June 21, 2011
This interview is an excerpt from The WildLife with Laurel Neme, a program that explores the mysteries of the animal world through interviews with scientists and other wildlife investigators. "The WildLife" airs every Monday from 1-2 pm EST on WOMM-LP, 105.9 FM in Burlington, Vermont. You can livestream it at theradiator.org or download the podcast from iTunes, laurelneme.com or laurelneme.podbean.com.
Dr. Laurel A. Neme is also the author of ANIMAL INVESTIGATORS: How the World's First Wildlife Forensics Lab is Solving Crimes and Saving Endangered Species.
This interview originally aired June 7, 2010.
As a group, dung beetles are incredibly diverse, with many specialized to eat feces from specific animals. For instance, the species Uroxys gorgon clings to the furs of sloths waiting for their ride to defecate. When it does, the beetles drop down with their prize. An Australian dung beetle is adapted to do the same with koalas.
Dung beetles are a critical part of nature’s “clean-up crew.” They not only remove manure, which is a magnet for flies and other pests, but also recycle nutrients back into the soil. In parts of Texas, for example, dung beetles bury 80 percent of the cattle dung. Given that a single domestic cow drops about 12 “pats” a day, the range would be littered with feces were it not for these “workhorses.”
Despite their big job, dung beetles themselves are relatively small. The smallest is less than 0.1 inch (2 mm) and the largest around 2.5 inches (6 cm). Yet they’re surprisingly strong for their size. They typically can roll 50 times their own weight, but often it’s much more. For example, a Onthophagus taurus male can pull over 1,140 times its body weight. That’s the equivalent of a 180-pound man pulling over 200,000 pounds (90 metric tonnes).
Doug Emlen with Japanese rhinoceros beetle (Trypoxylus dichotomus). Photo credit: University of Montana.
Douglas Emlen is a biology professor at the University of Montana. His research on the evolution and development of extreme morphologies in insects started with his PhD research, when he studied a Panamanian dung beetle specialized in howler monkey scat. Emlen even “mimicked” the dung beetles when he learned to track the monkeys by smell so that he could locate their first feces of the morning – and thus find the dung beetles that were the real object of his desire. He has studied dung beetles all over the world, and has a particular interest in the patterns of their weaponry. More recently, he has expanded his focus to the evolutionary forces that make animal armaments – from dung beetle horns to elk antlers to rhino horns – so diverse.
DUNG BEETLE DIVERSITY
Laurel Neme: What are dung beetles?
Doug Emlen: Technically, they are a group of scarab beetles in the Scarabaeidae family. The entire group of these beetles feeds on dung in one way or another, although there are a few who have branched out into fungi and other crazy things. But most of them feed on dung, both as adults and also when they are larvae. …
Dung is actually a very nutritious food source compared to plant leaf material or other things like fungi that they might feed on. Dung is nutrient rich. In particular, it is rich in nitrogen, which an immature animal needs to grow. You need nitrogen to build proteins, and you need proteins to build tissue. Growing animals need nitrogen, and dung is a far better source of that than anything they can feed on except maybe carrion.
Laurel Neme: Do different types of dung beetles feed on different types of dung?
Doug Emlen (in green shirt on right) in the Serengeti collecting dung beetles with students. Photo credit: Don Christian.
Laurel Neme: How many dung beetle species are there?
Doug Emlen: There are probably over 10,000 species of dung beetles, all told. In the genus I work on, Onthophagus, there are over 2,000 species already described by taxonomists, and they estimate there's another 2,000 species yet to be described. As far as I can tell, it is the largest single genus of anything anywhere. Period.
ROLE OF DUNG BEETLES
Laurel Neme: What role do dung beetles play in the ecosystem? Would we miss them if they were gone?
Doug Emlen: The simple answer to that is: yes, we would miss them a lot. For the simple reason that there are a lot of animals, particularly large herbivores, producing copious quantities of dung and it would pile up. In fact, historically it did pile up.
