The question of which invertebrate is the most intelligent is complex because this largest group of animals is incredibly diverse. Intelligence is not a single measure; it manifests as either sophisticated individual problem-solving or highly complex social organization. Researchers must compare the cognitive feats of animals separated by vast evolutionary distances, revealing a wide spectrum of abilities that challenge our traditional, vertebrate-centric view of what it means to be smart.
Defining Intelligence in Invertebrates
Researchers evaluate invertebrate intelligence by focusing on specific behavioral and neurological hallmarks that go beyond simple, fixed-action instincts. True cognitive function is demonstrated by the capacity for learning, such as associative learning where an animal links two separate stimuli, or operant conditioning where behavior is modified by consequences. The ability to store and retrieve information, showing both short-term and long-term memory, is a strong indicator of advanced cognition. Cognitive abilities are also measured by an organism’s adaptive behavior, specifically its capacity for flexible problem-solving in novel situations. This flexibility allows the animal to adapt its actions based on past experience or to improvise a solution to a new challenge, distinguishing it from instinct.
The Leading Candidates: Cephalopods
The consensus candidate for the most intelligent invertebrate belongs to the class Cephalopoda, which includes the octopus, squid, and cuttlefish. The octopus is particularly notable for possessing the largest and most complex nervous system of any invertebrate, containing hundreds of millions of neurons. This system is unlike that of vertebrates, as only about one-third of the neurons are centralized in the brain; the remaining two-thirds are distributed throughout the arms in a decentralized network. This unique structure allows each arm to act with a degree of autonomy, sensing and reacting to the environment without constant input from the central brain.
Octopuses are renowned for sophisticated individual problem-solving, such as unscrewing jars to access prey, escaping complex enclosures in laboratories, and tool use, like carrying coconut shell halves for future shelter. Their high intelligence also manifests in highly adaptive behaviors like their camouflage, which involves instantaneous changes to their skin color and texture. Some species, such as the mimic octopus, engage in active mimicry, changing their shape and behavior to impersonate over 15 different marine animals, including venomous lionfish and sea snakes. Cuttlefish, close relatives of the octopus, have also demonstrated advanced cognitive control, showing an ability to delay gratification for a better reward, a feat of impulse control comparable to that seen in large-brained vertebrates.
Social Insects and Other Clever Contenders
While cephalopods display remarkable individual intelligence, another group of invertebrates excels in collective and social cognition: the social insects. Ants and bees showcase swarm intelligence, a form of problem-solving that emerges from the complex interactions of many relatively simple individuals. This collective decision-making allows colonies to solve problems far beyond the capacity of a single insect.
Honeybees exhibit highly sophisticated communication through the waggle dance, a symbolic movement that translates the distance and direction of a food source to other foragers. Their navigational abilities rely on the sun’s position and the polarization patterns of light in the sky to maintain a compass sense even on cloudy days. Ant colonies utilize pheromone trails for efficient foraging and are capable of complex engineering feats, such as constructing living bridges with their own bodies to cross gaps.
Individual Hunters: The Portia Spider
In the realm of individual hunters, the jumping spider genus Portia is a standout, displaying strategies that require memory and planning. These arachnids specialize in hunting other spiders, often relying on detour behavior where they select a path that temporarily moves them away from their prey to reach a better ambush point. The Portia spider can remember the location of prey for up to an hour, demonstrating an ability to plan several steps ahead in its hunting routine. It also uses specialized web-shaking techniques to mimic the vibrations of a trapped insect, thereby luring its victim closer.
How Researchers Measure Invertebrate Cognition
Researchers employ a variety of controlled experimental protocols to quantify the cognitive abilities observed in invertebrates. Maze running, such as the Ant Visual Discrimination Y-Maze, is commonly used to test spatial learning and an animal’s ability to associate visual cues with a reward. Visual discrimination tasks involve training animals to recognize and respond to specific shapes or colors, which has been used to demonstrate that bees can categorize stimuli and that cuttlefish can recognize symbols.
The cuttlefish delayed gratification experiment, adapted from the human “marshmallow test,” is a prime example of measuring impulse control and the ability to plan for a future reward. This task required the mollusks to choose between an immediate, less-preferred food and a delayed, highly-preferred food, proving their capacity to exert self-control. For self-awareness, the mirror self-recognition (MSR) test is sometimes applied, although it is often debated for species with different sensory priorities. However, one study showed that ants of the species Myrmica rubra are the only invertebrates to date that exhibit mark-directed behavior, attempting to remove a spot of paint only after viewing it in a mirror.