Buzz
The human brain is truly astonishing, with over 100,000 neurons packed into areas as small as a single grain of sand. While our intelligence is beyond question, we’re not the only species with brains that have evolved to meet the demands of survival. From remarkably intelligent elephants to the humble bony-eared assfish (yes, really), the bizarre and fascinating array of brains in the animal kingdom provides a stunning example of evolution in action. Let’s take a brief journey through some of the most unusual examples.
10. Octopus
Being an octopus presents a unique challenge—how to manage all eight of your arms. Luckily for the octopus, its nervous system allows each arm to operate independently from the rest of the body. Unlike humans, octopuses lack a centralized nervous system, although they do have a main brain situated between their eyes. Over 60% of the animal’s neurons are found in the arms, with each tentacle containing an impressive 40 million neurons. These neurons function like a 'mini-brain,' allowing each limb to process sensory input and control its movements.
As a result, octopuses essentially have nine brains, making tasks like foraging much more manageable. The central brain functions as a command center, making critical survival decisions while delegating less significant tasks to the individual arms. Intriguingly, studies have shown that octopus arms continue to process and respond to stimuli even after being severed, highlighting the autonomy of the creature’s mini-brains. In 2001, researchers found that detached limbs still perform the same movements in reaction to stimuli as arms still attached to a living octopus.
9. Spiders
Weaving intricate webs requires significant brainpower, and many spiders have incredibly large brains relative to their body size. Researchers from the Smithsonian Tropical Research Institute aimed to understand how even the smallest spiders manage these complex tasks. They found that smaller spider species have disproportionately large brains, with the tiniest species dedicating up to 80% of their body mass to housing their central nervous system.
Some tiny arachnids are so brainy that their brains actually extend into their limbs, occupying up to a quarter of their legs. The young of these species even have swollen, bulging bodies to fit their surprisingly large brains.
8. Cockroaches
You’ve likely heard the theory that cockroaches can survive nuclear war. These resilient creatures can also survive decapitation—at least for a while. If you remove a human head, the rapid blood loss and disconnection from the brain would result in almost instant death.
The cockroach doesn’t face these problems. Its circulatory system operates at a much lower blood pressure compared to humans, meaning there’s no catastrophic blood loss when its head is removed. Additionally, clusters of nerve tissue in each body segment allow the cockroach to maintain basic functions, such as movement. They also breathe through tiny openings called spiracles found throughout their bodies. Even their heads can survive for some time if kept in the fridge and provided with a steady nutrient supply.
This raises an interesting question—why bother experimenting on decapitated cockroaches in the first place? Despite their repulsiveness, these grim experiments allow us to understand how neurons function in other species. Studies also suggest that cockroach brains might hold the key to new antibiotics, potentially aiding in the fight against infections like MRSA.
7. Sea Squirts
While their appearance might not suggest it, soft and squishy sea squirts are distant relatives of humans, belonging to the phylum Chordata, which includes vertebrates like humans. These humble marine creatures begin life with two primitive brains, a spinal cord, and neurons responsible for movement. However, as they attach themselves permanently to the ocean floor or another fixed underwater object, they no longer need the brain that controls movement. Consequently, one of their two brains is reabsorbed by their bodies, along with their nerve cords.
The way sea squirts degenerate their nervous system has unexpected implications for humans. Research reveals that the genes responsible for neurodegeneration in sea squirts are similar to those linked to diseases like Alzheimer’s and other neurodegenerative disorders. Studying this process in sea squirts could help scientists gain insights into the development of conditions like Alzheimer’s in humans.
6. Giant Squid
Giant squids have a beak located at the front of their heads, meaning that any food they consume must pass through their head before reaching their digestive system. This unique digestive configuration results in a fascinating brain structure. To allow space for the esophagus, the squid’s brain is shaped like a donut, with a hole in the center through which the esophagus passes. Thus, everything the squid eats travels through the middle of its brain.
As you can imagine, the process of swallowing large pieces of food could potentially harm the squid’s brain as the chunks move through the esophagus. However, the squid is well-equipped to deal with this challenge, thanks to its impressive mechanisms for breaking down food. Despite lacking traditional teeth, the squid uses its beak to tear food apart. Additionally, it has a radula, a tongue-like organ covered with rows of tiny teeth, which grinds the food into small particles to shield its brain. This radula is constantly regenerating to replace worn-out teeth.
