Teamwork Between Species Is The Key To Life Itself

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Codependency sometimes gets a bad rap, when you become so entwined with your other half that you cannot survive without them. But as it turns out, in nature, there’s a lot of codependency, species that rely on each other for survival.

And my next guest argues that these mutualistic relationships across species are the story of life on Earth. Joining me now is Rob Dunn, ecologist and author of the new book The Call of the Honeyguide. He’s based in North Carolina. Rob, welcome back to Science Friday.

ROB DUNN: Oh, it’s so great to be on the show, Flora. Thank you.

FLORA LICHTMAN: This book includes a lot of cool animal stories, but it goes way beyond that. I feel like you’re really offering a kind of different philosophy of how we should think about biology and evolution. Will you just give us your vision?

ROB DUNN: So for most of my career, I’ve just been interested in the positive relationships among species. So the mutualisms where two or more species benefit each other. And for me, that’s always been the most obvious thing to look at.

As I’ve gone through my career, I’ve realized it’s not the thing we most often study. And I became fascinated with these relationships, but also the reality that we don’t seem to tell the story of our human positive relationships with other species, those co-dependencies.

And so I just started elbowing out, well, what are these stories about us? Who are we? Which species are we codependent with? And what does that mean about us and our story and our history and our future? And it became so much fun that it became a book.

FLORA LICHTMAN: I mean, you say that humans are the most mutualistic of animals. That’s a big claim.

ROB DUNN: We have the largest number of mutualistic interactions, in part because we’re everywhere. And in each place we’ve partnered with different species. And so I’m in the United Arab Emirates right now, and historically, people here depended heavily upon camels and hunting falcons and date palms.

And those are different relationships than people would have a little bit farther north. And so in each place, we had these different dependencies. And even as we’re now global, we still connect with those ancient dependencies as well as the ones that account for more of our calories. And so collectively, we depend on thousands of animal species and tens of thousands of plants and nobody knows how many microbes. And so there’s just this web that connects us to the rest of life.

FLORA LICHTMAN: Well, let’s get into some examples. What is the honeyguide?

ROB DUNN: So the honeyguide, this amazing and yet drab bird that lives all across Sub-Saharan Africa. And it has this fascinating natural history where it really depends upon eating wax from honeybees, but it can’t get into honeybee hives on its own.

And so it evolved the ability to go to human settlements and make a specific call on a specific flight that basically says, for the love of God, I found a honeybee hive, just follow me. You can have the honey, I want the wax. This will be a good deal.

FLORA LICHTMAN: What’s the call sound like?

ROB DUNN: Oh, I can’t–

FLORA LICHTMAN: Come on. I wish I could do it. One could imagine that on radio. We could play it. [BIRD CHIRPING] It’s a slightly melodic call, and the flight that goes along with it is key. And those two things together say to people that I found the honeybee hive, follow me. And in each human culture that partners with the honeyguide, the humans respond in some culturally typical way. And the honeyguide knows that human response to.

And so a group of scientists has documented this in great detail in the last years. But the peoples who responded to the honeyguide have known about this for thousands, perhaps tens or even hundreds of thousands of years. And so it’s really this ancient set of relationships. And to me, what’s most amazing about it is it’s initiated by the honeyguide. And so the honeyguide says to the humans, do you want to partner?

FLORA LICHTMAN: Right. Not the other way around.

ROB DUNN: And the humans get to choose. Yeah.

FLORA LICHTMAN: Right?

ROB DUNN: Yeah. As I started to write about this story, it became clear that there were more and more of these stories where nonhuman species had more agency in these relationships than we thought. And so no fewer than four different human populations have a pretty similar relationship with dolphins, where dolphins will swim into bays and signal with their swimming that they’ve trapped a group of fish, and that the humans should throw a net over them.

And then there’s a population of killer whales historically in southeastern Australia, who partner with a group, the Aboriginal, to a mob, in which the human whalers would wait for a killer whale group to trap a baleen whale. And then the killer whales would come to the humans and signal that they’d trapped a baleen whale.

And so in all of these cases, the nonhumans are really kind of in charge. And part of the threat of the book is thinking about which species are calling to us today, and are we listening? And which species should we be partnering with to respond to the changes at hand in our moment?

FLORA LICHTMAN: What are the dolphins and the killer whales get out of the deal?

