Mangrove rivulus fish hate enforced water aerobics. Despite her best efforts, Giulia Rossi, a biologist who recently received her doctorate from the University of Guelph, cannot coax the fish to swim against a current in a laboratory tank. “They refuse to exercise in water,” she told me. “They just let themselves hit the back mesh.”
When plucked out of water, however, the stubborn swimmers quickly whip themselves into shape. Rossi boops their snoots with a clicky ballpoint pen, and the fish—which are amphibious, and can survive on land for weeks at a time—backflip out of reach.
In goading the rivulus into these workouts, Rossi becomes a trainer of both body and brain. Terrestrial CrossFit, she and her colleagues have found, is an excellent way to juice up the piscine mind. Just a few minutes of flopping each week was enough to spark a round of brain-cell growth and improve their ability to solve tricky mazes for a worm reward. The team’s findings, published today in Proceedings of the Royal Society B, are a ringing endorsement of the benefits of cardio. They also provide a potential glimpse, the researchers argue, into the colonizing tactics of our long-gone fish ancestors, who might have eased their transition from water to land by first hopping back and forth—and reaping the cognitive rewards.
Exiting the aquatic arena is, after all, more than conjuring new limbs to move, and new lungs to breathe. “An organism that comes onto land also has behavioral and logical problems it needs to solve,” says Alice Gibb, a fish morphologist at Northern Arizona University who wasn’t involved in the study. A well-stimulated brain could have given certain wayfaring fish a leg up in the race onto land.
Rivulus, which inhabit the temperamental, root-infested swamps that shroud mangroves, don’t mind being fish out of water. They’ll deliberately hurl themselves onto the soil to hunt, reproduce, or escape a deteriorating home. But on land, fish cannot swim. Rivulus solve that problem by traveling via a series of spectacular jumps, bending themselves into a C shape and shooting upward like a spring. The demands of these open-air acrobatics, Rossi’s past work showed, clearly rejigger the muscles, transforming the fish, she said, into better “terrestrial athletes.”
Thrust into a brand-new environment, the fish also have to build entirely new mental maps of their surroundings as they cavort—a feat that likely requires revving up their brains. To test whether terrestrial workouts help that process along, Rossi assembled a cadre of about 90 lab-raised rivulus to complete an eight-week exercise boot camp. A third of the fish spent the two-month stint in water-filled bowls, an intentionally dull environment. Another group had the water drained from their containers every few days, so that they split their time between swimming and lounging on the bottom of their newly dried habitats. The final cohort spent nearly all their time in their aquatic apartments, but were plucked out a few times a week to be gently poked with pens—a nuisance that kept them jumping for three minutes straight—then returned to the water to rest, like a very lazy version of high-intensity interval training.
When the training period was over, Rossi dissected the noggins of a few rivulus from each group. She found that the fish that had spent at least some of their time in the air had experienced a growth spurt in a part of their brain called the dorsolateral pallium—the equivalent of the human hippocampus, which helps people navigate new environments. The rest of the animals were plopped into an underwater maze, where both groups of air-exposed fish located a tasty bloodworm more quickly than their purely aquatic peers. These same fish also seemed better acquainted with the maze by the experiment’s end; after entering the structure several more times, they meandered less than the rivulus that had been relegated to water.
In most of Rossi’s experiments, the pen-prodded fish got the biggest brain boosts—even though they’d spent less than an hour and a half exercising in air over the span of the eight-week boot camp, an average of 90 seconds a day. “That is not very much,” says Emily Standen, an evolutionary biologist at the University of Ottawa who has previously collaborated with Rossi’s team but wasn’t involved in the study.
The idea that exercise makes for brainier creatures is nothing new. When animals move around, they’re usually hunting, foraging, fleeing a predator, or seeking or attracting a mate—all prime uses for neural real estate. But the links between physical and cognitive swole aren’t well understood among fish, especially ones that toggle between such drastically different habitats. Rossi’s work, Standen told me, underscores that even short physical forays can be enough to sprout new tissue in the rivulus brain, an asset that these fish can use in air and water alike. “It makes you think you should go out and try something new every day,” Standen said.
What’s still unclear is whether the rivulus’ laboratory education hinged on working out in air. As they were designed, Rossi’s experiments can’t guarantee that a few vigorous swims underwater, for example, wouldn’t have spiffed up the rivulus’ brains to the same extent. Gibb, of Northern Arizona University, told me she thinks the change of scenery was important: Even the fish who had alternated between air and water seemed a bit brighter in the end, despite the fact that they weren’t leaping as exuberantly as their exercised peers.
Other experts I spoke with weren’t ready to jump to conclusions about the importance of air. Based on the evidence so far, “I think it’s more about the exercise,” Prosanta Chakrabarty, a fish biologist at Louisiana State University, told me. Miriam Ashley-Ross, who works with mangrove rivulus at Wake Forest University, said she’d like to see data showing that aquatic aerobics were a poor substitute for a workout on land. That would help clinch the connection between air exposure and intelligence. “A lot remains unexplored,” she told me. (While rivulus in tanks can sometimes be loath to move, she said, they can still be pushed to burn some calories when they’re chased.)
It’s probably still true that exercise and novelty go fin in fin when breeding quick-witted fish. Perhaps those same evolutionary ingredients came together millions of years ago as well, helping our semiaquatic ancestors clamber atop the soil. Land-loving fish are, after all, uniquely poised to inject a healthy dose of peril into their day-to-day life, simply because they can explore so many types of landscapes, whereas pure water- or land-dwellers are limited to one.
But maybe it’s less about air itself, and more about mixing things up in general. Rivulus can survive on land for at least 66 days (they breathe through their skin), and could just as reasonably become savvier upon slipping back into water after an extended stint on shore. Animals also don’t need to venture into a completely different fluid to encounter complexity; past studies have shown that salmon raised in tanks outfitted with plants and rocks become brainier than those whose homes are poorly furnished. Wild rivulus similarly have plenty to contend with in their labyrinthine underwater homes, where they spend their days flitting through mangrove roots and weathering extreme fluctuations in temperature, oxygen concentration, and salinity. Virtually none of that is captured by a plastic bin in a lab.
Disentangling these enticing evolutionary hypotheses is tough. Clever behavior doesn’t fossilize, and we may never fully suss out the smarts of our indecisive ancestors as they flip-flopped between land and water. Either way, it would be overly simplistic, Chakrabarty told me, to assume that a terrestrial lifestyle is some sort of prerequisite for extra smarts: “There are definitely some real geniuses in the water too.”