Fish Have Feelings, Too: The Inner Lives Of Our 'Underwater Cousins' : The Salt Jonathan Balcombe, author of What A Fish Knows, says that fish have a conscious awareness — or "sentience" — that allows them to experience pain, recognize individual humans and have memory.

Fish Have Feelings, Too: The Inner Lives Of Our 'Underwater Cousins'

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This is FRESH AIR. I'm Terry Gross. We've become increasingly aware of the suffering of animals in factory farms. Many people have become vegetarians to do their part to spare animals from slaughter. But when it comes to fish - well, most people don't think about fish as having feelings. A new book makes the case they do. It's called "What A Fish Knows," and it's written by my guest Jonathan Balcombe, the director of animal sentience at the Humane Society Institute for Science and Policy. Sentience is the capacity to experience feelings.

Balcombe examines the scientific evidence that fish experience pain, have memory and are capable of learning. He says thanks to the breakthroughs in ethology, sociobiology, neurobiology and ecology, we can now better understand what the world looks like to fish, how they perceive, feel and experience the world. Jonathan Balcombe, welcome to FRESH AIR.

JONATHAN BALCOMBE: Thanks for having me, Terry.

GROSS: So you say the capacity to feel, to suffer pain, to experience joy - that those are the bedrock of ethics and that that's what qualifies one for the moral community. So do fish feel? Do they suffer? Do they experience joy? Do they have emotions? I mean, how can we tell if they have emotions?

BALCOMBE: Yeah, all of the above...

GROSS: 'Cause it's not like they purr or that they cuddle with us or that they rub up against us for attention like - like our pets do.

BALCOMBE: Well, actually some of them do. You can go into reef areas where there are large groupers who've been living there for years, decades in some cases. And it's a safe area where they say no fishing or spearfishing, and they become trusting. And they will approach trusted divers. Whether they're recognizing them as individuals, I don't know. But there was a new study this week showing individual recognition of human faces by fishes, so they probably do recognize individual divers.

And they come up to be stroked. It is almost like a dog. And I don't know that they roll over to have their belly petted, although some sharks will be sent into what looks like a euphoric state when they have their bellies rubbed. And I just watched a video the other day of a couple of intrepid divers rubbing the faces of tiger sharks. These guys go in the water regularly with sharks. They're on a mission, and the tiger sharks appeared to love it.

And you can also measure internal physiological changes, endorphins and pleasure compounds in the bloodstream. And you can measure cortisol, which shows that an animal's stressed. There's one study I really like of surgeonfishes where they stress them. I felt bad for the fishes in this study, although I'm happy to say they return them to the Great Barrier Reef when they were done. But they stress them for half an hour, and then they gave them the opportunity to swim up next to a wand which was modeled on a cleaner fish, which moved back and forth and could give them caresses. And when they were stressed, they would go right up and sidle up next to that wand and receive these caresses from this model. And they could measure that the stress hormones went significantly down.

Fishes who were in a - had an ability to swim up next to a model that wasn't moving and therefore couldn't deliver caresses was ignored. They didn't swim up to that, and they didn't show declining stress. So there are clever ways of studying internal states of animals, fishes included.

GROSS: I'm still trying to wrap my head around somebody tickling a shark's belly.


GROSS: Like, who would be doing that? OK. So how can we tell if fish feel pain? I mean, we're used to some kind of cry or scream from an animal if they experience pain. And if the fish is making a sound, we're probably not hearing it.

BALCOMBE: Yeah, it's curious. There's still - some people still question whether fish feel pain. Really there's no question in my mind based on the evidence that I've looked at. And I - I've looked at this quite extensively in my research for this book. A number of studies have been done on trout, very detailed, meticulous studies in the lab where the animals are terminally anesthetized and then different anatomy was found. They have pain receptors that are sensitive to heat, to different chemicals and different sensors to mechanical, such as piercing sorts of pain. And these signals are sent to the brain, and they affect the hormones in the animals. They affect the behavior. The animals change their behavior. They may stop eating.