ROB DUNN: In the dolphin case, the dolphins get to pick some fish off of the edge of the net. And so these are fish that are just– they’re schooling. They’re a little bit hard for the dolphins to get. And the nets slow them down. And so they grab a few. And there’s a study in Brazil that documents that the human fisher people get more fish when they partner with the dolphins. And the dolphins get more fish, too.

In the killer whale case, the killer whales can’t trap a big baleen whale on their own. It’s very rare. They can trap it, but they can’t kill it. They rely on the humans to kill the baleen whale. And then the law of the tongue in southeastern Australia was that the humans would leave the tongue for the killer whales, and the humans would take the rest of the whale.

FLORA LICHTMAN: You mentioned that part of the book is about, are we listening to these calls? I mean, are there places where animals have just sort of thrown in the towel on humans and said, OK, well, no one’s listening anymore, I have to find another way?

ROB DUNN: So the honeyguides have, in some places, thrown in the towel where people farm honeybees. The honeyguides no longer go to the humans to call them. They’ll often actually try to get into the honeybee hives that the people are taking care of. Where people have turned to sweeteners instead of honey, the same thing happens. And so the honeyguides stopped calling.

And I think in many places, wild nature is so quiet relative to the loudness of our daily lives, that we don’t know that species are still calling to us. There’s no one there to listen. And then for me, the other piece of this is that the honeyguide is conspicuous because it makes a call that can be heard.

But species call in many ways. And so if we think about yeast in a fruit that are calling with their smells. If we think about the microbes in the soil, there are all kinds of calls around us, and which of those portends something beautiful for our future.

FLORA LICHTMAN: Well, tell me about some of these other calls. I mean, you say that fruit is a mutualism. Will you explain?

ROB DUNN: So fruit evolved for no reason other than to attract animals to eat it, so that the seeds in that fruit would be carried slightly farther. And so can you think about a forest traveling in the gut of a primate that eats a fig, because eventually, the fig seeds will be deposited with the primates feces and grow a new tree in some new place.

And so that’s the only reason fruit exists. It’s the only reason berries exist. We have these wonderful foods in our stores because those species needed some animal to come to them, to carry them. And so each one of those fruits in your store is a kind of call. It’s a call to somebody. And in some cases, it was to ancient humans. In other cases, it was to extinct species. In many cases, it was to birds. But you can decode the fruits in your grocery store as a function of their size and color, and think about who they’re singing to or calling to.

FLORA LICHTMAN: I mean, do you still view them as an example of mutualism now in the time of mass agriculture, or is their history a story of mutualism?

ROB DUNN: I think that’s an interesting question. I think ecologists don’t have a firm answer to it. Historically, when we talk about mutualisms with nonhuman species, we measure them in terms of evolutionary fitness. And so did the fungus-farming ant have more offspring when it partnered with the fungus than it did on its own? Did the fungus have more offspring when it partnered with the ant than it did on its own?

But in our modern lives, what we’re looking for is typically not more fitness. We’re looking for well-being, happiness, longevity. And so an open question is, what do we want out of these partnerships? And how do we measure them?

And so what would we call a mutualism today depends on what things we want out of life and what things we think other species deserve out of life. And so it’s a tricky philosophical question that we ecologists are not sufficiently trained to answer on our own. It’s a broader societal question.

FLORA LICHTMAN: That’s complex. There’s a lot of nuance there, it sounds like.

ROB DUNN: Yeah, for sure. So I interviewed the astronaut Christina Koch at one point, and she was describing that when she was on the International Space Station, that one of the jobs people vied for was to water the little plants on the Space Station because they like the smell of the Earth that would rise up when you watered the plants. And that smell is the smell of specific microbes.

And if you think about fitness, well, the astronauts fitness didn’t increase when they smelled those microbes. But something fundamental rose up out of that soil and affected their lives in a beneficial way. And so do we count that or do we not count that? And from the microbes perspective, they benefited because we took them to space, and so they colonized a whole new realm. And so each partner is getting something there. But is that the right kind of metric? And how should we think about it?

FLORA LICHTMAN: What about pets. Talk to me about pets.

ROB DUNN: Pets are amazing. We clearly benefit psychologically from them. I think it’s questionable when and if we benefit in terms of longevity or health from them. There’s some evidence that in some places, people seem to benefit from some kinds of dogs. I know of no evidence that people benefit from cats. And I’m going to get nasty emails about that one, but–

FLORA LICHTMAN: Wait, excuse me?

ROB DUNN: People find something in cats, but it’s not that– it’s not a life expectancy benefit. It’s not a reduced cardiovascular risk benefit. What is it? What do you get out of your cat? What’s your cat offering you?

