In June 2025 a research team published drone footage of southern resident killer whales biting off lengths of bull kelp stalk, positioning the piece between their own body and a partner’s, and rolling it back and forth for as long as fifteen minutes. It was the first documented case of tool manufacture and use in any marine mammal. It happened in the Salish Sea, to a population of fewer than eighty animals that has been individually photographed, named, catalogued, and followed since the 1970s, by researchers who know these whales by the notches in their dorsal fins.
Sit with the arithmetic on that. Fifty years of continuous observation of the most intensively studied cetaceans on the planet, and a behavior involving manufactured objects and a fifteen-minute duration went unrecorded until somebody put a camera above them instead of beside them. The whales did not learn this in 2024. We learned to look down.
That is the honest shape of almost every recent addition to the animal tool use catalog, and it is why the list keeps getting longer without the animals getting any smarter. The list was never a measure of what animals can do. It has always been a measure of what we have managed to see, filtered through a definition we wrote for our own convenience, and the underlying capacity turns out to be far more widely distributed than the catalog ever suggested. There is a specific neural mechanism behind that distribution, it was worked out on monkeys that barely use tools in the wild, and it is the reason the lecture title says extended self rather than clever animals.
What counts as animal tool use, and who decided
The working definition most of the field still uses descends from Benjamin Beck: the external employment of an unattached or manipulable attached object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself, with the user holding or carrying the tool during or just prior to use.
Read that carefully and notice how much work the qualifiers are doing. The object must be unattached, which is why a chimpanzee using a rock as an anvil sits in a different category from a chimpanzee wielding a rock as a hammer. The user must hold or carry it, which excludes a great deal of interesting behavior. A bearded vulture dropping a bone onto rocks to shatter it is not using a tool under this definition, because the rock is part of the landscape and the bird never held it. Sea otters smashing shellfish against a stone on their chest are sometimes classified as tool users and sometimes demoted to proto-tool users depending on whether the stone or the shellfish is doing the moving.
None of that is arbitrary in the sense of being random. It is arbitrary in the sense of being a human decision about category boundaries, made to keep a research literature tractable, and it has consequences. Every few years someone proposes a revision, most recently a framework distinguishing tooling from mere object use on the basis of whether the animal is dynamically managing a mechanical relationship rather than statically placing something. Under different definitions, different animals join and leave the club.
There is a second layer of gatekeeping underneath the first, and it concerns manufacture. Using a found object is one category. Modifying an object to suit a purpose is another, and modifying it before you can see the situation it will be used in is a third that people treat as the interesting one. The whale biting a length off a kelp stalk crosses into manufacture. So does the crow trimming a twig into a hook. So does the palm cockatoo shaping a drumstick. Whether that boundary tracks anything real in the nervous system, as opposed to tracking our intuitions about foresight, is an open question that the field has largely declined to answer while continuing to use the boundary.
This matters because it means a meaningful fraction of the growth in the animal tool use list is definitional rather than empirical. Behavior that was known and excluded gets reclassified as included. And it means the reflexive question people ask about a new finding, whether it really counts, is usually a question about our filing system rather than about the animal.
Worth noticing what the definition was built for. It was written to discipline a literature that had a real problem with overclaiming, in an era when a single anecdote about a clever animal could circulate for decades without anybody checking it. As a filter against nonsense it earned its keep. As a description of what nervous systems are doing it was never meant to be load-bearing, and it has been quietly carrying that weight for forty years because nothing better arrived.
Where the extended self is actually located
Here is the mechanism, and it is the part that reframes everything else.
In the mid 1990s Atsushi Iriki and colleagues trained Japanese macaques to retrieve distant food with a rake and recorded from bimodal neurons in the caudal postcentral gyrus, cells that respond to both touch on the body and vision near the body. Those neurons carry something like a map of the hand and the space immediately around it. When the monkey used the rake, the visual receptive fields of those cells changed. They stretched to include the length of the rake, or to cover the expanded region the animal could now reach. The subsequent review by Angelo Maravita and Iriki on tools and the body schema laid out the case that a tool in active use gets incorporated into the neural map of the body, as though the effector had been elongated to the tip of the implement.