There's one other study I think I'd like to describe to you, though. I think it's, for want of a better word, the most elegant study of fish pain that I've ever seen. It was done a few years ago by a biologist named Lynne Sneddon, who's in the U.K. And she used zebrafishes, which are very commonly used in research. And what they did was they put these group of zebrafishes - I don't remember how many, perhaps 30 - in a complex tank that had two chambers. One chamber was enriched. It had rocks and vegetation, and the other chamber was barren. It was open.

And you can probably guess which chamber these fishes spent all their time in. It was the enriched one. Fishes like places to hide. They like stimulation in their environments. And then they injected the fishes either with one of two things. One was with an acid solution, which is known to be caustic and presumably painful to these fishes if they can feel pain. And then the other ones - the other half of the fishes were randomly selected - were injected with saline, which causes just the pierce of the needle. And then the pain is not going to be lasting because it's not acidic.

And then they watched to see how they behaved, and they all remained swimming in the enriched tank. And then they dissolved a painkiller solution in the barren, undesirable chamber of this complex tank. And lo and behold, some of the fishes then started to migrate across and swim and hang out in that normally undesirable tank. And it was only the ones injected with the acid and not the ones injected with the saline. So I find that a pretty convincing demonstration of pain in fishes.

GROSS: The Humane Society, where you work, publishes a journal about animal sentience, animals' ability to feel. And you're an associate editor of that. It recently published an article about whether fish can feel pain. The article said they don't. And then there was this long, (laughter) long number of dissenting opinions, including one of yours, that came afterwards. So is this a big debate now in the scientific community?

BALCOMBE: Well, you could say it's a big debate in a paper like that. And responses are being published in the current era. I don't think it is a big debate for the simple reason that there are very few scientists who are actively arguing that fishes do not feel pain. I think if you surveyed scientists, the great majority would say, yes, of course, they feel pain. They're a member of the vertebrate clan. They have all of the physiological, behavioral attributes that we would expect of an animal that can feel pain.

And, yes, and as you say, the great bulk of responses to that article were rebuttals. And the primary argument being made by the gentleman who wrote that paper is that they lack a neocortex, the sort of cauliflower structure sort of part of our brain that's found in the mammals. And that's, I think, the crux of what weakens that argument is it is only found in the mammals. Birds don't have a neocortex. Their very effective brains evolved in a different path, and it's the paleocortex that became the sort of thinking part of their brain.

And yet, nobody's really debating whether birds can feel pain, never mind that they can also learn language. And they can do a lot of other sophisticated things. And one of the things I try to show in this book is that fishes really deserve equal consideration to all the other vertebrates. When you look at the bulk, at the cumulative evidence we now have, it's quite clear to me that they are full members of the vertebrates and they deserve the same sort of respect that we accord mammals and birds when we're at our best.

GROSS: So what's the next step? Fish deserve kind of, you know, equal moral consideration. What are the implications of that?

BALCOMBE: They're huge, of course. Depending on who's doing the estimate, we humans kill between 150 billion and over 2 trillion fishes a year. If you lined them up end to end, they'd reach the sun and back, at the higher estimate. So the numbers are astronomical. And the way they die, certainly in commercial fishing, is really pretty grim. They die most often from suffocation in air but also getting crushed in these huge fishing nets when they're drawn up, decompression - fishes have swim bladders, which are very useful for them, adding buoyancy, but they expand as the pressure drops when the fishes are being raised to the surface. And that can crush or may bring organs or push things out of their mouths. That's probably not a very nice way to die either. And then sometimes they may be bled out on the deck, which is perhaps a little faster but probably not - also not very pleasant for them. So there's a lot of change that would be needed to reflect an improvement in our relationship with fishes.

GROSS: So fish experience pain where actually the fishing industry has some very probably painful ways of catching fish.

