Animals can do math
SOME ANIMALS LIKE ORB-WEAVING SPIDERS AND TÚNGARA FROGS ARE BORN WITH AN INNATE UNDERSTANDING OF QUANTITY
Every night during breeding season, the male túngara 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 irst frog is likely to add
two chucks to the tail of his sweep. And should his rival respond like-
wise, 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-ire 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 túngaras 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,” says 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 ire and the counting process breaks down.
“It’s game over,” Rose says. “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.
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 silkwrapped 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 ish beneit from
living in schools, and the more numerous the group, the statistically better a ish’s odds of
escaping predation. As a result, many shoaling ish
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 ive, or eight
guppies and 10, and if given the chance will swim towards the slightly ishier crowd.
Attitudes about animal numerosity have changed greatly since the mid20th century when many researchers believed only humans had enough gray 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 ield with caution and
rigor, seeking to identify the speciic evolutionary
pressures that might spur the need for numeric judgments in any given species.
Social carnivores like spotted hyenas, for example, live in ission-fusion
societies, collectively defending their territories against rivals but in ever-shifting groups of widely roaming members. “You can never predict who you’ll ind in
which group,” says Sarah Benson-amram, an assistant professor of zoology and physiology at the University of Wyoming. “You might be alone or in a group of 10.”
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, ive squares
with 5, and so on. They can put those numerals in order.
The numeric working memory of young chimpanzees is astonishing: lash a random scattering
of numerals on a screen for just 210 milliseconds — half an eye blink — 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, says at the scientiic meeting in London on which the themed journal was based. “You can’t.”