Laurel Neme: Is there a true-life example of what happens without dung beetles?
Doug Emlen: The best story I can give on this one took place in Australia a number of years ago, when they cleared back forests and created pastureland for cattle. It just so happened that the native dung beetles in Australia were all forest specialist species, and none of them would go out into the open. They couldn't handle the dry open habitats of pastures. They weren't acclimated or adapted to those types of environments.
Dung beetle in Gabon. Photo credit: Rhett Butler.
The fly density got really bad and Australia realized they had to do something to get rid of the manure. The Australian government’s equivalent of the US Department of Agriculture here went to great lengths to find ecologically sustainable species of dung beetles that they could pull from Europe or Africa and introduce into Australia to get rid of the problem. My recollection is that it was in the late 1960s or early 1970s when they went actively into the process of mastering and releasing these beetles. They've now been established in many parts of Australia for the last 30-40 years. So, we actually know from human history precisely what happens when you remove the dung beetles, and it’s not pretty.
Laurel Neme: Was there any negative impact from that introduction?
Doug Emlen: I'm sure there must’ve been some. But I'd have to say, compared to other types of biological introductions that I’m familiar with, this is the best success story I can think of. The reason I think it was a success story is that they actually paid attention to the biology of the animals. They didn't just, quick and dirty, go grab some beetles and throw them into the ecosystem. They did their homework, and what they found out is that the species in Australia were all forest specialists and none of them would cross that barrier into the pastures. So they went to Africa and Europe and said, “let’s find pasture specialists that won't cross into the forests.”
They [also] did all these trapping grid tests. They tried to find species they could find in the open. Then they would also set traps up in the woods and if they ever caught these same species in the woods, they'd reject that species as something that wouldn’t stay in the pastures. They were able to find the inverse – pasture specialists – species that were reluctant to go into the shaded, more humid understory of forests. These were the species they focused on.
They also went through all kinds of quarantine procedures. I think they treated all the eggs with formaldehyde and things to make sure they didn't introduce any pathogens or soil microbes that would have been detrimental.
They brought these things in and reared them in captivity until they had huge numbers and released them. And as far as I know, it’s worked – they've been able to establish non-native dung beetles in the pastures, but the non-native species have not had a detrimental impact on the native species because they haven't crossed that barrier and moved into the forest.
Now, some of the native species are in trouble but it’s not because of the new beetles. It’s because of habitat destruction and the fact that there’s not as much forest as there used to be. But the species they introduced, as far as I know, worked. They did the job that was asked of them, and they did not detrimentally impact the native dung beetle species.
THE STOUT LITTLE DUNG BEETLE
Laurel Neme: What do dung beetles look like?
Doug Emlen: ...The genus [of dung beetles] that I work on stands out because they are like little squat tortoises. They are very stout, round beetles and they waddle in this deliberate, awkward way. We've looked for these beetles on five continents and in 15 different countries, and once you know the look, it doesn't matter what species your dealing with out of the 2,000, you can spot them in a heartbeat. They are these squat beetles that waddle in this very characteristic awkward style that makes me think of little turtles walking around under the manure.
Laurel Neme: How big are they?
Doug Emlen: Often they’re tiny. [A lot of the species in this genus] are the size [and shape] of an eraser on a new pencil. In some environments and habitats they get quite a bit bigger, so you could be dealing with ones that are the size of a marble, but most of them are pretty small.
DUNG BEETLE HORNS
Laurel Neme: What is unique about the Onthophagus genus of dung beetles?
Doug Emlen: This genus is the most diverse.… [Also] noteworthy is how many of the species have these really cool horns. … Males compete with each other over access to females, and this has generated very strong selection for weapons. Analogous to antlers in deer or elk or moose, the beetles have male horns. The horns vary a lot. They can be curved or straight, and branched and forked. They can come from the front of the head, the middle of the head, or the back of the head, and from the thorax. There are so many kinds of horns coming off of these beetles.
