The human brain is truly amazing, with over 100,000 neurons packed into segments the same size as a single grain of sand. While our smarts are undeniable, we’re not the only creatures with brains cleverly adapted for survival. From ultra-clever elephants to the lowly bony-eared assfish (no, really), the weird and wonderful variety of brains found in nature provides an incredible illustration of evolution in action. Let’s take a whistlestop tour through some of the strangest.
Being an octopus throws up a unique challenge—how to control all of your eight arms. Fortunately for the octopus, its nervous system allows each arm to act semi-independently of the rest of its body. Unlike humans, octopuses don’t have a centralized nervous system, although they do have a main brain between their eyes. Over 60% of the animal’s neurons are inside the arms, with each limb containing a whopping 40 million neurons. These neurons act like a “mini-brain,” enabling each tentacle to process sensory information and determine its own movements.
Therefore, octopuses effectively have nine brains, which makes activities like searching for food much easier. The central brain acts like a control center, making decisions affecting the animal’s survival while leaving less important decisions to individual arms. Fascinatingly, experiments show that octopus arms continue processing and reacting to data after being chopped off, demonstrating the autonomy of the creature’s mini-brains. In 2001, scientists discovered that severed limbs perform the same movements in response to stimuli as arms attached to a living octopus.
Spinning complex webs takes a lot of brainpower, and it turns out that many spiders have simply enormous brains in comparison to their bodies. Researchers from the Smithsonian Tropical Research Institute set out to find out how the tiniest spiders manage these complex behaviors. They discovered that smaller species have proportionally larger brains, and the teeniest species devote up to 80% of their bodies to housing their central nervous systems.
In fact, some small arachnids are so brainy that their brains bulge into their limbs, filling up to a quarter of their legs. The babies of these species even have distended, bulging bodies to accommodate their comparatively enormous brains.
You’ve probably heard the hypothesis that cockroaches can withstand nuclear war. Well, these hardy beasties can also survive decapitation—for a while, at least. Remove the head from a human, and rapid blood loss and disconnection of the body from the brain ensures pretty instantaneous death.
The cockroach has no such issues. Its circulatory system maintains much lower blood pressure than humans, so there’s no catastrophic blood loss when you remove the head. Furthermore, clumps of nerve tissue in each body section allow the body to continue certain basic functions, such as movement. They can also breathe through tiny holes called spiracles distributed throughout the body. Even the heads can survive for a while if you keep them in the fridge with a steady supply of nutrients.
All this begs the question—why bother experimenting on decapitated roaches in the first place? Disgusting as they may be, these grim experiments help us learn about how neurons function in other species. Research also suggests that roach brains could contain new antibiotics, providing another tool to help combat infections like MRSA.
7 Sea Squirts
While you wouldn’t guess by looking at them, squishy sea squirts are distantly related to humans and belong to the phylum Chordata, a group encompassing vertebrates such as humans. These unassuming marine creatures start life with two primitive brains, a spinal cord, and neurons to control movement. They then attach themselves permanently to the ocean floor or another stationary, underwater object. As they no longer need the brain responsible for movement, their bodies reabsorb one of their two brains and their nerve cords.
How a sea squirt degenerates its nervous system has surprising implications for humans. Research shows that the genes that cause neurodegeneration in sea squirts are similar to the genes causing neurodegeneration in Alzheimer’s disease and several other brain disorders. Studying brain absorption in sea squirts could help scientists understand how conditions like Alzheimer’s develop in humans.
6 Giant Squid
Giant squid have a beak at the front of their heads, which means their food must pass through the head to reach their digestive systems. This interesting digestive setup leads to some pretty unusual brain anatomy. To accommodate the esophagus, the giant squid has a donut-shaped brain, allowing the esophagus to pass through the hole in the middle. Therefore, everything the creature eats goes through the middle of its brain.
As you might imagine, swallowing large pieces of food can damage the squid’s brain as the chunks travel through the esophagus. Fortunately for the squid, it has some formidable equipment designed to break down meals into manageable pieces despite not having any teeth in the traditional sense. First, the creature uses its beak to rip its food apart. It also has a radula, which is a tongue-like structure covered in row upon row of tiny teeth. The radula grinds the food down into small particles to protect the brain and constantly regrows to replace worn-out teeth.
