Columns

Enough with the murderous female arthropods. It’s time for something furry. 

Intelligence is hard enough to get a handle on in our own species. When it comes to other critters, even our fellow mammals, things get even trickier. Some are just not amenable to testing in a lab setting. I understand that at least one scientist has tested spotted hyenas in the field, with interesting results, but you can imagine how difficult the logistics would be. Interpreting the data is another can of worms. 

Take the raccoon. We think of them as anywhere from fairly bright to fiendishly clever. But when I started looking for lab studies, even Google Scholar failed me. We know that they’re very good with their hands and can count up to three, and that’s about it. (Long ago, there was an institution in Hot Springs, Arkansas called the IQ Zoo, whose inmates included a basketball-playing raccoon. I once saw him in action. He could shoot with reasonable accuracy, as I recall. But his defensive game was weak.) 

That data gap is one reason biologists have been tempted to use brain size, or some permutation thereof, as a proxy for animal intelligence. Among other advantages, that allows for comparison of living and extinct-even fossil-species, as long as you can measure their braincase volume. And it’s kind of intuitive, especially if you start with big-brained Homo sapiens as a reference point. 

The pioneer in brain-size measurement was Harry Jerison, who published the key work Evolution of the Brain and Intelligence in the ‘70s. What Jerison looked at was not just the size of the brain-larger mammals have larger brains-but the ratio of actual brain size to expected brain size based on body size. He called the resulting number the Encephalization Quotient, meaning, as Richard Dawkins explains it, “how much bigger, or smaller, the brain of a particular species is than it ‘should’ be, for its size…”  

If you plotted brain/body ratios for every mammalian species from pygmy shrew to blue whale, an animal whose values just fit the curve would have an EQ of one. With less than one, you’re below the curve; more than one, above. 

Humans, naturally, have the highest EQ around: 6.28 according to a later revision of Jerison’s formula. Primates as a group score higher than other mammals, as do dolphins, followed by carnivores, then hoofed mammals. Opossums, at 0.39, are near the bottom of the mammalian ranks, along with armadillos and moles. 

I’m sure Jerison’s book includes an EQ for the North American raccoon. But it’s too wet out, finally, to track it down at the UC library. Instead I’m going to rely on another set of calculations published in 2005 in the journal Brain, Behavior and Evolution by Dieter Kruska, who seems to have picked up where Jerison left off, although his terminology is a little different (Kruska uses Encephalization Index, or EI, rather than EQ, and a best-fit value of 100 rather than one.) The other caveat is that Kruska gives an EI for the tropical crab-eating raccoon (Procyon cancrivorous) rather than the North American raccoon (P. lotor). However, they’re very close relatives and it wouldn’t be a big surprise if their scores were similar. 

Anyway, Kruska assigns the crab-eating raccoon an EI of 162, equivalent to an EQ of 1.62. This is higher than the typical range for its next of kin, the other procyonids (ringtails, coatis and the like), bears, and pandas, who score between 92 and 135. Mongooses, hyenas, and weasels are lower than the carnivore average. In fact, according to Kruska, a weasel’s brain shrinks as it matures. He provides no numbers for cats, and I’d rather not get into that sensitive subject in front of Matt. 

There’s a lot more in Kruska’s article, including a fascinating discussion of brain size under domestication. The gist is that we’ve bred domestic mammals, even recent domesticates like mink and gerbils, for smaller brains. A dog’s brain is significantly smaller than a wolf’s. Moreover, when domestics go feral their brains don’t get larger. The dingo, which has been on its own in Australia for thousands of years, still has a dog-sized brain. 

Not that this has anything to do with what raccoons are doing with their relatively large brains. There’s another way of looking at intelligence-specifically social intelligence, the mental skills required by group living-and brain size, and I’ll get to that next time.