In 2006, a diver named Scott Gardner was ascending from an 18-meter dive in the Keppel Islands region of the Great Barrier Reef when he heard a cracking noise. He looked over and saw a blackspot tuskfish hovering above a sand patch, holding a cockle shell in its jaws. The fish was rolling onto its side and slamming the shell against a rock—alternating left and right blows, aimed at the pointed section of the rock for maximum impact—until the shell cracked open. Scattered around the rock were broken shells from previous meals. This wasn’t an isolated event. It was a feeding station. The fish had a preferred anvil, and it had been using it long enough to accumulate a midden of shattered prey.

Gardner photographed the sequence. The images were published in Coral Reefs in 2011, and the paper posed a question in its title that a generation of biologists had considered already answered: “Tool use in the tuskfish?” The question mark was doing heavy lifting. By the definitions that Jane Goodall had established—the use of an external object as a functional extension of mouth or hand in the attainment of an immediate goal—the tuskfish was using a tool. The external object was the rock. The goal was food. The behavior was deliberate, sequential, and repeated. The only reason anyone hesitated to call it tool use was that the animal doing it was a fish.

Why this matters more than it should

For most of the history of comparative cognition, the assumption was straightforward: fish are simple. They operate on instinct. They have small brains, short memories, and minimal behavioral flexibility. Tool use—the cognitive capacity to identify an external object, recognize its functional utility, and deploy it to achieve a goal—was reserved for the clever animals: primates, corvids, maybe elephants and sea otters. The hierarchy was implicit and rarely questioned. Mammals and birds think. Fish react.

The tuskfish broke that hierarchy not by being unusually smart but by doing something that forced the definition of intelligence to either expand or become incoherent. If tool use is a marker of advanced cognition, and a fish uses tools, then either the fish is cognitively advanced or tool use isn’t the marker we thought it was. Both conclusions are uncomfortable for the framework that produced the hierarchy in the first place.

The discomfort deepened as evidence accumulated. The tuskfish observation wasn’t a one-off. A 2025 study led by Macquarie University, published in Coral Reefs, documented anvil use in five species of Halichoeres wrasses across the western Atlantic—the first evidence of tool use for three of those species and the first video evidence for the other two. Through a citizen science initiative, researchers gathered 16 new observations of wrasses deliberately picking up hard-shelled prey and smashing them against rocks, corals, and other hard surfaces. The findings extended the known range of fish tool use from the Indo-Pacific to the Atlantic and from a handful of isolated observations to a pattern distributed across an entire fish family spanning 50 million years of evolution.

Culum Brown, head of the Fish Lab at Macquarie University and one of the foremost researchers on fish cognition, suggested that wrasses may be fishes’ answer to primates among mammals and corvids among birds—a lineage with a disproportionate number of examples of cognitive complexity relative to the broader group. Researchers at the Paris-Saclay Institute of Neuroscience found that wrasses have a larger telencephalon and forebrain region compared to other teleost fish, including a substantially enlarged inferior lobe—a brain structure with no direct analog in mammals or birds—that shows unique connectivity to the pallium, a region already linked to higher-order cognition in other animals.

The physics problem fish solved

The reason tool use is rare in fish isn’t necessarily cognitive. It’s physical. Water is 800 times denser than air. Try swinging a hammer underwater and you’ll understand the constraint immediately. The momentum required to crack a shell with an object held in your mouth, while suspended in a fluid medium that resists rapid movement in every direction, is orders of magnitude harder to generate than doing the same thing on land. A chimpanzee cracking a nut with a rock is operating in an environment that cooperates with the physics of impact. A fish is operating in an environment that actively resists it.

The tuskfish solved this by inverting the relationship: instead of swinging a tool against a stationary target, it swings the target against a stationary tool. The rock is the anvil, fixed in the substrate. The shell is the projectile, gripped in the fish’s jaws and slammed against the anvil through rapid body rotation. This isn’t just tool use. It’s tool use adapted to an environment where the conventional approach—wielding a hammer—is physically impossible. The fish engineered a workaround.