The tool does not feel like a held object to the nervous system doing the holding. It gets annexed. Peripersonal space, the zone the brain treats as immediately body-adjacent and worth defending, expands to the end of the stick. Related work found tool-use training driving immediate-early gene expression and neurotrophic factor expression in intraparietal cortex, which means this is not a transient perceptual illusion but an actual plastic change in tissue.
Now the detail that makes it important. Japanese macaques do not habitually use tools in the wild. They can be trained to be dexterous with them, and they are not a tool-using species in any natural-history sense. The neural machinery for incorporating an external object into the body schema was sitting there anyway, unused, in an animal whose ecology never called for it.
That is the finding that should govern how anyone reads a tool-use headline. The capacity is not the achievement. Body schema plasticity appears to be a general property of nervous systems that have to coordinate a limb with a visual field, which is most of them, and what varies between species is not whether the machinery exists but whether ecology, anatomy, and opportunity ever conspire to switch it on. Animal tool use is a behavior that gets expressed, not a faculty that gets evolved from scratch each time.
The human side of the same literature runs in parallel and is worth a beat, because it establishes that this is not a monkey curiosity. People using a tool show measurable shifts in how they judge distances and in how visual and tactile events get bound together across the extended reach, with the effects appearing after minutes of practice and decaying after the tool is set down. Patients with parietal damage show tool-related deficits that dissociate from ordinary grasping, which implies at least partly separate circuitry for acting through an object rather than on it. The consistent finding across species and methods is that the body model is a running estimate rather than a fixed inventory, continuously refitted to whatever the organism is currently doing.
Once that is on the table, the comparative question changes shape. Instead of asking which animals are smart enough to use tools, the productive question is which animals have a manipulator worth extending, an ecological problem that extension would solve, and enough tolerance for failure to get through the learning curve. Those are three separate constraints and they are all about circumstances rather than intellect, which is why they can be satisfied in a walnut-sized parrot brain and go unsatisfied in the largest brains on the planet.
The orcas, and the fifty years nobody saw it
Back to the whales, because the case study is unusually instructive.
Between April and July of 2024, researchers flying an unoccupied aerial vehicle over the central Salish Sea recorded roughly thirty instances of what they named allokelping, published in Current Biology as a report on the manufacture and use of allogrooming tools by wild killer whales. The whales detach a complete bull kelp stalk, bite off a short length of the stipe, maneuver it between themselves and a social partner, and roll it along their bodies. The kelp is firm but flexible with a slippery surface, which one researcher compared to a filled garden hose, and the leading hypotheses are skin hygiene, since orca skin accumulates scaly buildup, and social bonding, since grooming in other species is at least as much about relationships as about cleanliness.
The details that make it look like a real cultural behavior rather than an oddity: older whales with more skin sloughing were more likely to participate, participation was biased toward close kin, and specific pairs did it repeatedly, including a twenty-nine-year-old female and her five-year-old daughter, and grandmothers with grandsons. Cetaceans have long been known to drape kelp over themselves, a behavior called kelping. Doing it with a partner, using a length the animal shortened itself, is a different thing.
There is a conservation edge on this that deserves stating. This is a critically endangered population, bull kelp in its habitat is declining with warming water, and the researchers noted that the behavior’s persistence may be at risk. The southern residents are also the population whose vocal dialects made cetacean culture a serious research subject, which means we now have two independent culturally transmitted traditions in the same eighty animals, both of which could be lost with them.
But the methodological point is the one to carry forward. The behavior was invisible from a boat. It required looking straight down, in good light, with enough resolution to see a piece of kelp against a black-and-white animal, over enough hours to catch thirty instances. Every one of those is a technology problem, not a biology problem.
There is also a specific reason cetaceans were a hole in the catalog rather than a genuine absence, and it is anatomical rather than cognitive. Tool use as classically defined requires holding or carrying, and a whale has no hands. What it has is a mouth, a rostrum, and pectoral fins that do not oppose. Any cetacean tool behavior therefore has to route through the mouth or through pressing an object between two body surfaces, which is exactly what allokelping does, and which no primatologist writing a definition in the 1970s would have thought to accommodate. The sponge-carrying and shell-trapping traditions in bottlenose populations had already established that the mouth-and-rostrum route works. The deep-diving species whose social behavior is hardest to observe at all remain the obvious place to look next, and nobody has managed sustained aerial observation of them.
