Animal Numerosity Everywhere

The NY Times reports on a themed issue of the Philosophical Transactions of the Royal Society B, in that many animals have impressive built-in numerosity ability: being able to distinguish between quantities between one and five, say, or proportionally in how 25 is different than 30. In some cases, better than humans: monkeys can visually identify certain number sequences in a split-second.

We are reminded of when Noam Chomsky conjectured that enumeration might be the first mental stepping stone on the way to language -- or how apt it is that we call the "natural numbers" by that name.

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 ten, forty from sixty.

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 .8, which means they can distinguish at a glance between four guppies and five, or eight guppies and ten, 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.

New York Times.