We often think of mathematical ability as being uniquely human, but in fact, scientists have found that many animal species—including lions, chimpanzees, birds, bees, ants, and fish—seem to possess at least a rudimentary counting ability or number sense. Crows can understand the concept of zero. And a study published in April found that both stingrays and cichlids can take this rudimentary "numerosity" to the next level, performing simple addition and subtraction for a small number of objects (in the range of 1 to 5).

The latter study's conclusion doesn't surprise cognitive psychologist Brian Butterworth, an emeritus professor at University College London and author of a new book, Can Fish Count? What Animals Reveal About our Uniquely Mathematical Minds.

"There are lots of animals that can do addition and subtraction," Butterworth told Ars. "Bees can. Bees can represent zero as well. So it's not surprising to me that stingrays and cichlids can do it." His book explores how the ability to process mathematical information and extract numerical data from their environment is critical to an animal's ability to survive and thrive. In fact, there might just be an innate understanding of math at its most basic level that was passed down the evolutionary chain from our most distant common ancestors.

Butterworth's interest in the number sense of animals has its roots in his early work as a psycholinguist in the 1980s. When he met an Italian psychologist named Carlo Semenza at a conference, he became intrigued by human disorders in language, like aphasia, and mathematical cognitive disorders, particularly dyscalculia. Christian Agrillo, one of Semenza's students who came to work with Butterworth, was an expert on fish and gave a talk on his research demonstrating that some small fish have numerical abilities. Butterworth was fascinated and eventually developed a parallel research program focusing on the numerical abilities of fish. "And once you get into fish, there's all sorts of other animals that grab your interest," he said.

Butterworth is still studying the genetics and neuroscience underlying number sense in fish, with plans to conduct brain imaging studies later this year. And just how does one go about peeking inside a fish's brain as it counts? "First of all, you need to insert a [biofluorescent] gene, where when neurons connect, when synapses connect, they light up," said Butterworth. "Then you have to have a fish whose head is transparent—a larval version of the fish. That way you can see what’s going on in the fish’s brain using a microscope, while it is choosing the size of the tank with more objects in it."

Ars sat down with Butterworth to learn more.