What makes an animal clever? Is it the ability to learn? To reason? To use tools? All of these attributes and more have been used to describe animals as clever.
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But cleverness is hard to define - animals have to contend with different challenges in life and what might be clever or useful for one animal might be of little use to another.
Perhaps it can be as simple as the ability to face the problems that life throws at them, using their own unique adaptations. Whatever the definition, you might suppose that cleverness requires a brain, but the animals on this list may persuade you otherwise.
As you will see, some brainless animals are capable of some remarkable feats, including some that challenge our notions of intelligence and even what it means to ‘think’. Read on to learn more about these fascinating brainless wonders.
How clever are animals without a brain?
Caribbean box jellyfish (Tripedalia cystophora)
Smaller than a 5-pence piece, these tiny jellyfish can be found undulating through the Caribbean Sea. Despite not possessing a central brain - having only a few thousand neurons (nerve cells) clustered around their eyes instead - they are nevertheless avid learners.
A 2023 study found that these brainless creatures are capable of associative learning, that is they are able to associate a new response with a particular stimulus. In this case, they were able to learn to avoid objects in a laboratory experiment, quickly adapting their behaviour based on new information. These findings may challenge long-held beliefs that a central brain is needed for advanced mental processes.
Beadlet anemone (Actinia equina)
Another brainless animal that has been shown to learn in laboratory experiments is the beadlet anemone. Common in rockpools in many parts of the world, you may well be familiar with this ruby-red and tentacled sea creature. A study at the University of Nottingham examined their behavioural plasticity - that is the extent to which their behaviour is able to change in response to different stimuli- and found that they are able to change their level of aggression to other anemones based on familiarity and relatedness.
While they are normally very territorial and aggressive to other members of their species that encroach on their patch, they are able to habituate to those that are genetic clones of themselves. Over repeated interactions they learn to recognise these clones and contain their aggression, whilst still remaining hostile to new intruders.
Portuguese man o' war (Physalia physalis)
This creature, one of the deadliest animals in the sea, might certainly be described as clever despite its lack of cerebrum, if you consider extreme cooperation to be clever.
The Portuguese man o' war, though in appearance close to a jellyfish, is not one, and is technically not even an it but a they! It is a siphonophore, an animal made up of a colony of many smaller organisms working together. These organisms are called zooids, and although distinct individuals, within the same colonial animal are genetically identical as they originate from the same single fertilised egg. They remain connected through their lives and function as one body.
Brittle stars (Ophiuroidea class)
Closely related to starfish, brittle stars have no brain to speak of, just a main nerve ring and nerve cords running down the length of each of their five arms. There is no central processing centre, and each of the nerve cords can act independently of each other.
Despite this these echinoderms are capable of simple associative learning. As with the box jellyfish, this means that they can form associations between different stimuli via a process called classical conditioning (like Pavlovs salivating dog’s association between food and a bell).
In the case of brittle stars, scientists were able to condition them to associate a dimming of the lights with the arrival of food. Over time they formed a connection between the lights going low and dinner time, so that eventually just the change in light was enough to make them come ‘running’ over in anticipation of food. They were even able to remember this association after a two-week long break in the training.
Sponges (Amphimedon queenslandica)
Okay, it might be a bit of a stretch to consider sea sponges particularly clever. They are immobile and not only lack brains, but also any neurons at all! Surprisingly and paradoxically however, they can tell us something about the evolution of the brain.
Scientists have found that although sponges don't have synapses (the junctions between nerve cells that allow them to communicate with each othe), their DNA does contain the code for many synaptic genes. The discovery of the beginnings of a nervous system in such a primitive animal suggests that the common ancestor of all living animals was already pre-adapted for a future with nervous systems and eventually brains.
Green sea urchin (Lytechinus variegatus)
Again, it perhaps depends on your definition of clever, but despite having a very simple nervous system (very similar to that of the related brittle star), sea urchins do show some quite remarkable and resourceful behaviours.
For example, this common species of urchin is known for it’s habit of picking up loose objects from the sea floor - shells, rocks, seagrass, and even plastic - and using them as a covering. The exact purpose of this unknown, but theories include predator defence, protection from UV, and use as a ballast against strong waves and currents. Although an evolved and innate behaviour, rather than something ‘thought up’ this might still be called smart!
Brain Coral (Diploria labyrinthiformis)
This next creature may not have a brain but it sure looks like one! Although it doesnt exactly do anything that might be described as intelligent, it is however a very important part of coral reef ecosystems.
It is what is known as a reef-building coral, hard corals that over time form the foundations of coral reefs and provide intricate and invaluable structures that support a vast array of marine life. The deep valleys and ridges found across the surface of brain corals provide especially good shelter for other organisms, and in particular the Zooxanthella alga with which the coral has a symbiotic relationship. The algae gains protection from the coral and the coral gains nutrients from the algae.
Pond snails (Lymnaea stagnalis)
Cleverness can be hard to define. This is in part because like humans, most animals can be very ‘clever’ in one regard and yet perform very poorly in others. A dolphin may not be able to read a book like we can, but they can hunt using echolocation. This also happens at the individual level, with some members of the same species excelling in some tasks but not others. This is true of pond snails.
A 2015 study found that snails vary in their ability to remember and respond to training, with those snails that were good at forming memories related to food were poor at forming memories about predator threat and vice versa. These findings show that just as there is no universally smart human, there is also no universally smart snail.
Sea squirt (Polycarpa mytiligera)
Sea squirts are pretty simple creatures by all accounts. They are sessile filter feeders, which in common parlance means that they live their lives fixed to a rock, feeding by taking in water in one end and expelling it out the opposite, trapping tiny food particles as they do so.
Simple and brainless though they may be, in some ways they are also remarkable, and remarkably adaptive. One species, Polycarpa mytiligera, is able to regenerate all of its organs even if it is separated into three pieces!
Scientists dissected an individual in three, leaving parts that lacked a nerve center, heart, and part of the digestive system. Astonishingly, and contrary to their own predictions, rather than being left with one dead sea squrit, they instead had three renewed ones!
Slime moulds
Okay, so this next ‘creature’ may not actually be an animal at all, but belonging to the protist group (organisms that are neither fungus, animal, nor plant), but it is so clever it deserves a spot on our list.
These extraordinary single-celled gelatinous organisms can: solve mazes, ‘remember’ foraging routes, make decisions, and anticipate change, and all without a brain to speak of! They have even been found to solve complicated spatial problems, such as transport networks. One thing that they are really good at is finding the shortest path between two points.
Slime moulds in laboratories have effectively recreated Tokyo’s rail network at the micro level, as well as highways from Canada, the U.K. and Spain, finding the most efficient routes between urban areas (or in their case blobs of food).
