Practically every animal that scientists have studied — insects and cephalopods, amphibians and reptiles, birds and mammals — can distinguish between different numbers of objects in a set or sounds in a sequence. They don’t just have a sense of “greater than” or “less than,” but an approximate sense of quantity: that 2 is distinct from 3, that 15 is distinct from 20. This mental representation of set size, called numerosity, seems to be “a general ability,” and an ancient one. Many species have displayed a capacity for abstraction that extends to performing simple arithmetic, while a select few have even demonstrated a grasp of the quantitative concept of 0 — an idea so paradoxical that very young children sometimes struggle with it. Both monkeys and honeybees know how to treat 0 as a numerosity, placing it on a mental number line much as they would numerosity 1 or 2. And crows can do it, too.

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