BALCOMBE: Yeah, it's not pleasant for them. Nets, trawlers that go along the bottom, which are just rapacious. It's been likened to running a bulldozer over a rain forest if you had a big enough bulldozer. And there's this awful issue of bycatch, which are unwanted species that are caught by these largely indiscriminate nets and catching methods, and they get all crushed in there as well. And so they're either dying or dead when they reach the deck in many cases. Very few of by-caught animals survive if they're tossed back, which is routinely the case. They are typically tossed back into the oceans.

GROSS: If you're just joining us, my guest is Jonathan Balcombe. He's the author of the new book "What A Fish Knows: The Inner Lives of Our Underwater Cousins." And he's the director of Animal Sentience at the Humane Society Institute for Science and Policy. Let's take a short break here and then we'll talk more about fish. This is FRESH AIR.


GROSS: This is FRESH AIR. And if you're just joining us, my guest is Jonathan Balcombe, and he writes about animals. His new book is called "What A Fish Knows: The Inner Lives Of Our Underwater Cousins." He's the director of animal sentience with the Humane Society Institute for Science and Policy. And sentience is the ability to feel.

So I think a question people ask themselves if they ask do fish feel, should we consider fish in our moral universe, they likely ask themselves are fish smart? I mean, do they have intelligence? Is that, like, an appropriate question to ask? We talked a little bit about the controversy in science that you say is pretty much resolved about whether fish are intelligent. You say most scientists now think that they are, that they have some intelligence. Should that matter when we're thinking morally about how we treat fish?

BALCOMBE: I think it's relevant. You can certainly find parallels between intelligence or brain capacity and sentience. I think that may be fair. But I think more - it's probably - I mean, sentience is like pregnancy. You're either pregnant or you're not. You're either sentient or you're not. And if an animal is sentient, which means some kind of conscious awareness but particularly the capacity to feel pain and I would say by extension to feel pleasure, then to me that means that animal has moral traction, or it should have moral traction, that the animal is deserving of consideration of others because that animal can have a good day and a bad day and can have good or bad things happen to them. And that's, as I say, the bedrock of ethics.

GROSS: OK, so let's look at whether fish have intelligence. First of all, like, how do they perceive? Like, we have five senses - vision, smell, hearing, touch and taste. Fish live under water. What do they have to perceive the world around them?

BALCOMBE: Yeah, they have all those five. We also have another - a number of others that we often don't - we don't include on that vaunted list of five. We really probably should - a sense of balance, a sense of pleasure, a sense of pain. We've been talking about that. They, of course - they have all of those as well. But they have a couple of other pretty neat senses that are worth mentioning.

One is a sense of water pressure or movement in the water that's very acute thanks to a lateral line. We're talking about bony fishes now, not the sharks and rays, which are in a separate group, the cartilaginous fishes, which have really cool adaptations as well. But the bony fishes have a lateral line. You may notice a dark row of scales along the center line of a bony fish. And that's actually the shadow cast by these specialized scales because there's a depression in each one. And in that depression are specialized little cup-like chambers with gel in them and little hairs that stick out. And they detect pressure changes, so it's very useful for navigating at night, for avoiding dangerous things in limited-vision conditions and that sort of thing.

Another really cool sense is electrical. Some fishes, including sharks and I think rays as well, are electro receptive. They can detect electrical signals from other organisms. But there's also electric-producing fishes. The knifefishes of South America and the elephantnose fishes, named for a long projection on their faces, of Africa, appropriately. They - they're both electric producing. So they have EODs, which are electric organ discharges, and they use those as communication signals. And they communicate in some pretty cool ways. They will change their own frequency if they're swimming by another fish with a similar frequency so they don't jam and confuse each other. They also show deference by shutting off their EODs when they're passing by a territory holder. They don't want to - you don't want to piss off the territory holder, so it's probably better to sort of go, quote, unquote, "silent" during that time. So the perceptions and sensory abilities of fishes, well, they're the product of over 400 million years of evolution, so it might not surprise us that they've got some pretty cool ways of sensing their environments.

GROSS: One of the cool capacities that some fish have - let's look at flounders. Flounders - they're born with one eye on each side of their body. Then you say the eye - one of the eyes migrates. So eventually both eyes are on the same side. That just seems bizarre. I mean, I don't even understand how an eye can migrate like that. So can you explain what you know about flounder (unintelligible)?

