A Walk Through the Forest

The following is an abridged version of Alex Chadwick's discussion with Rex Cocroft and his colleague, Dr. Chung Ping Lin, as they search for treehoppers in the Amazon:

'Radio Expeditions' Series

Rex Cocroft: If you come to a tropical rainforest, most people's first priority is above all, avoid any contact with insects. And with some insects that may be true, but this is a particular group of insects that are very engaging. Even people that don't like insects say, "Oh, well that's kind of cute." They're harmless to people, they have fascinating behavior, and are well worth going out of your way to see.

Alex Chadwick: And beautiful, when you look at them up close, very beautiful.

RC: Last night I was just walking along with my headlight. It's pretty difficult to find at night but I did just see this insect which looks a bit like a thorn on a plant. But what makes it stand out is that it's a plant that completely lacks thorns. It's about half as long as your fingernail. It's a little thorn shaped creature, and it's feeding on the sap of the plant. This is a really great place to find them. It's just the fast growing end of a vine which is nutritionally very good for these insects.

AC:You haven't gotten very far, Dr. Cocroft, before you've stopped three times. We've only traveled 20 feet.

RC: Well, I knew there was a good reason to come to Tiputini, because it's got more different kinds of plants than anywhere else. And it's going to have more different kinds of plant feeding insects. So here on this very new growth on this vine, there's a group of about half a dozen adults of two species of the kinds of insects I'm looking for. So here's another genus. Well, if you want to blindfold me for a minute, we'll see if we can make it out of the camp. [Local guide] Henri has just pointed out a large aggregation of very beautiful, very colorful species of membracids that are tended by ants.

AC: Wow, I can see them from here.

RC: They are about the size of a pine nut, but with orange faces and then zebra-striped backs, and they're in a group of 25 or so, with ants very actively running all over them. Down here, one of the big predators of small insect is ants. And you'll notice there are ants everywhere. So they have various ways of dealing with these ubiquitous predatory ants, and one of the ways is essentially to buy them off. These insects have access to sugars and amino acids inside the plant that the ants don't have access to, so they'll provide and excrete the substance called honeydew that has these sugars that the ants find useful, so the ants will harvest all secretions, and in return they protect them as a valuable resource. So one of the things we're looking for here, especially in a lowland area where there are so many ants is to look for a branch that has ants running up and down on it, and then to go and look more closely, and quite often there will be a group of treehoppers being tended. Mind you, the ants will defend them against you as well.

AC: Are you saying they are like cattle to the ants?

RC: Yes, they are very much, and the ants even sometimes move them around from place to place. The protection by the ants is not entirely benign, because the ants' colony needs sugar. Then they'll harvest the sugar. Sometimes they need protein, so things can turn, and they can harvest some individuals for their protein as well.

AC: This trail that we're walking is fairly narrow. There are blocks of wood set out in it, so it's a comfortable way to get through the forest. There are these wooden steps for a footpath, and all around us the forest is growing up, but it's not especially dense. There's light here. You could walk off the trail. You could get through it. It's not encumbered with a lot of brambles, but it's really better to go on the forest path.

RC: Oh, beautiful — there's just a gorgeous adult of the genus membracis. There seems to be an egg-laying female, and there are several egg masses on this plant. The adults have something shaped like half a dinner plate, a sail up over their back, in this case salmon, yellow stripe and black before and behind with a big white spot. A really spectacular creature. And the egg masses look like patches of white meringue, so the eggs are laid just into the stem and covered over with this white secretion which helps to protect them. I would say that this forest, so far, is everything I had hoped it was. After one morning and a few hundred meters down the trail, there's a very diverse interesting membracid fauna. None of the species that are here are ones that I have seen before.

AC: And you think that will mean new signals as well?

