This is interesting:
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 two is distinct from three, that 15 is distinct from 20. This mental representation of set size, called numerosity, seems to be “a general ability,” and an ancient one, said Giorgio Vallortigara, a neuroscientist at the University of Trento in Italy.
Now, researchers are uncovering increasingly more complex numerical abilities in their animal subjects. 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 “zero” — an idea so paradoxical that very young children sometimes struggle with it. In fact, experiments have shown that both monkeys and honeybees know how to treat zero as a numerosity, placing it on a mental number line much as they would numerosity one or two. And in a paper published in the Journal of Neuroscience in June, researchers reported that crows can do it, too.
The fact that those three species are from diverse taxonomic groups — primates, insects and birds — suggests that certain numerical abilities have evolved over and over again throughout the animal kingdom.
Wild monkeys can do something similar. While monkeys watched, scientists placed several pieces of bread in a closed box, then periodically removed one or more of them. The monkeys could not see how many pieces remained, but they continued to approach the box until the last piece was removed — which suggested that they performed subtraction to inform their foraging.
That honeybee study raised eyebrows, not just because it showed that an animal with fewer than one million neurons in its brain (compared with the human brain’s 86 billion) could treat zero as a quantity, but because bees and mammals diverged in evolution 600 million years ago. Their last common ancestor “was [barely] able to perceive anything,” Avarguès-Weber said, much less count. According to Nieder, who was not involved with the insect work, this implied that the ability to grasp the empty set and other numerosities evolved independently in the two lineages.