For a biologist like me, that’s amazing. It tells us that something special is happening. They’re evolving and changing much faster than all the rest of the beetle bodies. That is what we're trying to figure out: what is going on with these weapons? How is it that they change so fast when the rest of the beetles look pretty much the same.
Laurel Neme: How big are their horns?
Doug Emlen: The horns can be pretty impressive. They can be huge. Again, the analogy I'd like to use is that some of these horns are easily 15-25 percent of the body weight of the animal. That’s not trivial. If you do the math, that is equivalent to a human producing another leg and wearing it around on their head for their entire adult lifetime. That's about how much weight and mass we're talking about that these beetles are allocating to the horns. They are not trivial structures.
Laurel Neme: How does that compare to other animals, like a moose or an elk?
Doug Emlen: I'm going to get in trouble here but I'll guess that even the best bull elk would have a rack that is probably less than five percent of it's body weight. So, in terms of bulk, the beetle horns are bigger but in terms of length, not necessarily. Some of the full six and seven point bull elk have antlers that are probably almost as long as the rest of the males’ body, and that is on par with these beetles. The horn lengths would be about the same. But in terms of the proportion of their body mass, the beetles take the cake. They win.
EXTREME WEAPONS OF DUNG BEETLES
Laurel Neme: What are some of the horn varieties you might find on dung beetles?
Doug Emlen: There are species that have a single horn, like a unicorn, on their heads. [The horn] comes up and wraps all the way over their backs. In some of these, the horns look like little toothpicks crawling up over the backs of their heads. The horn is actually almost twice as long as the rest of the beetle. It extends the full body length beyond the beetle behind it.
Laurel Neme: How can it then maneuver around and walk?
Doug Emlen: That's a good question. They’re not very agile animals. As I said, they walk like tortoises.
Laurel Neme: So, they’re not the most athletic of beetles.
Doug Emlen: It depends on how you define athleticism. They are athletic, in the sense that a weightlifter is athletic, because they are very strong for their body size. But they are not agile or fast.
Laurel Neme: What's your favorite dung beetle?
Onthophagus rangifer. Photo credit: Erick Greene..
Laurel Neme: What are some other strange horn types?
Doug Emlen: [There] is a species that is African but it's been introduced to Hawaii, and it has a horn that comes from the thorax. … If you picture yourself like a tortoise-shaped dung beetle, the thorax is the shoulder blade behind your head. [The horn would] come from between those shoulder blades and extend over your head. It’s a long curved horn, again like a toothpick, that extends in front of the beetle. In this case, these horns can be longer than the rest of the beetle. It's like having a jousting javelin or spear in front of your body all the time.
There are others that have a horn coming from the thorax. In these beetles the horn is like a big wide shelf, instead of a spear. It’s like a huge board coming forward over their heads that ends by splitting into a series of branches or tines, but it’s really like a wide shelf.
There are [also] horns that are like tusks that come off of the front of the head. They look like the tusks you'd have on a wild boar. And there are species that have a horn that looks like a spatula coming out from the front of the faceplate or the lip of the front of the head. Honestly! It’s like a curved spatula. You wouldn't use it for pancakes because it curls around, but it’s the shape of a big triangular spatula and it sticks in front of the face of these things. There are all kinds of variation in the shapes of the horns.
WHAT’S THE LOGIC?
Laurel Neme: What's the purpose of these different horn shapes?
Doug Emlen: You're not allowed to ask that! After all these years, I still don't have a good answer. [Laughs] Not for a lack of trying! That question is the thorn in my side. But it’s okay. That’s what scientists want. It keeps us challenged.
There are some things we can answer and there are some things we can't. One thing we're fairly confident about, that we think we understand, is why they're big.
Laurel Neme: Why are dung beetle horns so large?