BALCOMBE: Yeah, evolution is a boundless innovator. And one of the real joys of researching and writing books about animals is to see what sort of things that evolution comes up with. And certainly eye migration in flounders is one example of that. In some flounders the eye migrates in just I think as short as a day. It almost makes you wonder if it hurts. But it is quite remarkable, and it's all coded in the genes.

And it allows adult flounders to be much more effective at A, hiding from enemies, and B, hiding from their prey because what they do is they - they're literally called flatfishes and flatfish. And they sit on the bottom often under the sand, but they're also brilliant at disguising themselves by mimicking the substrate. And I have a photo in the book of the same flounder taken a few minutes apart in four locations who blends in beautifully to the background. So they are sort of chameleons of the sea.

And by having both those eyes on one side, they can lie flatly on the bottom with both eyes up, and they can swivel their eyes around. And they can watch out without being seen unless a shark with that electroreception comes by and might detect their heartbeat under the sand. So there are tradeoffs, of course.

GROSS: And each eye moves independently.

BALCOMBE: Yes, a number of fishes have eyes that move independently, again, also chameleon-like. That's something I love about chameleon lizards is they have those eerie and very cute pointy eyes that swivel independently. And being able to do that is useful. And I - you have to wonder how does that affect the perceptual experience of a fish or a lizard for that matter if you're taking in two visual fields at the same time? I don't know if anyone's really asked that question.

GROSS: Right. Just getting back to the idea of eye migration, that one eye on the side of the face migrates to the other side of the face in flounder, is it that one eye closes and a new eye is born on the other side, or does it literally, like, travel across (laughter) the body of the flounder to get to the other side?

BALCOMBE: My understanding is the eye actually migrates. It moves. It doesn't disappear and then pop out on the other side.

GROSS: Wow, that's just so hard to fathom. That's really fascinating.


GROSS: So you say that fish can hear and that they make sounds. They're under water; we're on land, so I don't know if divers hear any sounds underneath. But what kind of sounds do they make, and what are some of the ways they make those sounds?

BALCOMBE: Yeah, I have a list of words that I couldn't resist stringing together somewhere in the book - you know, clicks, pops, rasps, buzzes, burrs, purrs. I forget the other ones. I think there were about 15 or 20 words on that list. It is quite a symphony of different sounds they can make. They're not necessarily really tonal sounds. Some of them are, and some fishes are named for the sounds they make - grunts, for instance, and I think toadfish is maybe named for that, although they maybe look a bit like toads. But there certainly are quite a few fishes named for their sounds.

They produce them in different ways. Some fishes grind their teeth together. Some fishes grind their bones together. Limb movements I think may be used in some cases. And the swim bladder I mentioned earlier is quite useful for making sounds. You know, we use our lungs as an air producer. So swim bladders have gas in them, and they can be rubbed and air can be expelled or gases can be expelled. And so swim bladders are often involved in sound.

There is one really curious example, though, involving herrings that I can't resist mentioning. I think if you were to come up with a phrase that best captures at least a delicate phrase - flatulent communication would be perhaps the right phrase. They live in big schools, and they emit gases from the anus in large numbers. And it makes a sound, and they appear to use this as a communication device to maybe - to signal to others that it's time we moved up or down in the water column because it's that time of day when the predators are coming out and this sort of thing. And the researchers who studied it use the more technical term frequent repetitive ticks. And I'll leave it to the listener to make an acronym out of that, which is quite appropriate behavior.

GROSS: Oh, that's very funny. OK, so if fish produce, you know, what you've described in the book as hums, purrs, clicks, moans, chirps, buzzes, growls, snaps - I mean, are they audible to the human ear, or is this, like, waves in the water that could be interpreted as sound but aren't exactly sound?

BALCOMBE: Real sound. It's a real sound.

GROSS: It's a real sound?

