Every night during breeding season, the male tÃºngara 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 tÃºngaras are doing the same: listening, counting and ultimately mating with the male of maximum chucks.
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, USA. 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," Gary said. "Just as in human communication, an inappropriate comment can end the whole conversation."
The story of the frog's neuro-abacus 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, 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 towards the slightly fishier crowd.
Three-spined sticklebacks are more discriminating still: with a contrast ratio of 0.86, 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. 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.
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.
Attitudes about animal numerosity have changed greatly since the mid-20th century when many researchers believed only humans had enough grey matter to think quantitatively. They cited as an object lesson the 1907 case of Clever Hans, the horse that supposedly could solve arithmetic problems and would tap out his answers by hoof; as it turned out, he was responding to unconscious cues from the people around him. Since then, researchers have approached the field with caution and rigour, seeking to identify the specific evolutionary pressures that might spur the need for numeric judgments in any given species.
Social carnivores like spotted hyenas, for example, live in fission-fusion societies, collectively defending their territories against rivals but in ever-shifting groups of widely roaming members. Sarah Benson-Amram, an assistant professor of zoology and physiology at the University of Wyoming, USA and her colleagues tested hyena numerosity skills in the field, playing back the recorded whoops of spotted hyenas living in South Africa and Namibia to hyenas in Kenya. The Kenyan carnivores reacted to the whoops of strangers as predicted, approaching the hidden source of the sound when they had a home team advantage, retreating when they heard a few too many distinctive voices in a row. Or sometimes, when the local hyenas were outnumbered, they'd call for backup.
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. They can put those numerals in order. The numeric working memory of young chimpanzees is astonishing: flash a random scattering of numerals on a screen for just 210 milliseconds and then cover the numbers with white squares, and a numerically schooled young chimpanzee will touch the squares sequentially to indicate the ascending order of the numbers hidden beneath.
Don't bother trying to do this yourself, Tetsuro Matsuzawa, a primatologist at Kyoto University, said at the scientific meeting in London on which the themed journal was based. "You can't." By the look of it, cerebral property in humans once dedicated to numeric memory has, in the six million years since we diverged from chimpanzees, been co-opted for grander purposes, like the ability to judge whether a sentence like this is true: "There is no non-vanishing continuous tangent vector field on even dimensional spheres."