Doug Emlen: For most of these beetles, the way the mating system works is that the females will find the dung and they'll dig a tunnel into the ground underneath the dung. This is a species with parental care. Once they've dug the tunnel, the female will spend days pulling pieces of dung down into the ends of the tunnel and packing it into a sausage-like brood mass or brood balls. … [Then,] once she's made it, she'll hollow out a chamber at the end and make a little tiny stalk, and then lay an egg in this stalk. So [a single egg sits] inside this little chamber, not touching the sides of anything. … Once the egg hatches, the larvae lives inside that sausage-like brood mass and it eats it completely as it develops. It does its entire developmental period on that amount of food that the female has stored underground. When it’s done developing, it will crawl to the surface and repeat the cycle.
A female can spend a lot of days and make hundreds of trips pulling dung into the tunnel and making these different brood balls. Then she will fill the tunnel behind her and she'll make another, and fill the tunnel in behind that and make another.
As this type of provisioning behavior is going on, a male will try to guard that tunnel and try to keep everybody else away. He'll [also] mate with the female as often as he possibly can. Every time she comes up to the top to get dung, he'll try to mate with her. He'll often try to keep her in the tunnel. This is one of the ways that you get bi-parental care in this system. The males will go out into the dung and pull fragments of it back and cram it into the tunnel just [it appears] to keep the female trapped. The males will help provision the tunnels, and then the females will take the dung from the top of the tunnel all the way to the end of the tunnel and actually make the brood balls.
So, you've got this tortoise-shaped beetle guarding this cylindrical tunnel, and the job of that male is to keep all other males out. That’s where the horns come in. They use the horns in fights. Rival males will challenge these [guard] males and try to get past them into the tunnel. That’s when the fights take place. These beetles fight surprisingly viciously. They’re really fun fights to watch.
DUNG BEETLE BATTLES: NOT AN ELEGANT DANCE
Laurel Neme: Can you describe a fight? [Laughs] It sounds like Gladiator or Robin Hood...
Doug Emlen: Except they’re not quite as agile. They fight like turtles. They aren't the most impressive things to watch because you're looking at two eraser-sized black animals in the dark. But the fights are pretty interesting. The best descriptor I can use is chaos.
Onthophagus nigriventris. Photo credit: Doug Emlen.
They push and they pry and they twist and they try to dig past each other and the other ones are prodding and prying with horns and they scramble. Sometimes these beetles are so awkward in these tunnels that they can't even turn around. A big male can't turn around. Sometimes, if he’s facing down the tunnel and another male comes up and bumps into him from behind, he can't even turn around to engage that male. What he has to do is back out of the tunnel, forcing the other male with him until he gets to the surface. Then he can turn around and back into the tunnel and then they can actually fight proper. So they twist and pry. And sometimes they can push the sides of tunnel so far apart that they can turn around or twist.
DOES HORN SHAPE MATTER?
Laurel Neme: What does that mean for the horn shapes?
Doug Emlen: We thought when we saw those fights we'd get magical answers as to why you'd want a twisted horn or a long horn or a horn with branches or a spatula horn on the front of your face and [laughs], we've come up empty. The male with the bigger horns usually wins.
Laurel Neme: So size does matter? [Laughs]
Doug Emlen: Size does matter. Body size really matters. The bigger beetle always wins, to a close approximation. But once you control for size, if you have two beetles the same size, then weapons matter, and the males with the bigger weapon wins. There have been a number of studies done by a number of people who have found the same things. The big horns really do matter, and they help the males win.
We have a system we would call directional sexual selection for weapon sizes that are big. If your weapons are bigger than everybody else's, then you've got an edge. You can win these contests over the tunnels and keep everybody else at bay. You can mate with the female and, presumably, there’s a reproductive advantage.
Laurel Neme: Does horn shape make a difference?
Doug Emlen: We have no way of knowing why one horn shape works better than another. If they locked together in that lock and key fashion, then you can envision how horn shape could matter a lot. … But these things don't do that. … There’s no obvious reason for a particular shape. As near as I can tell, horns are like bars on a jail cell. They help anchor the beetle into the tunnel and make it more difficult for the other beetle to get past. Pretty much anything that can be stuck into the side of the tunnel helps. With these things, bigger weapons seem to work better than smaller weapons, but why one species has a triangle and another has a branched horn we don't know yet.