5. Leeches
A common misconception is that leeches possess 32 brains, but this is not accurate. In fact, leeches have several ganglia, which are nerve clusters that send signals throughout their nervous system, forming their nerve cord. There are two primary 'brains' located at the head and tail, along with 21 smaller nerve clusters scattered throughout their body. These clusters act like mini-brains, giving the leech a total of 23 brain-like structures, not the widely believed 32.
Scientists have found that leeches don’t require all their brain components to carry out certain functions—yes, that includes cutting off their heads. While leeches are naturally good swimmers, propelling themselves through the water in search of prey, removing the ganglion from the top of their 'head brain' doesn’t hinder their ability to swim. In fact, studies revealed that leeches without their first ganglion actually swim better than those with it intact.
4. Manta Rays
Manta rays are equipped with exceptionally large brains, holding the record for the highest brain-to-body ratio among cartilaginous fish. These massive brains are linked to impressive intelligence. In a 2016 study, manta rays demonstrated self-recognition when exposed to mirrors—an advanced cognitive trait only seen in a handful of non-human species. When presented with their own reflections, the rays exhibited unusual, repetitive behaviors, suggesting a level of self-awareness.
However, possessing such an intelligent brain presents unique challenges for cold-blooded species. Manta rays often dive deeper than 980 feet (300 meters) into cold waters, which are far below their optimal thermal range. To counter this, they have a fascinating adaptation known as retia mirabilia, which functions as a natural heat exchanger. This structure surrounds the ray's brain, containing a network of veins and arteries. It's believed that by basking in warmer water before a deep dive, the ray warms the blood in the arteries, which then transfers heat to the veins, keeping the brain warm in colder waters.
3. Elephant
When it comes to brain size, bigger doesn’t always mean better. Elephants are renowned for their intelligence, exhibiting remarkable memory, social skills, and problem-solving abilities. However, their intelligence doesn’t quite match that of humans—at least not by our own criteria. The elephant’s brain weighs three times more than a human brain and contains 257 billion neurons compared to our mere 86 billion, so why don’t they outsmart us?
A 2014 study reveals that it’s not just the total number of neurons that determines an animal’s cognitive capabilities. Elephants may have more neurons than humans, but most of these neurons are located in the cerebellum (the part of the brain responsible for motor control). In contrast, humans have about three times as many neurons in our cerebral cortex, which is responsible for higher-level cognitive functions like learning, reasoning, and language production.
2. Bony-Eared Assfish
The poor bony-eared assfish. Not only does it possess one of the most unfortunate names in the animal kingdom, but it’s also believed to have the smallest brain-to-body ratio of any vertebrate. Despite its strikingly bulbous head, most of its large skull contains little more than empty space, with very little of it devoted to actual brain matter.
While the bony-eared assfish may lack a large brain, it compensates with extraordinary hearing. Its skull contains large otoliths, or “ear stones,” which help it detect low-frequency sounds. The brain itself is adapted to help the assfish navigate its environment, with a notably large cerebellum. These adaptations may allow the creature to sense its own movements and detect nearby objects. Perhaps a smaller brain isn’t so detrimental for the assfish, as its advanced sensory system may be a significant advantage in the pitch-black depths of the ocean.
1. Woodpeckers
Repeatedly slamming your head into a tree would be a surefire way to end up with a concussion... unless, of course, you're a woodpecker. Scientists have long been intrigued by how these quirky birds manage to peck at hard surfaces up to 12,000 times a day without causing brain damage. For a while, it was believed that a soft, spongy section of the skull behind the bird's beak might act as a cushion, absorbing the blows. Some even speculated that the woodpecker’s tongue, extending from the nostrils to the beak, could hold the brain in place like a seatbelt.
However, none of these theories are entirely convincing, as any form of cushioning would interfere with the woodpecker’s ability to strike with the force it requires. Eventually, science seems to have found the answer. In 2022, a team of researchers analyzed footage of woodpeckers in action and discovered that there is no shock-absorbing mechanism at play.
Instead, the shape and size of the woodpecker’s brain protect it from dangerous intracranial pressure. Additionally, the bird has very little cerebrospinal fluid, limiting the movement of its brain inside the skull—what is commonly referred to as 'brain slosh.' According to the team's calculations, a woodpecker could double its usual pecking speed before risking brain injury.