RC: What seems to be the pattern in this family is that it looks like there is a whole evolutionary radiation of social signaling. And what I mean by that is that it looks as if closely related species or groups of species have shifted from one habitat to another — and in this case meaning from one using one kind of plant to using another. And then being on this new kind of plant with different growth forms requires a different kind of social life, and a different kind of social grouping perhaps. Along with that will be differences in signals, and it may be differences in what they are communicating about. For example, with a couple of species I've studied in detail, one is a member of a group in which they're on woody stems on large trees, still towards the end where the plant is growing, but back on the woody part of the stem. And they grow up in family groups from 80 to 100 individuals with the female who laid the eggs guarding them the entire time and defending them from predators. And they stay on that one stem for their entire development to adulthood. In fact, the female enhances their food supply by using her ova pother to make incisions in the bark about a week before the eggs hatch. Then when the young come out, they feed on this new growth. So essentially they invest in this one stem.

So they don't communicate about food resources because they're always pretty much on this one place, and the food resources are there. However, because they're stationary and they're on a plant stem, always exposed, they're very vulnerable to predators, all kinds of predators. So their communication has to do with predation. The offspring detect a predator approaching and they begin signaling. The signaling is very contagious within the aggregation, and they produce these coordinated group signals and alert the mother of the presence of a predator. In another species that I've studied, they live on plants that are quite quickly growing, and they also like to feed on the newest growth. And they like to feed at the base of new growing leaves, but these leaves mature in a few weeks, and it takes them a month to grow to adulthood, so they need to move as a group.

They still have a mother there defending them from predators. It's still a group of 40 or 50 individuals growing up as a family together. But when the stem that they're on is maturing and becoming no longer suitable, they need to find a new feeding site. And there are individuals who behave almost as scouts that will leave the group and go and explore other parts of the plant and find a new feeding resource and advertise it with vibrational signals. And then other individuals orient to those signals. They signal once they get there, and they produce this growing chorus, and eventually everybody ends up in this new site.

AC: And the signal is strong enough to go down a stem from where the scout has found the new supply and up another stem to the base of the leaf where the rest of the group is.

RC: Yes, it's all on the same plant, and their communication is mostly restricted to within the same plant. And the plants they are on are maybe one, two, or three meters tall, and the signals travel a few meters through the plant. So in that species, when predators arrive, often they'll scatter. So they're not communicating so much about predators, but they are communicating about food resources. So that's what we mean about species utilizing different kinds of plants, having different kinds of social life, and then evolving communication systems that function in that new context.

Now that's just two species out of 3,200 or so described species in this family, and there's a wide variety of different kinds of social behavior, and so what I've had some indication so far from my previous work and my field notes is that in each of these different kinds of social groups, they use different signals and in different ways. And in some cases, it's still quite mysterious what they're communicating about. But what I'm trying to do by coming to a place like Tiputini is very much a process of discovery to look at this diversity of species with different kinds of social behavior to see what kinds of signaling they're doing. And ultimately to try to get some clues about what they're signaling about. To put this together and be able to see how the ecology of these species relates to their forms of social behavior and to the kind of communication signals that they evolve.

And in a sense when, under what circumstances, do they evolve a new signal that functions in a new context. It's kind of amazing to think about why something that would live for weeks needs to have such a complicated communication system, but in fact in that time period, they do have complex communication systems. They cooperate. In fact, they are so complex, it can take years really to understand all the subtleties of what's happening in one species in which the individuals are living for weeks.

Let's try to find that same nymph we were looking at earlier.

AC: Very good, Rex, you just plucked him off that stem, transferred him to this one, and he hasn't hopped off.

RC:That's their whole world, essentially. They live on the surface of the plant the way we live on the ground, and almost never leave it, except briefly to fly between one plant and another once they become adults.

AC: Well, I think this guy is one extremely handsome membracid with a very interesting anterior structure.

RC: You're definitely developing an eye for membracids. Honestly, it's a bit like developing an eye for art. The aesthetic pleasure to be gained from something like this is there, to be able to appreciate even the differences between species, to know how to look at an insect. They're just marvelous, and the more you understand about them, the more fascinating and even beautiful they become.