Elephants, hoses, and one elephant turning off another’s shower
The elephant result is more recent and considerably funnier, and the comedy is doing real analytical work.
Researchers observed a female Asian elephant at a zoo using a hose as a flexible shower head, and not simply holding it: adjusting her grip and trunk posture to direct the spray at different parts of her body, switching techniques for different regions, apparently handling the hose as a manipulable object with variable behavior rather than as a fixed water source. That is a tool by any reasonable reading of the definition, deployed on the user’s own body, which is the clause in Beck’s formulation people usually forget is there.
Then the second elephant. A companion animal was observed interfering with the water supply, kinking the hose and disrupting the flow while the first elephant was showering, in a pattern the researchers were careful to describe cautiously and which the coverage immediately and irresistibly called a prank. Whether it represents intentional interference or something less interesting is not settled, and the honest reading is that a single individual’s behavior in a captive setting is a weak base for strong claims.
What is not weak is the general point about elephants and tools, which has an anatomical wrinkle. An elephant trunk is roughly forty thousand muscle units with no bone, capable of grip, suction, precision manipulation, and demolition. It is already the most versatile manipulator in the animal kingdom, which means the ecological pressure to extend it with objects is lower than it would be for an animal with less capable hardware. That the populations studied in the wild still use branches as fly swatters, scratch with sticks, and plug water holes anyway is more interesting given that they hardly need to, and the long-term behavioral records from different ecological contexts keep turning up local variation in what they bother to pick up.
The trunk case generalizes into a principle that explains several gaps in the catalog. An animal with an extremely capable native manipulator has less to gain from an external one, which predicts low tool-use rates in elephants and in cephalopods relative to their cognitive capacity, and both predictions hold reasonably well. Run it the other way and the prediction is that tool use should be concentrated in animals whose native anatomy is almost but not quite sufficient for the job, which is a decent description of a chimpanzee facing a termite mound, a crow facing a beetle larva in a hole, and a sea otter facing a shell it cannot crack with its teeth. Necessity is not the mother of invention here. Near-sufficiency is.
The working-animal record contains a version of this too. Elephants employed in Burmese teak extraction and in wartime logistics learned to handle objects and equipment in ways nobody trained explicitly, and the best-documented individual cases come from handlers whose survival depended on noticing what the animal figured out. Those observations were never collected as tool-use data. They were collected as work notes, which is another way material gets lost.
Cockatoos, and the arrival of the tool set
Parrots have quietly become the most productive experimental system in the field, and Goffin’s cockatoos are the reason.
Goffin’s are not tool users in the wild in any documented systematic way, which makes them the same kind of case as Iriki’s macaques: latent capacity without ecological expression. In the laboratory they innovate tools, and in a task requiring the use of one object to control the movement of a second, a setup the researchers called the Golf Club Task, individuals worked out composite tool use, which is the simultaneous coordinated use of more than one tool and which had been reported in very few non-human animals, mostly specific nut-cracking techniques in chimpanzees and capuchins.
More striking, Goffin’s have been shown to transport tool sets. Given a task solvable only with two different implements, and given a distance to cross, birds carried both tools together rather than making two trips, which implies some representation of the requirements of a job that has not started yet. Tool sets were for a long time a signature of great ape technology, particularly the multi-implement termite and honey extraction kits documented in central African chimpanzee populations.
The wild-parrot side keeps producing too. Sulphur-crested cockatoos in Sydney worked out how to open kerbside waste bins and the technique spread geographically as a social innovation, then the same population was documented operating public drinking fountains. And the palm cockatoo, which manufactures a drumstick from a branch and beats it against a hollow trunk with individually distinctive rhythms, remains the only known non-human case of manufactured instrumental sound production, which is a category with an audience of exactly one species and no competitors.
The kea’s reputation for dismantling anything left unattended belongs in the same conversation, and so does the awkward fact that parrots achieve all of this with a beak and one foot, in a brain the size of a walnut with no cortical layers, which is a fairly direct problem for anyone who wants tool use to be a story about cortex.