BALCOMBE: It's real vibrations in the water. And if you have - if you have dived or snorkeled on reefs, particularly the U.K., you can often hear this cacophony of snapping and popping sounds going on. And it's constant sounds. And some of it may not be directly produced by the fish in terms of trying to make a sound to communicate. Sometimes it's the sound of parrotfishes' mouths crunching away on corals. But a lot of those sounds are made in a - sort of a communication role.

GROSS: My guest is Jonathan Balcombe, author of the new book "What A Fish Knows." We'll talk more after a break, and we'll hear from Ellie Kemper, who stars in the Netflix series "Unbreakable Kimmy Schmidt" and co-starred in "The Office" as the receptionist Erin. I'm Terry Gross, and this is FRESH AIR.


GROSS: This is FRESH AIR. I'm Terry Gross. Let's get back to my interview with Jonathan Balcombe, author of the new book "What A Fish Knows." The book makes the case that fish experience pain, have memory and are capable of learning. The book is based on recent research. Balcombe says that what we're learning about fish has implications for how we catch and consume them. He's the director of animal sentience at the Humane Society Institute for Science and Policy.

So getting back to fish intelligence. Give us an example of a fish that does something that you perceive as intelligent.

BALCOMBE: One of my favorite examples is from a humble little pouty-mouthed fish of intertidal zones called the frillfin goby. A series of experiments in captivity actually done in New York City were done over several decades that reveal a pretty remarkable skill - some form of intelligence involving spatial memory. As I say, these fishes live in intertidal zones, so they live in that area between sort of rocks between high and low tide. And so at low tide, it's a bunch of rock pools, and they hang out in those rock pools. And we notice that to escape danger, say, an octopus prowling around through the rock pools, they will jump with great accuracy to neighboring pools. And they can actually work their way out to the open sea again by jumping into successive neighboring rock pools. Well, why - how are they doing that? How do they know how far to jump and which direction to jump in?

And it turns out through a series of these captive experiments where this guy set up fake rock pools, it turns out that these gobies memorize the topography of the rock pool at high tide when the water is in, and they can swim over it. And so they're sort of translating an aerial view into a - sort of a horizontal view for want of a better term. It's what you might call a mental map. And they're able to use that information to accurately leap and to avoid making a leap of faith.

And this - in the study, I remember a couple of numbers. When they had not had a chance to swim over these rock pools, they had about a 15 percent success rate when he poked a stick in or tried to scare them into jumping. And they would jump, and they would be stranded on the rocks in those situations most of the time. But they had about a 97 percent success rate if they had had an opportunity to swim over it. So a clever experiment that shows a pretty remarkable form of intelligence in a very small fish.

GROSS: So they can learn what's there by swimming over it once and then remember it as well.

BALCOMBE: That's right, learning and memory goes on there. And just about any study of social behavior in a fish has found individual recognition that they recognize each other's faces. They remember them over the course of their lives.

I describe a number of other memory experiments. One of the most interesting, most impressive feats of learning in a fish is the archerfish, which is named for its ability to catch prey - often flying insects or perched insects - by shooting water from their groove-shaped mouth at these animals above the surface. They have to account for the refraction index of the water surface and a number of other things. And they have to account for the speed of the flying insect.

And studies show that naive, novice archerfishes who are new to this, they're very poor at it. They can't even hit a moving object going at just half - a half an inch per second. They can't hit that with their stream of water. But if they're given the opportunity to watch other experienced adult archerfishes plying their trade and shooting at objects accurately without any personal experience themselves, they - when given a new opportunity, they show a marked increase in their accuracy. So they can learn very well by observational learning.

And there are a number of examples of that that that I point out in the book of what biologists call audience effects, where fishes changed their behavior according to who's watching, how many are watching and what the circumstances are.

GROSS: So since you firmly believe that fish have the capacity to feel, that they have intelligence, that they feel pain and that we therefore shouldn't punish them, what do you think of fish as pets?