The sixbar wrasse took the same approach in captivity. Given food pellets too large to swallow and too hard to break with its jaws, the wrasse carried the pellets to a rock in its aquarium and smashed them. The researcher who observed it, Łukasz Paśko at the University of Wrocław, watched the wrasse perform the behavior 15 times and described it as “remarkably consistent” and “nearly always successful.” The behavior only appeared after many weeks in captivity, suggesting the fish learned it through individual experience rather than instinct—it tried other approaches first, found them inadequate, and developed a new strategy.

Anvils, middens, and long-term site fidelity

A 2023 study on graphic tuskfish in New Caledonia found that specific anvils showed evidence of being used by one or more tool-using fish for years. The anvils accumulated debris. Other fish species learned to recognize the visual and auditory cues of tool use in progress—the body movements, sand clouds, and the “clack” sound of shell hitting rock—and gathered as scavengers. In 94 percent of observed tool-use events, attendant fish from six different families showed up to pick up fragments: surgeonfishes, triggerfishes, butterflyfishes, wrasses, angelfishes, and damselfishes. The tuskfish’s tool use had created a micro-ecosystem around its feeding station—a social and ecological structure generated by a fish banging a clam on a rock.

The wrasses also showed flexibility in their tool use, selecting different types of anvils for different prey and sometimes switching anvils mid-session when the first choice wasn’t working. This isn’t stereotyped behavior—the kind of fixed action pattern that “instinct” describes. It’s decision-making under uncertainty, adapted in real time to the properties of the specific prey item and the available tools.

The archerfish problem

The wrasses aren’t the only fish that complicate the tool-use question. Archerfish—four-inch tropical marksmen from estuaries and mangroves between India and the Philippines—hunt by shooting precisely aimed jets of water at insects sitting on vegetation above the water’s surface, knocking them into the water where they can be eaten. The archerfish accounts for refraction at the water’s surface, adjusts for the target’s distance and position, and can hit prey up to three meters above the waterline. Researchers have demonstrated that archerfish can learn to recognize human faces and can be trained to hit specific targets, showing a capacity for visual discrimination and precision that wouldn’t be out of place in a primate cognition lab.

Whether the water jet constitutes a “tool” depends on how strictly you define the term. The archerfish isn’t wielding an external object—it’s producing a projectile from its own body, more analogous to a spider’s web than a chimpanzee’s stick. But the functional outcome is the same: an organism using a mechanism beyond its own body to obtain food that would otherwise be inaccessible. The boundary between tool and technique blurs when the organism in question can’t hold anything in its hands, because it doesn’t have hands.

What 600 species of wrasse haven’t told us yet

There are over 600 species of wrasses worldwide. The Macquarie University team’s citizen science initiative is explicitly calling for divers and snorkelers to report observations of anvil use, acknowledging that the documented cases almost certainly represent a fraction of the actual prevalence. Brown put it directly: “For a long time, tool use was thought to be exclusive to primates and birds. We are still far from knowing how many species of wrasses use tools.” The field of fish cognition itself is young—69 percent of published studies used captive-reared subjects, only 9 percent conducted experiments on wild fish in their natural environment—meaning we’ve been studying fish cognition primarily by watching captive fish in artificial environments and then drawing conclusions about what fish can’t do.

The tuskfish cracking a cockle on a rock doesn’t prove that fish are as smart as chimps. It proves that the cognitive hierarchy we built—mammals on top, birds below them, everything else at the bottom—was a projection of our anatomy onto our definition of intelligence. An animal that solves the same problem a primate solves, in a medium 800 times denser than air, without hands or arms, using a body plan that hasn’t shared a common ancestor with primates in over 400 million years, isn’t failing to be smart. It’s being smart in a way we weren’t looking for.

We cover fish cognition alongside dolphin communication, elephant memory, and primate social intelligence across our Animal Culture & Knowledge course—including why the most important discoveries in comparative cognition keep coming from the species we assumed had nothing to teach us.