The tool-set finding deserves one more paragraph because of what it implies about representation. Carrying two implements across a distance to a job you have not started requires holding something about the structure of the task while the task is not in front of you, which is the kind of claim that used to be reserved for apes and which the experimental design was specifically built to test rather than to assume. Birds also adjusted their transport behavior when the task only required one tool, which is the control that makes the result interesting: they were not simply carrying everything available. Chimpanzee tool sets in central African populations had established the behavior in a primate lineage with a plausible evolutionary story attached. Finding it in a parrot removes the evolutionary story and leaves the capacity.
Corvids, and what is actually new
New Caledonian crows have been the flagship for two decades and the recent work has shifted from whether they use tools to how they think about them. The established repertoire is genuinely impressive: hooked tools manufactured from specific plant species, stepped cuts in pandanus leaves, tool selection by task, and in one much-discussed result the assembly of a functional long tool from separate short components that were individually useless, which is compound tool construction with no obvious template.
What has changed is the framing. The species is now studied less as a curiosity and more as a system for asking about planning, memory for tools, and whether the birds represent a tool’s function independently of the specific object. The Hawaiian crow, extinct in the wild and maintained in captive breeding, turned out to be a habitual tool user as well when anyone finally tested it, which is a reminder that absence of evidence in a poorly studied species means very little.
For ravens, the most cognitively flexible of the widely distributed corvids, the tool-use record in the wild is thinner than their reputation implies, and the laboratory record is strong. That gap between wild behavior and demonstrable capacity is now such a consistent finding across corvids, parrots, and macaques that it has stopped being an anomaly and started being the pattern.
The pattern has an uncomfortable implication for how the catalog gets read. If most tested species turn out to have more capacity than their wild behavior displays, then the documented distribution of animal tool use across the tree of life is not a map of ability. It is a map of ecological opportunity crossed with research attention, and the honest version of any such map would need error bars wide enough to swallow most of its own conclusions. The songbirds whose learning has been characterized in the most detail have never been seriously tested for object manipulation, not because anyone thinks they would fail but because nobody has had a reason to ask.
The primate updates are not about sticks anymore
Chimpanzee tool use has been documented since Jane Goodall, so the interesting recent work has moved to the level of material selection and to categories nobody was filing under tools at all.
At Gombe, analysis of termite-fishing implements found that chimpanzees are not grabbing whatever stem is handy. They preferentially select plant species with mechanical properties suited to the job, favoring materials with the flexibility to navigate a curved termite tunnel, which is a materials-engineering decision embedded in a foraging behavior. The long-term study populations in the Mahale mountains show their own local technological traditions, and the between-population variation in what gets used and how is one of the strongest lines of evidence for chimpanzee material culture.
Then the category that is genuinely new. Chimpanzees have been observed catching insects, applying them to open wounds on themselves and on other individuals, which is either topical medicine or something that looks remarkably like it. In 2024 a wild Sumatran orangutan named Rakus was documented chewing leaves of Fibraurea tinctoria, a plant with known antibacterial and anti-inflammatory compounds, and repeatedly applying the resulting material to a facial wound, which then healed without infection. A single individual is a single individual and nobody should build a theory on one orangutan, but the behavior was targeted, repeated, and directed at a specific injury.
Self-medication with an applied substance sits awkwardly against the classical definition, since a chewed leaf poultice is not exactly an unattached object employed to alter another object. It is also obviously the same underlying competence: using something external to change a physical situation. Which brings us back to the definitions doing more sorting than the animals.
The medicinal cases also arrive with a methodological trap attached. A single wild individual doing something once, observed by researchers who were already watching closely, is exactly the observational situation that generates both genuine discoveries and durable myths, and the two are indistinguishable at the time. The appropriate response is neither dismissal nor a press release, but the field’s incentive structure rewards the press release. Rakus may well turn out to be the first documented instance of a widespread behavior nobody had caught. He may also turn out to be one orangutan who happened to chew a leaf near a wound. Both remain live, and the papers involved were considerably more careful on this point than the coverage.
Underwater, and in animals with no hands at all
The invertebrate and aquatic cases are where the concept gets stress-tested hardest, because the anatomy is wrong for everything our intuitions expect.
