Some Dung Beetles Carry Parasites On Their Genitals, And It’s Not A Bad Thing

11:48 minutes

dung beetle on palm of a hand
A dung beetle. Credit: Daniel Peterschmidt

Nematodes and dung beetles interact a lot in nature. The small worm-like parasites hitch a ride on their insect partners as they fly from one dung patty to another. And until recently, scientists had assumed this relationship wasn’t as positive for the dung beetle. To get around, the nematodes actually ride on the beetle’s genitals. The parasite can even be passed on to future offspring or other other sexual partners, like an STD.

[Must the stars and planets align for your pie crust to emerge perfectly flaky and golden brown from the oven? Science can help with that.]

While dung beetles put up with a lot of crap, it’s hard to imagine what good could come from a relationship with a parasite. But Cristina Ledón-Rettig, assistant research scientist at Indiana University says there may be one very good reason for letting nematodes climb aboard. She joins Ira to discuss her latest work.

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Cristina Ledón-Rettig

Cristina Ledón-Rettig is an assistant research scientist in the Department of Biology at Indiana University in Bloomington, Indiana.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. I’ve got a good idea. The weekend’s here. Let’s talk about nematodes and dung beetles. Why not? They interact a lot in nature. The small, worm-like parasites like to hitch a ride on their insect partners as they fly from one dung patty to another. Sounds like a pretty sweet deal for the nematode, right?

But perhaps not so much for the dung beetle. See, the nematodes actually ride on the beetles’ genitals, where they can be passed on to their offspring or mates like an STD. Now, dung beetles put up with a lot of dung.

But what do they stand to gain from this relationship? My next guest says, as icky as it sounds, there might be a very good reason for letting those parasites climb aboard. Dr. Cristina Ledon-Rettig is assistant research scientist in the Department of Biology at Indiana University in Bloomington. Welcome to Science Friday.


IRA FLATOW: So tell me about this ride-sharing arrangement between the dung beetle and the nematode. How do these two organisms interact?

CRISTINA LEDON-RETTIG: Gladly. Well, when a dung beetle is ready to reproduce, they tunnel underneath a dung patty. And they create these developmental chambers out of dung that we call a brood ball. And in each of those brood balls, they place a single egg. And from being an egg to an adult beetle, that developing beetle depends on this developmental chamber for its nutrients, for its shelter, and protection.

And nematodes ride along on these beetles. They’re small worms, and they end up in these developmental chambers along with the developing beetles. And we didn’t know before whether that was a good thing or a bad thing. But we did know that this association existed. And so this was something that we wanted to investigate more to understand the consequences of that interaction.

IRA FLATOW: So tell me, tell me, what did you learn?

CRISTINA LEDON-RETTIG: Well, we looked on all different parts of the beetle for nematodes, and we found them on various parts. But we were particularly interested in this really high concentration of nematodes that’s found on the genitalia. It’s a good place for nematodes to hide. But also, it’s a really compelling venue for getting to the next generation because it’s involved in reproduction.

And so what we found was not only are these nematodes passed between males and females, but they’re also passed off to the offspring. And the offspring that develop alongside these nematodes actually grow faster and larger than their counterparts that don’t have these nematodes.

And so there’s actually a fitness benefit to having this– I wouldn’t even call it a parasite because under any conditions that we tested, we didn’t find that the nematode was actually bad for the developing beetle. In the conditions under which we tested this, we always found that it was a good thing to have it along. So it’s more of a mutualist, really.

IRA FLATOW: Yeah. I was wondering about that. Usually, a parasite is something that sucks away the life of its host, right? But this is not what’s happening here.

CRISTINA LEDON-RETTIG: No. And that was– nematodes are almost infamous for being parasites. They’re parasites of humans, of cattle, of plants. And a lot of our attention is directed towards that sort of negative interaction. But less investigated is ways in which they can be either neutral or even beneficial to their hosts.

IRA FLATOW: So how can you tell all this is going on, especially when it’s taking place inside a dung patty?

CRISTINA LEDON-RETTIG: Right. So it’s actually taking place underground. So the mothers tunnel underground, take that dung into these tunnels, so it would be very hard to assess that under natural conditions. But fortunately, in the lab, we have a culture system where we can make little replicas of these brood balls and transfer eggs into them. And the power of this is that we can create conditions that we can either include or exclude nematodes. And that allows us to understand what the consequences of these nematodes really are.

IRA FLATOW: Were you surprised by anything you found?

CRISTINA LEDON-RETTIG: Yeah. Well, we didn’t really know what to find because there hadn’t been this type of system where you could so easily manipulate nematode presence before. On one hand, we were surprised. But on the other hand, people have kind of suspected this for several years because nematodes are so widespread on insects. You know, there was definitely the possibility that these close interactions during development could be beneficial.

But what thwarted our understanding of whether those consequences were good was actually having a system that we could bring into the lab and watch the development and interaction between these two species more closely.

IRA FLATOW: Now, these nematodes live on other insects too, right?

CRISTINA LEDON-RETTIG: That’s correct. They’re found on certain species of bees, termites, a lot of other beetles, and they’re not the same species of nematodes. There’s lots of different species of nematodes. But as idiosyncratic as it seems, genitalia worms are not a rare thing in the insect world.