Laurel Neme: Do you find any species fighting with another species?
Doug Emlen: Not as often as we'd like. That's one of the reasons we haven't been able to do really well [at figuring out] why horns have different shapes. We can't just pick a species with a thorax horn and a species with a head horn [and see who wins]. … Because we don't get multiple horn types within a single species, and because males of different species rarely engage in fights, we haven’t been able to test what happens when one horn type fights another.
Laurel Neme: Could you stage fights between species with different horn shapes?
Doug Emlen: We have tried to stage them that way, and we’ve tried gluing horns on the beetles. We probably will be able to see that with the rhinoceros beetles because they’re very big. But these little dung beetles [are too small]. We tried superglue but we have not been able to engineer or manufacture a biologically plausible version of a dung beetle with the wrong type of horn yet.
So the answer is no, we don't get species fighting each other as much as you would like. We haven't seen circumstances [that can tell us] if shovel horn will win over a curved horn or a straight horn.
DUNG BEETLE ADAPTATIONS
Laurel Neme: What adaptations do dung beetles have for their life underground?
Doug Emlen: They spend a lot of their life underground. First of all, as larvae. Their entire developmental period is spent underground. Then they come above ground and they have to disperse. These things can fly. These are flying turtles with horns.
Laurel Neme: [Laughs] You forgot to mention that part!
Doug Emlen: They definitely can fly. [Laughs] These aren't swallows or agile birds, but they can fly—and they need to fly. It’s essential because dung isn't a resource (except for in Australia after decades of no dung beetles) that is everywhere. You have to spend time [finding it]. Dung appears suddenly on the landscape – over here and then it appears over there on the landscape – but the beetles have to find it.
They’re able to use the odor cues to find the food sources, and they can find them really fast.
Laurel Neme: How fast?
Doug Emlen: In the tropics, where I was studying them, …15-30 seconds after howler monkey dung appeared on the ground you'd already have the first beetles coming in. They’re very good at finding this food source because they have to compete with all these other animals to get to it, and so they do fly. They’re above ground for that time period. But once they find the food [dung], they burrow into the ground and hangout underground.
All the interesting stuff that we were focusing on—who’s mating with whom, what are they doing in terms of parental care, who’s fighting with whom, why the horns, all that stuff – happens underground.
Laurel Neme: What happens after that?
Doug Emlen: Well, the dung only lasts a few days until its gone. At that point, once they've buried all they can, the process repeats. The males and females will disperse and the whole cycle repeats itself when they find another piece of dung. They alternate between being below ground and above ground.
SENSE OF SMELL IN DUNG BEETLES AND OTHER INSECTS
Laurel Neme: Do they have special adaptations for smelling, to be able to detect dung?
Doug Emlen: Yes. They definitely have an exquisite sense of smell. Although I haven't worked on this, and I'm not sure that I can think of anyone who has directly worked on this, they almost certainly have had to specialize in the chemical receptors that they have on their antennae that they use for smell. If one of the main ways for these dung beetles to diversify is by specializing on different types of dung – so that there are some that will feed on wildebeest dung and some on seabird dung and some on giraffe dung and some on elephant dung – if they’re specializing like that, it’s almost certain that the characteristics are different among the dung sources, especially with respect to smell. Since they use odor cues to find their food, one of the key traits that has got to be differentiated among these different species of beetles is the particular smells that they hone in on to find their food source. Although I'm guessing on this, I'm willing to bet there has been a ton of evolutionary specialization for their olfactory capabilities. They’re all good at finding dung. But the particular types of dung they find differ depending on the biology of each species.
Laurel Neme: Are hairs the chemical receptors or do they have noses?
