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Many Animals Can Count, Some Better Than You (nytimes.com)
An anonymous reader shares a report: 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 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. 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.
Recent research has shown that bird brains have a much higher neuron density than mammal brains, most prominently in the forebrain, so even though their brains are much smaller they are more 'optimized' and can therefor reach the same complexity as that of much larger animals like primates. See for example http://www.pnas.org/content/113/26/7255.full