We tend to attribute a certain degree of mental complexity to animals like dogs and cats. But fish generally don’t get that kind of praise.
“They don’t speak, they don’t bark,” says Caio Maximino, neuroscientist at the Universidade Federal do Sul e Sudeste do Pará, in Brazil. “We usually think: ‘Well, these are very simple animals. They’re like little robots that don’t do much.’”
But Maximino personally doesn’t believe that. “These animals have very rich behavior, mediated by these internal states similar to emotions,” he says.
Previous research has largely focused on fish’s negative experiences, driven by fear, anxiety, and discomfort. “It’s been shown that they feel pain, for example,” says Marta Soares, behavioral physiologist at the University of Porto, in Portugal. “And that was a big step, actually.”
But Soares and Maximino wondered if fish could also feel good. In a study published in the journal Proceedings of the Royal Society B, they and their colleagues concluded that fish can feel pleasure and actively seek it out.
“Fish like things, they want things,” says Soares. “Basically, it would be good to change people’s view of fish a bit.”
Very cooperative fish
To determine what fish might feel, the researchers turned to two coral reef species. The first was the blue-tailed cleaner wrasse. This small blue-silver fish with a black stripe feeds on blood-sucking parasites from other fish, including predatory species that might otherwise eat them. The whole system, Maximino says, is “a model of cooperation.”
“What they do is simply clean, clean, clean, from 6 a.m. to 6 p.m.,” says Soares. In the wild, “you have all kinds of different species” that stop by the wrasse’s territory for a cleaning before moving on.
That includes the second species the researchers studied — the linefin butterflyfish … The scientists wondered if these striking yellow, black, and white fish might be visiting the cleaners for something beyond health benefits. Especially because, in the lab, the butterflyfish didn’t need cleaning — they came parasite-free.
Soares had already shown that cortisol levels, a stress hormone, dropped in another type of reef fish during cleaning. “So we thought maybe there’s something more,” says Maximino. “Maybe there’s some pleasurable sensation produced by this massage.”
To find out, they conducted a series of experiments. First, Maximino observed that butterflyfish preferred to spend time in the part of the aquarium where they had previously interacted with a cleaner wrasse.
“Not only [did the fish have] a memory of being cleaned there, but it also wanted to go there,” says Maximino. “Like, ‘This was a really nice place where I got a wonderful massage from this fish and, well, I want that again.’”
Liking and wanting
Maximino and Soares knew that fish have an opioid system (just like us), which regulates both pain and pleasure. And they thought that system might be at least partially responsible for the butterflyfish’s interest in seeking a cleaning.
To test the idea, the researchers injected the butterflyfish with a low dose of an opioid mimic, a drug similar to morphine that slightly increased opioid activation.
The result was that “they spent much more time seeking out this place where they had previously experienced cleaning,” says Maximino. “So it increased their preference.”
But when they injected naloxone into the butterflyfish — a drug that blocks opioid receptors and is used in humans to reverse overdoses — they lost interest in the spot where the cleaners had been. This suggested that pleasure might be involved in the massage “and that it’s mediated by natural opioids in their brains,” says Maximino.
The scientists’ final question was whether there was a difference between liking the cleaning and wanting the cleaning. So they placed a series of barriers in the aquarium that made it harder for the butterflyfish to access the cleaner. This time, the same drugs — both the opioid mimic and naloxone — had no effect. The butterflyfish continued crossing the barriers to reach the cleaner, revealing that its motivation for the reward wasn’t affected.
This meant that this other sensation of desire to receive the pleasurable stimulus — think of it as the fish’s craving — might be governed by a different chemical, like dopamine.
Maximino explains it this way: “The opioid system thinks: ‘This is pleasurable,’ and the dopaminergic system thinks: ‘Go after what was pleasurable before.’ Opioids change how much you like something, but they don’t change how much you want something.”
“The main conclusion is that fish experience some kind of pleasure,” he says, “and they work very hard to get that pleasure again.”
Some scientists, however, may need a bit more convincing.
“Can we unequivocally say this is pleasure in fish?” asks Susana Peciña, biopsychologist at the University of Michigan-Dearborn, who was not involved in the research. “I’m not sure I’d say it in those words. That said, it’s very hard to measure pleasure in animals, period, let alone in fish.”
Still, she finds the results exciting. To her, they suggest we might need to rethink how fish are treated in aquariums and aquaculture. “Can we think of ways for them to have more positive experiences, better lives?” she asks.
Ultimately, what this and other experiments are revealing, Maximino says, is that fish have rich behaviors that can be controlled, at least in part, by feelings — both negative and positive.
“Pleasure, wanting, desiring, and all these positive emotions we feel — they don’t just apply to humans, chimpanzees, cats, and dogs,” he says. “Fish can feel them too. So this is a very ancient function.”
It’s a capability that Maximino argues was probably very important in animal evolution.
Peciña agrees. If the results hold up in additional studies, she says, they could suggest “something deeper about what it means to be alive on Earth.”
Source: npr.org by Ari Daniel