense, and so in some of these beetles, if they have really big horns, they have really small eyes. We found that in beetles that have horns on the back of their head, their eyes were 30-percent smaller as a result of having allocated everything to the weapons.
In another species, they actually trade off with the testes or with the wings. So there are costs associated with producing these big, bulky things.
GROSS: So what do these dung beetles say about evolution? What do you feel like you've learned about evolution from studying them?
Mr. EMLEN: Honestly I'd have to turn that on its head and say what haven't I learned about evolution from them. These beetles have taught us, and me in particular, an amazing amount of lessons, some good and some more frustrating. But the biggest question that I have been focusing on and that I think I've learned a ton from these beetles on has to do with the backdrop of animal diversity.
It's very hard to be an evolutionary biologist and not look around you and become sort of increasingly aware of the differences among animals that are out there. There's so much variation in the ways things act, the way they look, and it is sort of amplified, in a way, in these beetles.
The variation in shape, especially connected with these weapons, is truly stunning in this group of beetles. And so as a biologist, I'm obsessed with understanding where this diversity in form comes from. How did these big, gaudy structures arise, meaning what do the beetles use these for? How - you know, once we know how they're used in natural populations, we can ask questions like how did they evolve? Under what types of ecological or environmental circumstances are these weapons likely to evolve?
And then that leads us to the next question of why don't all these beetles have the same weapons? What is it about them or about their history or the ecologies and behaviors of these animals in the past that has caused weapon evolution to go one way in some populations and a different way in other populations - and for it to go in enough different ways in enough different species that we end up with the truly thousands of types of shapes and forms and locations for these weapons on their bodies.
And so there's sort of fundamental biological questions connected with animal diversity that we are tackling with these beetles.
GROSS: Well, Doug Emlen, thank you so much for talking with us about the beetles that you've been studying.
Dr. EMLEN: Oh thank you. It's been my pleasure too.
GROSS: Doug Emlen is a biology professor at the University of Montana. Our interview was recorded last May. If you'd like to see a dung beetle fight show and the video of two beetles battling go to our Web site at freshair.npr.org, where you can also download Podcasts of our show.