One Fish, Two Fish, Fish Can Count(ish?)

Most of us don’t think about the importance of counting beyond our Sesame Street days. Little did we know, that purple puppet was teaching us an essential cognitive survival technique. In the wild, counting allows individuals to join larger social groups, determine the number of mates available, and choose more plentiful food. But counting has long been considered the purview of smarter species with higher levels of perceived consciousness, the Clever Hans horses and celebrity lab chimps of the animal kingdom. Increasingly, though, scientists have shown that fish—often considered near the bottom of the spined-species hierarchy—are able to discern between discrete quantities much like their more cognitively complex counterparts. Moreover, the evidence shows that the way piscine brains “count” is similar to way our own brains process numerical quantities, suggesting deeper evolutionary origins for one of our most essential cognitive skills.

Building on the findings of a 2015 study conducted with guppies, recently published research in Animal Behavior shows that freshwater angelfish presented with two small quantities of food reliably chose the larger stack of snacks. The preference for larger quantities supports the idea that fish are able to process quantitative information in order to be more successful foragers in the wild. This isn’t “counting” in the “one, two, three” sense—fish likely have little use for The Count’s prescribed methods—but it shows that fish do know the difference between these quantities.

The idea that fish can “count” isn’t new—fish have been shown to be able to discriminate between different sized groups (or “shoals”) of their own species, which is especially beneficial for smaller fish that rely on large groups for protection—but calorie count is more immediately important to a fish’s individual survival than choosing a slightly larger group of friends.

“Whether a fish chooses the very large shoal or the somewhat smaller shoal makes [little] difference from a survival perspective,” says Robert Gerlai, a biologist at the University of Toronto and one of the authors of the paper. “But whether it eats more or eats less is very important.”

The new research demonstrated more than just fish’s ability to count for their lives. As the food quantities grew larger than four items, the angelfish in the study were less picky with their choice. Other vertebrates behave the same way when presented with large quantities. Vertebrates—including humans—and even some exceptional invertebrates like bees are thought to have separate systems of counting for small and large quantities, in which small numbers are perceived as exact quantities and larger numbers are more roughly estimated. And humans, just like the angelfish in the study, seem to switch from the exact system to the approximate one around the magic number four.

The connection between fish and humans may come in handy as scientists continue to explore the intricacy of human cognition. “Fish are easier to study than complex humans,” says Gerlai. “In the long run, ideally, we would like to know what the [human] brain can do, and you can study much better with fish.”

But the findings beget more evolutionary existentialism. Humans and fish diverged evolutionary over 400 million years ago (humans and apes, by comparison, are thought to have parted evolutionary ways between 4 and 13 million years ago). “If you find some numerical abilities in fish, then those abilities are more ancient than previously thought,” says Christian Agrillo, a biologist at the University of Padova, who was not involved in the current research, but published one of the earliest studies on fish counting in 2008. If such skills can be traced back to our fishy ancestors, it may change how we understand our own cognitive magnificence.

The scientific jury is still out on whether fish actually have two systems of numerical cognition. Agrillo points out that while fish represent half of the world’s vertebrates, most lab studies are conducted only on guppies, angelfish, and zebrafish. “To better understand the issue, we need to focus on a larger range of fish,” he concedes. But the research has already come a long way since Agrillo began studying fish cognition in 2004. “Until some years ago, nobody thought that fish could have numerical estimation,” he says. “When we started, we were the only one. It seemed like a very silly curiosity of science.”

But for some scientists and activists, our cognitive connection to fish is especially important considering the way fish are treated in our global economic system. Humans use—and often abuse—fish, recklessly mass-harvesting wild stocks for food, raising them in intensive aquaculture conditions, ripping them from reefs to keep as pets, and even conducting unfettered testing on them for scientific research. Yet fish receive fewer legal protections than their charismatic vertebrate counterparts. Depending on local laws, fish are often exempted from animal welfare protections entirely.

“Humans tend to give more empathy to animals they think are smart,” Culum Brown, a biologist at Macquarie University who studies the behavioral ecology of fish. “Because of that, people have had very little consideration for fishes because most people underestimate them.” Brown argues that emerging research supporting fish intelligence should qualify them for more ethical treatment than is afforded by current policies. Rather than take fish off the menu entirely, Brown at least hopes to see more consumers advocating for humane treatment of tuna, salmon, and their finned brethren, much like the way the free-range movement has taken off for chickens. After all, sharing cognitive roots with other vertebrates not only helps fish count, but also gives them the capacity to feel pain.

“What is striking about vertebrates is how conserved they really are—just about every aspect of human cognition has been observed in other animals,” Brown says. “The reason that humans suffer the way that they do is we inherited it from our fishy ancestors.”

Counting aside, studies show that fish’s cognitive abilities rival other vertebrates in many ways. Sharks’ sense of smell is 10,000 times more sensitive than humans’. Thanks to an extra cone in their eyes, some fish see colors more vividly than we do. Fish can recognize family members, inherit social traditions in the form of migration patterns, and use primitive tools. Contrary to beliefs popularized by Finding Nemo, many fish have fantastic memories, avoiding hooks for as long as a year after being caught once and building mental maps of their surroundings that they retain for weeks after being moved.

Brown, for his part, has “mixed feelings” about Finding Nemo. On the one hand, the movie led to a massive overharvesting of the starring species, and the ditzy Dory character feeds into the debunked (but prevailing) mythology that fish have two-second memory spans. But he also sees the positive impact it has had on public perception. “People like and can warm to fishy characters,” he says, “If you can have empathy for Dory and Nemo and everyone else, that’s got to be a positive thing.”

The recent findings on fish cognition may not immediately change human hearts and minds with respect to their intelligence, but every new study can be counted as a step in the right direction.

Editor's note 8/24/18: We've pushed back the date of Christian Agrillo's early fish numerosity study.