BALCOMBE: Well, they're lovely to look at and they're graceful and beautiful. And I remember visiting a couple of aquariums in people's homes during the research of my book. And I remember being invited to feed some discus fishes with some food that I held in my - between my thumb and finger. And these - one of these discus fish came up and just - I could feel the tug as this individual pulled the food from my hand. And I got - I get a lot of stories from people, anecdotes of people who live with fishes and become very attached to them. And the relationship is touching and often moving and maybe span a decade or more. And they grieve their loss. And so definitely there's some lovely parts of the relationship between captive fishes and their humans if they're well looked after.

Having said that, there's certainly problematic areas of the whole aquarium industry itself. The aquarium industry is very actually quite disturbing in terms of the capture of wild fishes, mostly from reefs because that's where the beautiful colorful little ones that are so popular in reefs are particularly sought after. Some of the methods used to catch them are pretty awful - cyanide poisoning, which often kills many of the fishes being targeted or ones that are not being targeted. And explosive devices are sometimes used, and then you have the vicissitudes of transport, where they're shipped over continents. And the mortality rates are quite high.

One of my affiliate organizations, Humane Society International, which is under the umbrella of the Humane Society of the U.S., for whom I work, they have an active campaign now to draw attention to the Blue Tang, which is the star of the sequel to "Finding Nemo," the new film "Finding Dory," which, by the way, my book was strategically time to come out around the same time as. And Dory is a blue tang, and they are going to be probably very popular in the aquarium trade because of the fact that this film will draw a lot of attention to that species. Well, unfortunately, blue tangs are caught in the wild. And they are subject to the - some of the ills of the industry.

And so we are campaigning actively to try to discourage - to discourage people from buying these fishes because it's like I said earlier, when you when you purchase a product, you tell the manufacturer to do it again. And we don't really want that happening.

GROSS: If you're just joining, us my guest is Jonathan Balcombe, and he's the author of the new book "What A Fish Knows: The Inner Lives Of Our Underwater Cousins." And he's the director of animal sentience - sentience is the capacity to feel - he's the director of animal sentience at the Humane Society Institute for Science and Policy. We'll be right back after a break. This is FRESH AIR.


GROSS: If you're just joining us, my guest is Jonathan Balcombe. He's the author of the new book, "What A Fish Knows: The Inner Lives Of Our Underwater Cousins."

You're vegan. We need things to eat. Not everybody wants to be or can be vegan. So assuming that people continue to eat fish, what do you think are the most humane forms of fishing now? Do you have any hope for fish farming, that there could be a way of doing that - sure, the fish die, but at least could they die less cruelly?

BALCOMBE: Yeah. Well, there's always improvements that can be made. I would go back to what you said earlier. I think most of us - a lot of people maybe don't want to become vegan. Although, I would suggest that that's just because they don't realize what an incredible diversity of plant-based foods are available. And there's a real revolution happening in plant-based foods with both meat substitutes but also - and even in-vitro meats is a rising phenomenon where you can make meat from animal tissue that never involved any slaughterhouse or factory farming or transportation trucks because the - it's all done in tissue culture. So the options there are becoming much broader. And they're - veganism is something I would encourage listeners to look into.

Having said that, yeah, I mean, there are certainly improvements we can make. And I think it is - it's not making it humane. It's making it less inhumane. I think we just have to be realistic that if we are raising animals for human consumption, the economies of scale and the scale that it needs to be done - if you're going to be feeding a few billion mouths - is such that there's going to be confinement. There's going to be - animals are going to be stripped of their opportunity to engage in normal behaviors.

Let me just mention one study came out this past week which found that farmed salmon - a lot of them become dropouts. That's the term they used. I don't mean to sound like teenagers who start smoking on street corners and taking drugs. I'm talking about essentially giving up on life. They become listless.

And measures of their physiology suggest that they're actually severely depressed. They fit the hallmarks both behaviorally and physiologically of severe depression. Can we ask them if they're depressed? No, and that's one of the challenges, of course, of any study of animal feelings is that we have to surmise based on evidence.

