Numerical competence, or numerosity, is a cognitive ability involving the perception, comparison, and manipulation of quantities without the use of human language or formal symbols. Evidence suggests this quantitative skill is not unique to humans or even primates, but is instead an ancient trait shared across a vast range of species, from insects to mammals. These non-verbal numerical skills serve as the foundation for the more complex mathematical operations observed in some animals under experimental conditions.
The Foundational Skills: Subitizing and Estimation
The perception of quantity relies on two distinct, non-verbal systems for handling small and large sets of items. The first system, known as subitizing, allows for the rapid and accurate assessment of very small numbers, typically between one and four objects. This process is immediate, almost perceptual, and does not require sequential counting, similar to how a person instantly recognizes the number of dots on a die face. Many species, including rhesus monkeys, domestic chicks, and free-ranging dogs, use subitizing to make swift judgments about small groups.
For larger quantities, animals shift to the second system, the Approximate Number System (ANS), which relies on approximation and magnitude estimation. This system represents numbers as mental magnitudes on a continuous scale, meaning the representation is inherently fuzzy rather than precise. The accuracy of estimation is governed by the ratio between the two quantities being compared, a phenomenon described by Weber’s Law. For example, an animal can easily distinguish between two items and six items, but struggles to tell the difference between 30 items and 32 items because the ratio is much closer. The ANS ability to estimate “more or less” is observed across nearly all animal groups, providing a tool for quick decision-making.
Demonstrating Complex Numerical Tasks
Beyond simple perception, many animals manipulate quantities in ways that resemble basic arithmetic and complex numerical concepts. Rhesus monkeys, for example, were trained to associate arbitrary symbols with quantities of liquid reward ranging from zero to 25 drops. When presented with two symbols simultaneously, the monkeys successfully chose the pair whose combined value was greater than a single comparison symbol. This demonstrated that they could combine symbolically represented magnitudes through calculation, rather than relying on memorization.
Primates can also grasp ordinality, the understanding of sequence and rank order. Rhesus macaques were trained to touch a sequence of visual arrays in ascending order, from one item up to nine items. They were able to extrapolate this rule to novel, larger quantities, indicating an understanding of the relationship that numbers exist in a fixed order. This ability to order quantities is thought to be a precursor to true counting.
Complex arithmetic has been demonstrated even in species with smaller brains, such as the honeybee. Researchers trained bees to use specific colors as symbolic operators: one color meant “add one” and another meant “subtract one.” The bees applied these abstract rules to novel numbers, using both short-term memory for the quantity and long-term memory for the operational rule. This capacity, which includes understanding the concept of zero as an empty set, suggests that advanced numerical cognition is not restricted to large-brained vertebrates.
Mapping the Brain’s Number Sense
The biological basis for numerical competence is rooted in specialized neural circuits mapped across diverse species. In primates, number sense processing is primarily localized to the parietal cortex, specifically the intraparietal sulcus, and the prefrontal cortex. Researchers have discovered “number neurons” in these regions of the monkey brain that fire selectively based on a specific quantity, regardless of the objects’ size, shape, or arrangement.
These number neurons exhibit a bell-shaped tuning curve, meaning a neuron tuned to the number five responds maximally to five items, but less strongly to four or six. This similarity in response to close numerosities provides a neural explanation for the ratio effect observed in the ANS. This neural architecture is not confined to mammals; birds like corvids, which lack a six-layered cerebral cortex, possess analogous number neurons in their nidopallium caudolaterale. The presence of such specialized cells in phylogenetically distant animals suggests a conserved evolutionary pressure for processing quantity information.
The Evolutionary Advantage of Numeracy
The widespread presence of numerical competence highlights its importance as an adaptive trait that enhances survival and reproduction. Numerical skills allow animals to make optimal foraging decisions, such as a fish choosing to join a larger school to reduce its risk of predation. Honeybees rely on counting landmarks outside the hive to accurately navigate and measure the distance to a food source, influencing their foraging efficiency.
Numeracy also plays a significant role in social dynamics and conflict resolution. Lionesses, for instance, use the number of roars they hear from intruders to assess the size of the rival group before deciding whether to engage in a confrontation. They compare the number of opponents to the number of allies in their own pride, only choosing to advance if they have a sufficient numerical advantage. The ability to process quantities allows animals to navigate complex ecological challenges and maximize their fitness.