IRA FLATOW: Do we know how they– why they chose that, how shall I put it, part of the anatomy to attach to?

CRISTINA LEDON-RETTIG: We don’t know precisely why, but the consequences of being on the genitalia are kind of twofold. I mean, it’s a nice place to be because nematodes like moisture, and they’re protected as they are in transit between different dung pats. But I think more importantly, in this case, is that it’s a very surefire way to get passed between individuals.

For instance, if you’re on a male, that’s not a great place to be because you might die there. But if you get transmitted to a female, then you have the potential to be passed on to the next generation with the eggs. And that’s a good thing for the nematode.

IRA FLATOW: So it’s helpful, healthful for the dung beetle to have the nematodes around? They are healthier. They grow bigger. They contribute to their lifestyle, so to speak.

CRISTINA LEDON-RETTIG: Yeah. That’s correct. They seem to– it seems to contribute to better health. And we searched for a mechanism for why that is so. And what we looked into was the ways in which they influence the microbiome, so the microorganisms that are also in these developmental chambers. And we found that nematodes are actually really good at engineering the bacteria and fungi that are in these developmental chambers in ways that we’re hypothesizing are good for the developing beetles.

IRA FLATOW: What do they– is it healthier? Do they give them nutrients in the dung? I mean, they produce the stuff that the dung beetles eat?

CRISTINA LEDON-RETTIG: Possibly. So what we think is happening is that some bacteria are bad for beetles, but some are good in the sense that they have this arsenal of metabolism that is– could be useful in breaking down plant material.

And plant material is not a particularly easy diet for any organism to eat. But bacteria are really good at breaking down plant material. And so the beetles secondarily benefit from that because the beneficial bacteria are breaking down the plant material that, then, the beetle can feed on.

IRA FLATOW: Not all dung beetles have nematodes though, correct?

CRISTINA LEDON-RETTIG: Not all species have them. From our own studies, we found that some species have them, some species don’t have them. The species that have them have them at different levels in different populations. So the incidence of a population that I’d found in North Carolina, the beetles there– many more of them have these worms than, for instance, the beetles in Indiana.

IRA FLATOW: People can get worms. Have you looked in ways these worms could help people?

CRISTINA LEDON-RETTIG: Not personally. But there’s actually a lot of really exciting science that’s coming out about how worms do benefit people. So they’ve been finding that in many cases– because obviously, worms have some bad effects on people, as well. But in cases where they’re not killing people and people are actually tolerating worms, those populations tend to have a lower incidence of autoimmune diseases.

And they’re actually starting to suss out the mechanisms behind this. In order to not be ejected from humans, worms have evolved a way to suppress our immune systems. And so people who are predisposed to having autoimmune disorders like multiple sclerosis, or Crohn’s disease, or type 1 diabetes actually benefit from this suppression of their immune system. So–

IRA FLATOW: So in other words, you’re saying people in societies where the people have these parasites in their system are getting less sick from these other diseases.


IRA FLATOW: So we’re not saying go out and get a, you know–

CRISTINA LEDON-RETTIG: No, no. So definitely, these worms also have bad effects, too. But what scientists have been trying to do is find the certain chemicals or proteins that the worms are providing to these hosts to suppress their immune systems and just give patients those chemicals or proteins instead of the whole worm.

IRA FLATOW: Did you always have this interest in dung beetles? Or did you follow them as a kid on the ground, or–

CRISTINA LEDON-RETTIG: I kind of follow all organisms that I can find, or catch, or see. And it wasn’t until I started talking to my nematologist friends that I realized that these organisms that are a little harder to see are actually really interesting and can be found all over the place. And we’re just starting to scratch the surface of why they’re important to human health and ecology in general.

IRA FLATOW: So where do you go with your research from here?

CRISTINA LEDON-RETTIG: That’s a great question. We are definitely in the process of finding how widespread this phenomena is by looking at other species of insects. But likewise, we’re really interested in how they’re engineering the microbiome and whether it’s because they’re eating certain bacteria, allowing other bacteria to grow, or if they’re emitting chemicals that suppress the growth of certain bad bacteria, or enhance the growth of good bacteria. That’s a little bit of a black box that we don’t really understand yet.

IRA FLATOW: Well, we wish you good luck.


IRA FLATOW: And do you have a lab that’s full of dung beetles? I mean, is–

CRISTINA LEDON-RETTIG: They’re very surprisingly easy to keep. You can keep–

IRA FLATOW: Are you recommending we at home– because we’ve got 2 million people listening. Should they go out and get some dung beetles as pets?

CRISTINA LEDON-RETTIG: Sure. If there’s a farm nearby, you probably can find a dung beetle if you’re willing to sift through a little bit of cow poop.

IRA FLATOW: I can’t top that for an ending to this segment. Of course, we have done a show on dung beetles. We have some video up on our website. So we’re very happy to talk about dung beetles. And they are fascinating creatures, are they not?

CRISTINA LEDON-RETTIG: They are incredibly diverse and amazing, I think.

IRA FLATOW: All right. Well, we think your research is amazing. Dr Cristina Ledon-Rettig, assistant research scientist in the Department of Biology at Indiana University in Bloomington. Have a great weekend.


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