The veined octopus collects discarded coconut shell halves, carries them stacked beneath its body in an awkward stilt-walking gait that is slower and more costly than normal locomotion, and later assembles them into a shelter. The carrying is the part that satisfies the definition, since the animal is transporting an object at a cost for delayed future use. Octopuses have also been shown to learn to use mirrors to locate food they cannot see directly, a capacity previously demonstrated only in birds and mammals, and there is a documented and thoroughly enjoyable literature on octopuses propelling debris at each other with jets of water.
Among fish, tuskfish carry bivalves to a specific rock and strike them against it repeatedly to break them open, returning to the same anvil site. Under Beck’s definition the fish is arguably not the tool user, since the rock stays put and the clam does the moving, which is exactly the kind of ruling that makes the definition look like a technicality rather than a biological distinction. The reef fish whose cooperative hunting arrangements with moray eels rewrote assumptions about fish social cognition sit in the same uncomfortable zone: obviously doing something sophisticated, awkwardly served by categories built for primates.
Dolphins in Shark Bay wear marine sponges on their rostrums to probe the seafloor without abrading themselves, a tradition transmitted primarily from mothers to daughters, and separately use empty shells to trap and extract fish in a behavior that spreads through the population horizontally rather than by descent. Bottlenose populations elsewhere show their own local behavioral traditions, and the beluga that spent years working the Norwegian coast demonstrated how quickly a cetacean will incorporate human objects into its own behavior when given the chance.
Insects belong in the list and rarely make it. Certain ant species drop soil particles into liquid food to soak it up and carry the saturated grains back to the nest, which is object use for transport with no other interpretation available. Some wasps use small pebbles to tamp down nest closures. Neither behavior involves anything resembling a brain in the sense the rest of this discussion assumes, and both satisfy the definition as written, which is either a reason to revise the definition or a reason to stop treating the definition as a proxy for cognition. It is probably the latter.
Why the animal tool use list keeps growing
Four things are driving the expansion, and only one of them is about animals.
Instrumentation is first and largest. The orca finding came from a drone. Camera traps have produced tool-use records in species nobody could follow on foot. Biologgers and accelerometers detect stereotyped movement patterns in animals underwater and at night. Higher frame rates and better resolution catch fast manipulations that a human observer registers as a blur. Each new sensor produces a wave of first documented reports, and the wave says more about the sensor than the species.
Observer effort bias is second and it cuts both ways. Tool use gets found where people look, and people look at charismatic, accessible, diurnal animals. The distribution of documented animal tool use across the tree of life correlates disturbingly well with the distribution of research funding and field station locations. The Hawaiian crow case is the cleanest demonstration: a habitual tool user that went undocumented because nobody had run the test.
Captivity is third and it is genuinely double-edged. Goffin’s cockatoos, Iriki’s macaques, and a great deal of the strongest experimental work involves animals with time, safety, and nothing to do, which is a condition that reveals latent capacity and also a condition no wild animal occupies. A laboratory result establishes what a nervous system can do. It does not establish that the behavior is part of the species’ natural repertoire, and conflating the two is the most common error in popular coverage.
Definitional drift is fourth, and it quietly reclassifies old observations as new discoveries without anything being discovered.
There is a fifth factor that belongs on the list even though it is awkward, which is that some of the growth is real behavioral change driven by us. The Sydney cockatoos opening waste bins are exploiting an object that did not exist in their environment a century ago, and the technique spread through the population in a documented geographic wave. Urban animals encountering novel manipulable objects at high density are a genuinely new selective and learning environment, and behavior that emerges there is new behavior rather than newly observed behavior. That is a small share of the catalog and it is the only share that reflects animals actually doing something they were not doing before.
What is not on the list of drivers: animals acquiring new cognitive abilities. On the timescale of the last twenty years of publications, essentially none of the growth in animal tool use records reflects the evolution of new capacity. It reflects epistemics, instrumentation, and in a few urban cases a novel object supply.
What tool use predicts, and what it does not
The folk model treats tool use as an intelligence trophy, a rung on a ladder, and the comparative data will not support that reading.