Doug Emlen: One of the chemical receptors you find in insects generally is on the hairs that they have on the outsides of their bodies. The hairs often have pores in them, and the chemoreceptors inside those pores are hooked up to neurons. ... Another thing that’s neat about insects in general, and these beetles for sure, that’s very different from ourselves and vertebrates is that we smell with our noses and taste with our mouths. Our chemoreceptors are concentrated within our mouths and our noses. [In contrast,] insects can taste and smell all over their body. There are insects with chemoreceptors on their heads or their wings, so they can pick up chemicals from the air as they fly. Many insects have pads on their feet covered with these sensory hairs, so they can essentially taste and smell and respond to chemicals on everything that they land on. [For example,] the housefly that lands on the edge of your glass can taste the residues on the edge of that glass through their feet. They will often have hairs on the undersides of their body, all around their mouths, because that’s a very logical place for them to be tasting.
But the primary organs of smell in insects are thought to be the antennae. The antennae are thought to be covered with sensory hairs that all have these pores, these chemoreceptors, on them.
Some of the studies on insects, for example male moths, [have found] they’re phenomenally sensitive to chemicals. If I'm not mistaken, they've found moth antennae that are so sensitive that they can detect individual molecules of the mating pheromone of that species. I don't think we've been able to engineer artificial sensors even within orders of magnitude of that level of biological sensitivity. So, insect antennae are impressive.
I'm not sure dung beetle antennae have been studied as extensively as the moths, but they clearly are master organs of smell. They really live and die by smell. They find their food by smell. They spend a lot of their lives trying to track down food. There is so much competition for dung that they've got to find food sources very fast. That means there is going to be a selective premium on individuals that are unusually good at detecting smells. You put that on to a history of animals that for the last forty million years or so have been specializing on all different types of dung and they have an incredible sense of smell.
DUNG BEETLE BULLDOZERS
Laurel Neme: What adaptations help dung beetles dig?
Doug Emlen: This is where the turtle shape comes in because they actually are like little bulldozers.
Laurel Neme: How does their turtle-like shape and biology help dung beetles dig?
Doug Emlen: A turtle is a robust animal with a big stiff shell on the outside. If you're going to work your way down into the soil you need to have some tough architecture or armor on the outside or you're going to get torn to shreds. Dung beetles, too, have very robust, thick exoskeletons. They have very strong forelegs, and the forelegs of the dung beetles have blades or “teeth.” One of the diagnostic [characteristics] of the genus Onthophagus is that there are four teeth on the front legs of these beetles, they look like triangular blades, and they literally carve out the clay and the soil underneath them. They’re very, very fast and efficient at it.
A bulldozer is a good analogy because a bulldozer has a big curved blade down the front and it can push through the soil, and you don't get all that dirt clogging the engine, tearing up the axles on everything, and destroying the path where you are because the blade protects it. The dung beetles’ head is really like a bulldozer blade. If you look at them closely under a microscope, the front of these animals really is a bulldozer blade, and that’s the top of the head. You can see a tiny bit of the eyes. There is a little slit that pokes through on the front of that blade, but pretty much everything else that matters is underneath—its compound eyes, mouth parts and antennae are all things that can fold up underneath that blade.
These things are built like bulldozers. They've got the front legs that they can carve into soil with, and they've got this big, curved flat slab of a head that’s like the blade of a bulldozer they can use to plow through the soil or to push again the walls of the tunnel, and it protects all of the delicate sensory structures of their head underneath.
Laurel Neme: How strong are they?
Doug Emlen: These things are strong. You can take a pencil eraser-sized dung beetle and hold it in your hand as tightly as you possibly, and I guarantee you that beetle can crawl out of your hand no matter how many pounds you can bench press. I’ve done this and I’ve challenged bigger, stronger men than me to do this. I promise you, that beetle is so strong that it can force its way out of your hand. These are things that are used to digging into hard packed clay.
Laurel Neme: I'm imagining you challenging this beetle [Laughs] …
Doug Emlen: [Laughs] Who’s stronger! It's a very humbling challenge because I always lose.