FLORA LICHTMAN: Humility.

ROB DUNN: Oh, that’s a good one. Yeah.

FLORA LICHTMAN: They’re cute, too. They’re very cute.

ROB DUNN: They’re super cute. And they look just like giant cats. They look like a tiger. They look like a lion. And so one of the things people think cats might offer us is sort of a fulfillment of our morbid curiosity, that we like the instance of predation, we like the idea of a scary animal.

And so a scary movie, a cat is a reminder of something fearful in the world that has moved into a safe landscape, and so that– the cat’s not actually threatening, but it reminds you of being threatened. And so there’s something a little bit pleasing about that.

FLORA LICHTMAN: Yeah, like a horror film.

ROB DUNN: Yeah, right. It’s a bedroom horror film. It’s very reliable.

FLORA LICHTMAN: I mean, what about dogs? We were talking about who initiated, right, in some of these mutualisms. Do we know whether wolves initiated that partnership or whether it was humans?

ROB DUNN: I mean, that went so far back in time that it’s subject to great bar room discussion among the people who study it. But I think it’s what we know is reconcilable with the idea that both humans and the wolves were playing some role in that relationship. And it’s not hard to imagine the wolves coming to the humans in the way that the honeyguide came to the humans. And that there was some way in which humans and wolves could hunt together that they couldn’t on their own.

And so we can imagine something similar there. Among the 20 people who study the evolution of dogs from wolves, there’s ferocious debate about this. And so I think it’s unwise to come down hard on any one hypothesis. But it seems like the same kind of relationship, and that the co-hunting was part of it, just in the way that the honeyguide and humans are co-predating the honeybees.

But it’s also fascinating because that’s a million miles away from the Schnauzer in a sweater in your apartment. And what they’re doing now is clearly something very different. But we do know that people who have dogs, especially dogs that are historically associated with hunting, seem to get some cardiovascular and health benefit from having the dogs.

And it could just be that they’re getting people out to walk. I think the other possibility is that they’re actually forcing us to pay attention. When they stop to sniff something in our lives today, pausing for a minute is such a hard thing to do, that being forced when a dog sniffs to pause might be existential in its value.

FLORA LICHTMAN: We’ve harnessed microbes for fermentation, for beer, for bread. Who’s in charge in that relationship?

ROB DUNN: I think for most of human prehistory, the microbes were in charge. We used to think about fermentation for beer and bread starting after agriculture. It’s now pretty clear that our ancestors started to tangle with alcohol and lactic acid bacteria more like eight million years ago. And we know this because our genes changed.

And so every single listener right now has a version of the alcohol dehydrogenase gene that breaks down alcohol 40 times faster than that of our ancient ancestors. And so that change was so important that everyone inherited it, who survived.

And the reason for that appears to be that they were ingesting enough alcohol, that it was beneficial to be able to break it down into sugar faster. We all also have a version of a receptor in our bodies that intercepts some of the products of lactic acid bacteria, and those more remote ancestors didn’t have that either. And so those are two genetic changes in our body that seem to have related to ingesting fermented fruit.

And what’s amazing about that is that the microbes didn’t change in that time period. And so we usually talk about domestication as being one organism takes control of another organism in such a way that its genes change.

Well, so in this case, our genes changed when we started to tangle with yeasts and lactic acid bacteria, and theirs didn’t. And so from a standard storytelling, we would say, well, they domesticated us. And then you could keep going and say, well, eventually, what we provided to them with fruit was not enough. And so they forced us to start to farm grains, and then to malt those grains to feed them even more.

And then they forced us to colonize even more of the Earth to provide even more sugar to them. And so there’s a story that’s equally reconcilable with our standard story, where we’re not in charge at all. The yeast and lactic acid bacteria are running the show. And might have even led me to write this book, yeah.

FLORA LICHTMAN: Look, that feels resonant to me. Not being in charge at all feels very resonant to me.

ROB DUNN: Yeah, I think it’s our time.

FLORA LICHTMAN: That’s all the time we have. Thank you, Rob.

ROB DUNN: Thank you so much. What an absolute pleasure.

FLORA LICHTMAN: Likewise. Rob Dunn, ecologist and author of the new book The Call of the Honeyguide. He’s based in North Carolina. And if you want to read an excerpt from the book and learn more about the honeyguide, head to sciencefriday.com/honey. That’s sciencefriday.com/honey.

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