A bottlenose dolphin never fully loses consciousness. Not once in its entire life. One hemisphere of its brain sleeps while the other stays awake, the two sides trading off in cycles that distribute the daily sleep quota roughly evenly between them. The eye connected to the awake hemisphere stays open. The eye connected to the sleeping hemisphere closes. When researchers selectively deprived one hemisphere of deep slow-wave sleep, only that hemisphere showed a rebound increase during recovery—the non-deprived hemisphere didn’t compensate. Each half of the dolphin’s brain maintains its own independent sleep debt, as if two separate organisms are sharing one skull and taking turns resting.

This is unihemispheric slow-wave sleep—USWS—and it’s not a curiosity or an edge case. It’s a fundamental alternative to the way sleep works in every terrestrial mammal including humans, and it appears independently in cetaceans, pinnipeds, birds, and possibly reptiles. It raises questions about sleep that the study of human sleep can’t answer, including the most basic one: what, exactly, is sleep for, and why does it apparently need to happen one hemisphere at a time if the whole brain can’t go offline?

How it works neurochemically

When you fall asleep, both hemispheres of your brain transition together into slow-wave sleep—high-amplitude, low-frequency EEG activity that characterizes deep non-REM sleep. Acetylcholine release drops bilaterally. Serotonin and norepinephrine decrease. The whole brain enters a coordinated state of reduced responsiveness. A dolphin does something different. During USWS, acetylcholine release drops in the sleeping hemisphere but remains elevated in the awake hemisphere—a lateralized neurochemical pattern that maintains arousal on one side while the other side generates the characteristic slow-wave oscillations of deep sleep. Noradrenergic neurons continue firing in the awake hemisphere, producing a measurable temperature difference: the awake hemisphere runs slightly warmer than the sleeping one.

The EEG signature is unmistakable. One hemisphere shows the high-amplitude, low-frequency waves of slow-wave sleep. The other hemisphere, simultaneously, shows the desynchronized, low-amplitude activity of alert wakefulness. It’s not drowsiness. It’s not light sleep. One half of the brain is genuinely asleep by every electrophysiological measure while the other half is genuinely awake.

Whales and dolphins exhibit only USWS—they never show bilateral sleep of both hemispheres simultaneously, and whether cetaceans experience REM sleep at all is still unclear. Northern fur seals and sea lions, which live both on land and in water, switch between systems: USWS while swimming, bilateral slow-wave sleep plus REM sleep while hauled out on land. The fur seal essentially runs two different sleep programs depending on whether it’s in an environment where both hemispheres can safely go offline.

Why dolphins can’t just sleep normally

A dolphin that lost consciousness bilaterally would drown. Cetaceans are voluntary breathers—unlike humans, who breathe automatically even during sleep, dolphins must consciously decide to surface and inhale. Bilateral unconsciousness means no surfacing. No surfacing means death. USWS solves this by keeping one hemisphere awake to maintain swimming patterns and control respiration while the other hemisphere sleeps.

But breathing isn’t the only function the awake hemisphere serves. The open eye monitors the environment—and the direction it monitors is revealing. In pods of Pacific white-sided dolphins, animals on the left side of the group keep their right eye open, and animals on the right side keep their left eye open. You’d expect the open eye to face outward, scanning for predators. Instead, the open eyes face inward, toward the center of the group. The dolphins are watching each other, not the surrounding ocean. Researchers concluded that pod formation and social cohesion during sleep matter more to this species than predator detection—the group stays together because each sleeping dolphin is watching its neighbors with its awake hemisphere.

Birds: sleeping on the wing and at the edge

Unihemispheric sleep in birds was noted by Chaucer in 1386—”smale fowles slepen al the night with open ye”—and confirmed by EEG nearly 600 years later. In birds, the phenomenon is called unihemispheric-monocular sleep, and it serves a function distinct from the cetacean version: not breathing, but predator detection.