AC: When you talk about the aesthetic qualities of these membracids, I think that you mean how physically striking and beautiful they are — but there is also the aesthetic quality of how they sound.

RC: Yes — so at a first glance you see that some of them are beautifully colored or beautifully shaped or just fascinating in some respect, and that's before you ever listen to them. But yes, there is this entirely other aspect that comes through very different senses of their communication signals. And their social signals are very interesting. You can hear these very intense social interactions going on, and their mating signals are often maybe in some cases very beautiful or in some cases very eerie or surprising or even humorous to our ears, but a rich complex set of sounds.

AC: Why is it that a signal from one membracid to another, especially one membracid that's courting another, would evoke in us a sense of wonder, or mournfulness or a sense of understanding.

RC: The insects like membracids and some others communicate with very different kinds of signal structures. Some of them use rhythmic patterns, but many of them use melodic patterns, where they use fairly pure frequencies where they slide up and down the frequency scale, and that's much more like what we experience in our own music, for example. So I think we respond to that very differently than we do to something that's going OOOOooooohhhhh, as opposed to something that's going shhhhhhh – shhhhhh – shhhhhhh. In the first case, it has a more haunting musical quality that we associate more with things we're used to appreciating for their beauty like bird song or whale song.

AC: Why is it they have these harmonics that we like?

RC: Part of the appeal, I think, and what makes that vibrational world so strange to us, is that when we are in this world of airborne sound, we get used to a certain relationship between the size of the animal and how deep its voice is. And just because of the biophysical constraints of communicating in the atmosphere, very small insects or other animals are not very efficient at radiating a low-pitched sound. So we get this relationship where very small creatures produce very high-pitched sounds, and large creatures like bears produce lower pitch sounds. But when we move into the vibrational world, that particular constraint is gone. And although there seem to be some others, on the whole, they use much lower frequency sounds. So a very small insect that might be the size of a sunflower seed can use frequencies like those used by a large bullfrog, or a large mammal. And that's part of what makes them eerie, and gets out attention.

The other is that they seem to use not only rhythm to communicate information. So not just a different number of pulses, or a different rate, but they also use pitch. And part of that may have to do with the way signals transmit through plants. Maybe particular frequencies that transmit quite well. And so some of it may have to do with a signal that transmits well in this plant being in some cases one that is a very pure tone. And so just by coincidence they happen to produce in many cases signals that converge on what for us are very melodious, very attractive signals. And I won't say that the signals of all these insects are exactly beautiful. Some of them are more raucous; they produce surprising combinations of different acoustical elements. But some of them are, and that is something of an unanswerable question, I suppose, or at least hard to answer now – why we find so many natural sounds, bird song or whale song, to be beautiful.

My colleague, Dr. Chung Ping Lin has more field experience with membracids in more parts of the world than anybody in history.

AC: You're such a young man to have acquired so much experience. Where have you worked?

Chung Ping Lin: Most of the time in Central America, and North and South America. Panama, Guatemala, Honduras, and the Caribbean Islands and Ecuador.

AC: And what is it that got you interested in membracids?

CPL: I started my first year of graduate school by just looking at the shape of their horns. There is just such a diversity of colors and shapes. And it's really hard to imagine how an ancestral treehopper would evolve. In the present day, there is such a diversity of different kinds of shapes and colors, and it is very difficult to sort out which treehoppers are closely related to different treehoppers. So recently with the advance of biotechnology, we can start to use DNA sequencing to figure out their relationships.

Rex is primarily working in the New World tropics, and maybe half of the treehoppers are living in the Old World. And most of their behavior and the kind of communication systems they have is largely unknown. So I'm starting to work in the field to record their signals and figuring out their ecology behavior, and trying to put a picture together to see whether there will be a similarity between the Old World and the New World in terms of their communication system, or if they are different, and why they are different.

