Animals can count, new research shows

They have keen sense of quantity and are able to distinguish two from four, four from 10, 40 from 60

E very night during breeding season, the male tungara frog of Central America will stake out a performance patch in the local pond and spend unbroken hours broadcasting his splendour to the world.

The mud-brown frog is barely the size of a shelled pecan, but his call is large and dynamic, a long downward sweep that sounds remarkably like a phaser weapon on Star Trek, followed by a brief, twangy, harmonically dense chuck. Unless, that is, a competing male starts calling nearby, in which case the first frog is likely to add two chucks to the tail of his sweep. And should his rival respond likewise, Male A will tack on three chucks.

Back and forth they go, call and raise, until the frogs appear to hit their respiratory limit at six to seven rapid-fire chucks.

The acoustic one-upfrogship is energetically draining and risks attracting predators like bats. Yet the male frogs have no choice but to keep count of the competition, for the simple reason that female tungaras are doing the same: listening, counting and ultimately mating with the male of maximum chucks.

Specialised brain cells

Behind the frog’s surprisingly sophisticated number sense, scientists have found, are specialised cells located in the amphibian midbrain that tally up sound signals and the intervals between them.

“The neurons are counting the number of appropriately timed pulses, and they’re highly selective,” said Gary Rose, a biologist at the University of Utah. If the timing between pulses is off by just a fraction of a second, the neurons don’t fire and the counting process breaks down.

“It’s game over,” Rose said. “Just as in human communication, an inappropriate comment can end the whole conversation.”

The story of the frog’s neuroabacus is just one example of nature’s vast, ancient and versatile number sense, a talent explored in detail in a recent themed issue of the journal Philosophical Transactions of the Royal Society B, edited by Brian Butterworth, a cognitive neuroscientist at University College London, C. Randy Gallistel of Rutgers University and Giorgio Vallortigara of the University of Trento in Italy.

Scientists have found that animals across the evolutionary spectrum have a keen sense of quantity, able to distinguish not just bigger from smaller or more from less, but two from four, four from 10, 40 from 60.

Orb-weaving spiders, for example, keep a tally of how many silk-wrapped prey items are stashed in the “larder” segment of their web. When scientists experimentally remove the cache, the spiders will spend time searching for the stolen goods in proportion to how many separate items had been taken, rather than how big the total prey mass might have been.

Small fish benefit from living in schools, and the more numerous the group, the statistically better a fish’s odds of escaping predation. As a result, many shoaling fish are excellent appraisers of relative head counts.

Guppies, for example, have a so-called contrast ratio of 0.8, which means they can distinguish at a glance between four guppies and five, or eight guppies and 10, and if given the chance will swim toward the slightly fishier crowd.

Three-spined sticklebacks are more discriminating still: with a contrast ratio of .86, they’re able to tell six fellow fish from seven, or 18 from 21 — a comparative power that many birds, mammals and even humans might find hard to beat.

Chimpanzees are social scorekeepers, episodic warriors and number ninjas, too. They can be taught to associate groups of objects with corresponding Arabic numerals up to the number 9 and sometimes beyond — three squares on a computer screen with the number 3, five squares with 5, and so on. They can put those numerals in order.

Born with number sense

Despite the prevalence of math phobia, people too are born with a strong innate number sense, and numerosity is deeply embedded in many aspects of our minds and culture.

Researchers have determined that number words for small quantities — less than five — are strikingly similar across virtually every language group studied, and the words are among the most stable, unchanging utterances in any lexicon.

They are more conserved through time and across cultures than words for other presumably bedrock concepts like mother, father and most body parts, with a few puzzling exceptions like the words for tongue and eye.

“The sounds that you and I use to say ‘two’ or ‘three’ are the sounds that have been used for tens of thousands of years,” said Mark Pagel, a biologist who studies the evolution of language at the University of Reading.

“It’s not out of the question that you could have been wandering around 15,000 years ago and encountered a few of the last remaining Neanderthals, pointed to yourself and said, ‘one,’ and pointed to them and said, ‘three,’ and those words, in an odd, coarse way, would have been understood.”

That continuity, Pagel added, “should astonish us.”