There’s a commonly-held belief that leeches have 32 brains, but this isn’t entirely true. Instead, these blood-sucking critters have multiple ganglia, which are clusters of nerves carrying signals throughout the nervous systems and forming the leech’s nerve cord. The leech has two main “brains” at the head and tail with 21 mid-body nerve clusters that act like mini-brains. So, that’s 23 “brains” in total—not the 32 commonly touted.
Scientists have discovered that leeches don’t need all their brains to perform certain functions by—you guessed it—chopping their heads off. Leeches are decent swimmers at the best of times, propelling themselves through the water to find prey. Removing the ganglion at the top of a leech’s “head brain” doesn’t stop it from swimming. In fact, researchers discovered that leeches missing their first ganglion swim better than intact leeches.
4 Manta Rays
Manta rays have truly colossal brains and have the highest brain-to-body ratio of any cartilaginous fish. These big brains result in some impressive smarts. A 2016 study discovered that manta rays recognize themselves in a mirror, a sign of advanced intelligence only seen in a few non-human species. When shown their own reflections, the rays showed unusual, repetitive behaviors, demonstrating a degree of self-awareness.
However, possessing a super-smart brain presents challenges to cold-blooded species. Manta rays frequently dive deeper than 980 feet (300 meters) below sea level, exposing their bodies to temperatures well below their ideal thermal range. A fascinating structure called the retia mirabilia acts like a built-in heat exchanger, allowing the ray to keep its brain warm and functioning well in cold conditions. The structure wraps around the creature’s brain and contains a network of veins and arteries. It’s thought that basking in warm water before embarking on a deep dive heats up the blood in the arteries, transferring warmth to the veins via the retia mirabilia.
Smashing your head repeatedly against a tree is a surefire recipe for concussion… unless you’re a woodpecker. Scientists have long speculated about how these comical birds can peck hard surfaces up to 12,000 times per day without sustaining brain damage. Until relatively recently, it was thought that a spongy section of the skull behind the bird’s beak could act like a crash helmet and cushion the blows. As the woodpecker’s tongue stretches behind the skull from the nostrils to the beak, some even conjectured that it holds the brain in place like a seatbelt.
However, none of these theories are convincing because any form of cushioning would prevent the woodpecker from pecking with force, which is what it wants to do. Finally, it appears that science has the answer. In 2022, a team of scientists analyzed videos of woodpeckers pecking and discovered there’s no shock absorption at play.
Instead, the size and shape of the woodpecker’s brain prevent dangerous intracranial pressure. Furthermore, woodpeckers have minimal space for cerebrospinal fluid, limiting movement of the brain inside the skull, or “brain slosh.” According to the researchers’ calculations, a woodpecker could peck at twice its usual speed before risking brain injury.
2 Bony-Eared Assfish
Poor bony-eared assfish. Not only does this deep sea creature possess one of the most unfortunate monikers of any animal, but it’s also thought that it may have the smallest brain-to-body ratio of any vertebrate. While the assfish has an arrestingly bulbous head, relatively little of its enormous cranium contains any actual brain.
What the bony-eared assfish lacks in brains, it makes up for in hearing power. Its skull contains large otoliths, or “ear stones,” allowing it to hear low-frequency sounds. The brain itself shows several adaptations to help the assfish thrive in its environment, such as a large cerebellum. These characteristics may give the creature a keen sense of its own movement and the position of nearby objects. Maybe a small brain isn’t such a bad thing for the assfish, as an enhanced sensory system could be a major advantage in a dark, deep ocean environment.
When it comes to brain size, bigger isn’t always better. Elephants are known for their intelligence, displaying impressive memory, social, and problem-solving skills. However, their intelligence doesn’t match that of humans—at least by our own standards. Elephant brains weigh three times more than human brains and contain 257 billion neurons compared to our lowly 86 billion, so why don’t they outsmart us?
According to a 2014 study, it isn’t just the overall number of neurons that determines an animal’s cognitive abilities. Elephants may have more neurons than we do, but the vast majority are inside the cerebellum (the part of the brain controlling muscular activity). However, humans have around three times as many neurons in our cerebral cortexes. Our highly developed cerebral cortexes allow us to develop higher-level thinking skills, such as learning, reasoning, and language production.