AC: When you say that you and Lin are studying a previously unknown evolutionary-radiation, what do you mean?

RC: Maybe there's another way to put this. Like most biologists, we're interested in trying to understand the processes that generate and maintain the tremendous diversity of species. In a place like Tiputini, you see at a glance that plants dominate this environment, and in fact they create the environment for other organisms. And that's especially true for the organisms that we're studying, plant feeding insects, which themselves make up a huge fraction of the earth's species. It's estimated that if you collected together all the species of animals in the world, that four out of ten would be small specialized plant feeding insects. So, I've chosen one family, the membracidae, treehoppers, that has 3,000 plus named species, and undoubtedly many more not yet known to sciences.

To understand biological diversity, you can't just have lists of species, we need to understand their traits, their unique adaptations that enable them to survive and do well. In this group of insects, one of the things that seems to be important is the way they are able to survive on plants is to be social, to live in groups. Some of them might live in groups their entire lives, some might live in groups for a little while and then disperse as adults. Some of them live in groups that are defended by a female. Some of them might live in groups that are tended by ants. So that diversity of behavior seems to be related to their ability to exploit a range of different plants. What I've found is that within those different kinds of social groups, individuals are communicating with their other group members. And this communication seems to be important in their lives.

And in process of studying those species, I've also been making field observations and recordings, and I've found there's a surprising diversity in communication signals in these different social groups. So what we're trying to do here, we're very much in a discovery phase. It's very much like what a linguist or anthropologist might do to describe the diversity of languages in an area. And that gives you the big picture. It doesn't tell you the details of those languages or why there or so many, but that's a first step. And then ultimately we're interested in understanding why there is such a diversity of communication systems in this group. So on a trip like this, ultimately we want to see what kind of patterns emerge. There are lots of different species that are communicating about the presence of food resources. And if so, do they use similar signals or different signals? If different, are these just equivalent ways of achieving the same result, or different things in the group that might explain the differences.

Every time I go in the field, I find surprising more aspects of diversity, so we've been here at Tiputini for three days, and already I've found kinds of communication signals very different from any of the ones that I've recorded before. I've learned a lot about vibrational signaling in general and gotten a much clearer idea of the tremendous diversity of communication systems. And that's why we can refer to this as an evolutionary radiation. Starting off with one ancestral species of membracid, it has a set of descendants that have moved into a lot of new habitats. They've evolved different forms of social behavior, and very different kinds of communication signals that I'm convinced, from the ones that we have studied, are very important for understanding why these insects are so successful as herbivores. So I think their communication systems are key to understanding their biology.

AC: You don't think of treehoppers as primitive communicators.

RC: They have exactly as long a communication history as we do. People often do think of insects as simple, or primitive, or like robots. But they're not; they're very complex…they're just very different from us. But they have just as many challenges in their lives, and fabulous, very finely tuned adaptations for dealing with them. So they're not at all primitive or simple. They're actually very complex and advanced if you will.

RC: There's one thing that's unfortunate about this for you. When you come through here with the ability to tap into the vibrational soundscape within these plants, the next time you walk through you're going to be quite frustrated. You're always going to want to carry a phono cartridge and an amplifier and a set of headphones the next time you take a walk in the woods because you're missing 99 percent of the communication that's going on.

Rex Cocroft's collaborator in his project in Ecuador is Dr. Chung Ping Lin, an assistant professor at Tunghai University in Taiwan and a former postdoctoral research working with Cocroft at the University of Missouri. Dr. Lin uses DNA sequence information to reconstruct the evolutionary relationships among species of treehoppers, allowing researchers to make inferences about how treehopper communication systems have changed over time. The graduate students at the University of Missouri helping with research include Paul DeLuca, Jen Hamel, Gabe McNett, Karthik Ramaswamy, Laura Sullivan and Rob Snyder.

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