But the evidence supports that they are really miserable to the extent that they give up. They weigh about one-third what the other fishes weigh. And it's probably because they have no control over their lives and they - there's probably other aggressive fishes who attack them and chase them and they - it's probably terrifying. And so they become miserable and depressed, and they just wither away.

GROSS: So one of the subjects you're interested in, you know, for animals, is the question of do animals experience pleasure? I mean, we've talked about do animals experience pain - do fish experience pain? And you say, yes, fish experience pain. And we should take that into consideration when we decide if we're going to eat fish or not and when we figure out how we're going to farm fish or catch fish.

But why are you so interested in the question of whether and how animals and fish experience pleasure?

BALCOMBE: Yeah. Thanks for asking that. Pleasure is so often overlooked. And I think it's so important. That's why I've written two books about animal pleasure in the past. And I certainly made sure I included some discussion of pleasure in this book. Pleasure adds so much richness to life. It's pleasure and not pain that makes life worth living. And so I think it's so important to include pleasure in - when we evaluate other lives - or our own, for that matter.

And certainly, if you look at how fishes behave, pleasure is a big motivator for them just as it is for other sentient animals. It's the - the reason, I think, pleasure evolved is it's nature's way of encouraging good, adaptive behaviors just as pain is nature's way of discouraging maladaptive behaviors, behaviors that risk injury and/or death which are, in the genetic stakes, really bad outcomes.

Is it any wonder that we're motivated - that we love food? We love the taste of it. Is it any wonder that fruit - which plants produce to really distribute their own seeds - why they're attractive to look at? They've got bright colors. They've got nice smells, sweet tastes and a big, nutritional reward.

As for fishes, well, there's not so many fishes that eat fruit. There are some, actually. But they have their ways of feeling and expressing pleasure. And I think touch is perhaps the most key sensory realm where fishes can get pleasure.

It's a good opportunity to mention these - this wonderful, very complex cleaner-client mutualism or symbiosis that you find typically on reefs where you have cleaner fishes of different species but often it's a cleaner wrasse, and they may work individually or in pairs or even small teams. And they hang out at a particular station on the reef. And then client fishes will line up to wait their turn to receive a spa treatment, essentially. They swim in and they hover there.

I've watched this during dives myself. And it's been very well studied. And they get plucked over by these cleaners. And the clients cooperate. They open their mouths. And the cleaners swim in to look for parasites and algae and what have you. Clients never eat the cleaners because it's just not good to eat your business partner. And then they open their gills, and the cleaners go in there. So it's a trade-off. The cleaners get food, and the clients get this spa treatment.

And while it's certainly adaptive to have parasites removed, I suspect that the way - the reason, the main reason, the clients return time and again to these cleaning stations is because it feels nice. It feels good. And I think there's further support for that. The cleaners will curry favor with clients by pausing from their cleaning ministrations. And they will actually move - rapidly move their pectoral fins to give caresses to the clients on their skin. And it's sort of a way to say hey, look, you know, come to me, you get good service. But I did want to mention it partly to point out the - the role of the pleasure of touch in these animals' lives.

GROSS: So obviously you love animals. Do you have pets?

BALCOMBE: Currently, I don't. I travel a lot. I mean, I have an unknown number of intestinal parasites probably. But I wouldn't really call those pets.

GROSS: (Laughter).

BALCOMBE: But other than that no. I've certainly lived with cats and dogs. I've had rats. And I certainly recommend rats, despite their often negative reputation. They actually make wonderful companions. They're very social. And if you get them when they're young, they're very malleable. They can learn their names. They'll play games with you. But no because I do travel, I worry about them being left without me because they're - certainly dogs especially - are highly social and they can really miss their companions.

GROSS: Jonathan Balcombe, it's been great to talk with you. Thank you so much.

BALCOMBE: Terry, thank you for having me.

GROSS: Jonathan Balcombe is the author of "What A Fish Knows." After we take a short break, we'll hear from Ellie Kemper, who stars in the Netflix series "Unbreakable Kimmy Schmidt" and costarred in "The Office" as the receptionist Erin. This is FRESH AIR.

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