| We can count "one, two, buckle my shoe" and "three, four, knock at the door." We can quickly calculate how much to tip after a meal out, or how long a train downtown might take. We can see the injustice in someone getting three pieces of chocolate when we only get one. At the heart of these everyday calculations is an intuitive understanding of numbers, known as "number sense." Our brains can understand, connect and relate numbers. Humans aren't unique in having a number sense. Researchers have documented a wide range of animals, from ants and bees to monkeys and spiders, who also seem to grasp at least some numbers. Most of the studied animals can assess the number of items in a set. Honeybees, for example, can determine which areas have the most flowers to forage. Monkeys can judge which trees hold the most fruit (and the fewest hungry competitors). From the roars that they hear, lionesses can assess the strength of a new pride invading their domain and decide whether to fight them or retreat. "It simply pays off for the survival of an animal to be able to differentiate numeric quantities," Andreas Nieder, the chair in animal physiology at the University of Tübingen in Germany, told me last year. The great utility of number sense, and its widespread distribution among diverse species, are among the reasons that biologists think it's likely to have first appeared hundreds of millions of years ago — fairly early in the history of animals with brains. Yet the diversity of specific numerical abilities also suggests that some of them may have evolved separately and repeatedly throughout the animal kingdom.
What's New and Noteworthy Relatively little is yet known about the neurological foundations for number sense. Two decades ago, researchers discovered that certain neurons involved in these processes in monkeys are linked to "favorite numbers." Some neurons are tuned to the number 5, for example, while others are tuned to the number 10. A neuron will fire more often for its favorite number and less often for others. Studies in humans have determined that there is a limit on the number of items that our brains can accurately judge at a glance. That limit is four. We are much better at instantly estimating that a pile of books contains four volumes than nine. By examining the brains of patients with epilepsy, a group of researchers recently uncovered the neural underpinnings of why such a boundary might exist. As I reported for Quanta, they found that the brain uses a combination of two mechanisms to judge how many objects it sees. One mechanism estimates quantities. The other one sharpens the accuracy of the quick estimate, but only for small numbers. The new research "seems to be the beginning of a new leap" in our understanding of number perception, Pedro Pinheiro-Chagas, an assistant professor of neurology at the University of California, San Francisco, told me. But much else is still unknown about how the brains of humans and other animals perceive numbers, including how we process the abstract concept of zero. The "number zero is the most fascinating number of them all," Nieder said. "It's the eccentric uncle in the family of numbers." It takes children a few years to grasp the concept of zero even after they learn other small numbers. Nieder's team and others have found that some animals, like monkeys, honeybees and crows, also have the ability to understand zero. This diversity suggests that certain numerical abilities may have evolved repeatedly throughout the animal kingdom, Quanta reported in 2021. Once "you realize that almost every animal, or maybe even every animal, has some ability to do a numerical task, then you start wanting to know … what's the threshold? What's the limit?" Scarlett Howard, a postdoctoral research fellow at Deakin University in Australia who studies numerical cognition in honeybees, told Quanta. In some cases, the threshold seems to be much higher than was once believed. Bees, for instance, know that zero is less than 1, which is less than 2, which is less than 3. They can also add and subtract. There are over 600 million years of evolution between humans and bees, yet we can do similar numerical tasks, Howard said. The evidence that some understanding of numbers might be fundamental to brains throughout the animal kingdom is really adding up. | 