Brain size does not track it. Cetaceans have the largest brains on Earth and, until 2025, no documented tool manufacture at all. Parrots do sophisticated composite tool work in a brain a few grams in mass. Elephants have three times our brain mass and use tools casually rather than centrally. Meerkats and African wild dogs run intricate cooperative societies with essentially no object technology, and the bowerbird constructing and decorating an elaborate display structure is doing something architecturally sophisticated that the definition mostly excludes.
What does predict it is a mundane trio. Manipulative anatomy: a hand, a beak plus a foot, a trunk, a set of arms with suckers. An extractive foraging niche, meaning food that is embedded, encased, or otherwise not immediately available, which is the ecological problem tools solve. And opportunity, in the form of enough slack in the daily energy budget to fail at something repeatedly without starving.
The macaque troop that famously washes its food is a useful check on the intelligence framing, since the behavior spread socially through the population without any object being employed at all, which means the transmission machinery and the tool machinery are separable. Culture does not require tools and tools do not require culture, even though the two travel together often enough that people assume otherwise.
One more correlation deserves killing. Sociality is often invoked as a driver, on the theory that living in groups creates opportunities for observational learning that accelerate technological accumulation. The theory is reasonable and the data are messy. Octopuses are close to asocial and manage object use. Some highly social primates use almost no tools. The cooperative hunters and pack societies that run the most complex coordination in the mammalian world do it entirely without objects. What sociality plausibly does is speed the spread of an innovation once it appears, which is a claim about transmission rather than invention, and the cases where a behavior demonstrably moved through a population support the transmission half while saying nothing about where the innovation came from.
The extended self, taken literally
Put the mechanism and the catalog together and a cleaner picture emerges than the list-of-clever-animals version.
Nervous systems maintain a model of the body: where the limbs are, what they can reach, which region of space counts as adjacent and worth monitoring. That model is not fixed. It updates continuously, and it will absorb an external object that is being actively used to act on the world, remapping receptive fields to the tip of the implement and expanding the defended zone outward. This appears to be a general property of the relevant parietal machinery rather than a specialization, which is why it shows up in an animal that never uses tools in the wild.
On that account, animal tool use is what happens when an existing plastic body model meets a manipulator, an extractive foraging problem, and some free time. It is not a threshold that gets crossed. It is a capacity that gets recruited, which is why it keeps appearing independently in lineages that separated hundreds of millions of years ago and share almost nothing about their neural organization.
The same principle runs the other direction in a way that ought to be uncomfortable. Human prosthetic and brain-interface work depends on exactly this plasticity, on the nervous system’s willingness to treat a manufactured object as part of the body given adequate sensory feedback and practice, and the engineering succeeds precisely to the degree that it exploits machinery a macaque has too. The attempts to push that integration further are not adding a new human capability. They are leaning on a very old vertebrate one.
Which leaves the self as something less solid than advertised. The boundary between organism and environment is not a fact the brain discovers. It is a hypothesis the brain maintains, revises when a rake is in hand, and revises again when the rake is set down. An orca rolling a length of bull kelp along her daughter’s back has, for those fifteen minutes, a body that includes a piece of seaweed. So does anyone who has ever driven a car into a parking space they could feel the edges of. An orca has a body that ends where her attention says it ends, and so do you. The mechanism does not care about the species, and the 24-lecture Neurozoology course works the tree of life on that assumption throughout, from the animals whose knowledge visibly moves between individuals to the working animals whose capacities were discovered by people who needed something from them and the first edition’s survey of what nervous systems are actually built to do.
The list will keep getting longer. Fifty years of watching eighty whales missed a fifteen-minute grooming ritual until a drone went up, which is a reasonable estimate of how much else is being missed right now, in animals nobody has flown a camera over yet. There is a last thing worth extracting, and it concerns what to do with the next headline. When a new species joins the animal tool use catalog, the productive questions are not whether it counts or how smart that makes the animal. They are: what instrument caught it, and how long had people been watching without seeing it. What manipulator is the animal extending, and what was almost-but-not-quite sufficient about it. Whether the object was found, modified, or made before the problem was visible. And whether anyone has tested the obvious neighboring species, or simply not gotten around to it. Those four questions will tell you more about a finding than any amount of argument about the definition, and they are the questions the course applies to every capacity it examines rather than only this one.
The mechanism was always there. We are just finally in a position to catch it running.