MARY LOUISE KELLY, HOST:
Well, here's something I never thought I would say on air. Fish may feel pleasure and actively seek it out. Science reporter Ari Daniel tells us more.
ARI DANIEL, BYLINE: We tend to credit animals like cats and dogs with a certain level of mental complexity. But fish?
CAIO MAXIMINO: They do not talk, they do not bark. We usually think, well, these are very simple-minded animals. They are like little robots that do not do much.
DANIEL: Caio Maximino is a neuroscientist at the Federal University of the South and Southeast Para in Brazil, and he disagrees with that assessment.
MAXIMINO: Those animals - they have very rich behavior.
DANIEL: Previous work has largely focused on the negative experiences of fish, driven by fear, anxiety and discomfort, says Marta Soares, a behavioral physiologist at the University of Porto.
MARTA SOARES: It has been demonstrated that they feel pain, for example. And that was a huge step, actually.
DANIEL: But Soares and Maximino wondered whether fish could feel good too.
SOARES: It's almost like - it was the next question, really.
DANIEL: So they turned to two types of coral reef fish. The first was the bluestreak cleaner wrasse, a silvery blue little number with a jet black stripe, which helpfully eats the blood-sucking parasites off other fish.
SOARES: They just clean, clean, clean from 6 a.m. to 6 in the night.
DANIEL: In the wild, the wrasse are visited by all kinds of fish who stop by for a cleanup before moving on. This includes the second species the researchers studied, the threadfin butterflyfish. They wondered if these striking, yellow, black and white fish might be visiting the cleaners for more than just the health benefits, especially because in the lab, the butterflyfish didn't need to be cleaned. They came parasite-free.
MAXIMINO: Maybe there's some pleasurable sensation that is being produced by this massage.
DANIEL: Maximino observed that butterflyfish preferred spending time in the part of the tank where they had previously interacted with a cleaner fish.
MAXIMINO: Not only he had a memory of being cleaned there but that he wanted to go there.
DANIEL: Now, fish have an opioid system, just like us, which regulates both pain and pleasure, and Maximino thought perhaps it might explain the fish's behavior. So he injected the butterflyfish with a low dose of an opioid mimic, a drug kind of like morphine that worked to boost opioid activation slightly.
MAXIMINO: And they spent much more time looking for this place where they experienced cleaning before. So it increased their preference.
DANIEL: But when he injected them with naloxone, a drug that blocks opioid receptors and is used in people to reverse an overdose, the butterflyfish lost interest in the spot where the cleaners had been, suggesting there may well be pleasure involved with the massage...
MAXIMINO: And that this is mediated by those natural opioids in their brains. The main takeaway is that fish experience some type of pleasure.
DANIEL: And in another experiment, the researchers observed that the fish work hard to experience that pleasure again, although some worked harder than others. The findings are published in the journal Proceedings of the Royal Society B. Susana Pecina is a biopsychologist at the University of Michigan-Dearborn who wasn't involved in the research.
SUSANA PECINA: Can we inequivocally (ph) say that this is pleasure in fish? I'm not sure that I would say it in those words.
DANIEL: Still, she finds the results exciting. To her, they suggest we may need to rethink how we treat fish in aquariums and aquaculture. And she was struck by what this might mean for animal evolution.
PECINA: This ability to experience positive emotions and joy are maybe something deeper about what it means to be alive on Earth.
DANIEL: For NPR News, I'm Ari Daniel.
(SOUNDBITE OF DAVID WISE'S "AQUATIC AMBIENCE") Transcript provided by NPR, Copyright NPR.
NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.