The most dramatic evidence comes from the “group edge effect.” Mallard ducks sleeping in a row show significantly more unihemispheric sleep at the ends of the row than in the middle. The ducks on the edges keep their outward-facing eye open—the one pointed toward the direction from which a predator would approach—while the ducks in the protected middle of the group sleep with both hemispheres. The edge ducks are literally sleeping with one eye on the threat. They can switch which hemisphere sleeps by turning around, rotating 180 degrees to rest the previously awake hemisphere while activating the other.

Frigatebirds, which can spend weeks aloft over the ocean without landing, sleep primarily unihemispherically in flight—one hemisphere at a time, presumably to maintain aerodynamic control and avoid collisions with other birds. Their sleep is more asymmetric in flight than on land. The total amount of sleep they get in flight is substantially less than on land, but they function with it, which raises questions about how much sleep a bird actually needs versus how much it takes when safety allows.

A 2025 study in Current Biology showed that when sleep pressure builds in birds, they trade asymmetric sleep for symmetric bilateral sleep—essentially, when the need for rest becomes strong enough, the survival advantage of keeping one eye open yields to the biological imperative of getting both hemispheres the deep sleep they require. Sleep need can override vigilance. The bird’s brain chooses rest over safety when the debt gets high enough.

Crocodiles: the evolutionary bridge

Birds are technically reptiles—they’re dinosaurs in the clade Dinosauria—and their closest living relatives are crocodilians. If birds sleep unihemispherically, their reptilian cousins might too. Research on juvenile saltwater crocodiles confirmed unilateral eye closure during behavioral sleep. The crocodiles increased the amount of one-eye-open sleep in the presence of a human, and preferentially oriented their open eye toward the stimulus—the same behavior seen in edge-sleeping ducks and dolphins monitoring pod mates.

Unilateral eye closure during rest has been observed across all three orders of reptiles that have been studied: crocodilians, lizards and snakes, and turtles and tortoises. The EEG evidence for whether this represents true unihemispheric slow-wave sleep (as opposed to simply closing one eye) is less conclusive in reptiles than in mammals or birds. But the behavioral pattern—one eye open, directed at potential threats, during apparent sleep—is consistent enough across the reptilian lineage to suggest that unihemispheric sleep may predate the divergence of mammals and birds. If so, it may be the ancestral condition, and bilateral sleep—the kind humans do—might be the derived state. We might be the weird ones.

What it tells us about sleep

The most important thing unihemispheric sleep demonstrates is that sleep is not a whole-organism phenomenon. It’s a brain-regional process that can occur independently in different neural structures. Each hemisphere accumulates its own sleep debt. Each hemisphere can be deprived and recover independently. The function of sleep—whatever it is—operates at the level of neural tissue, not at the level of the animal.

This has implications far beyond marine biology. In 2016, researchers at Brown University found that humans sleeping in an unfamiliar environment show asymmetric slow-wave activity during the first night—one hemisphere sleeps more lightly than the other, with the lighter-sleeping hemisphere showing greater responsiveness to deviant auditory stimuli. It’s not true unihemispheric sleep. Humans don’t keep one eye open. But it suggests that the capacity for hemispheric asymmetry during sleep isn’t unique to dolphins and ducks—it’s a latent capability in the human brain that emerges under conditions of environmental uncertainty, as if our sleeping brain retains a vestigial version of the sentinel mode that dolphins and birds use as their primary sleep strategy.

The dolphin that never fully loses consciousness, the duck that watches for predators with half its brain, the frigatebird that sleeps on the wing across the Pacific, and the crocodile that keeps one eye on you while it rests—they’re all running variations on the same solution to the same problem: how do you get the benefits of sleep without accepting the total vulnerability that sleep normally requires? The answer, across 500 million years of evolutionary divergence, is the same: you don’t have to shut down the whole system. Half at a time is enough.

We cover unihemispheric sleep alongside octopus distributed cognition, mirror neurons, and the full landscape of comparative neuroscience across our Neurozoology course—including why the most fundamental question in sleep science might be answered not by studying humans who sleep badly, but by studying dolphins who never sleep at all.