Back to the raccoons. Last week I wrote about one way of indirectly assessing intelligence in non-human mammals: the Encephalization Quotient, the difference between actual brain size and expected brain size based on body size. It’s a measure on which humans outscore all our fellow mammals, bottle-nosed dolphins outrank the great whales, and dim creatures like opossums occupy the cellar. Based on data from the German biologist Dieter Kruska, carnivores have higher indices than hoofed mammals, and raccoons outdo their closest relatives, the bears, pandas, and coatis.
But what are they using those big brains for? Anthropologist Robin Dunbar of the University of Liverpool has published extensively on brain size and social intelligence—the cognitive skills needed to sort out relationships within a group, keeping track of kinship ties, alliances, and grudges. In mammals, that’s the work of the neocortex, the part of the brain involved with sensory perception, motor commands, spatial reasoning, and, for some of us, conscious thought and language. Dunbar hypothesized that neocortical function would be important for social primates, and that the size of the neocortex would correlate with the size of the social group.
His magic number is the Neocortex Ratio: the volume of the neocortex divided by the volume of the rest of the brain. A ratio of one means forebrain and hindbrain are equal in volume. In general, group-living primates like chimpanzees and baboons turn out to have higher-than-average NRs, and solitaries like many of the lemurs score low. There are a few anomalies, though. Gibbons live in monogamous pairs, and orangutans are loners; but these apes have high NRs. Although behavior doesn’t fossilize, it can be argued that gibbons and orangutans evolved from more social species.
Humans, of course, have the highest NRs of all. Based on that Dunbar proposes that the “natural” human group size is around 150, and offers supporting evidence from New Guinea villages, Hutterite communities, military units, and workplace social networks. (For the full Dunbar, try Grooming, Gossip, and the Evolution of Language.)
Trying to extend the method to nonprimates, Dunbar and his colleague Julian Bever calculated NRs for a number of carnivores (both highly social species like lions, wolves, and spotted hyenas, and nonsocial species like weasels) and insectivores (moles, shrews, and the like, mostly nonsocial as far as is known.) Some of their results paralleled the primate studies. Weasels tended to score low. Wolves, with an NR of 1.83 and an average group size of seven, outranked coyotes and jackals. Ultrasocial spotted hyenas, at 1.94, had higher ratios than striped or brown hyenas. And the Neocortex Ratio of the African lion was a whopping 2.11; oddly, no data was given for the other cats.
Raccoons are somewhere in the middle of the pack (NR 1.35.) And that’s a problem: raccoons are not all that social. Mothers and their kits form the largest units, and these are only temporary associations. But the raccoon’s ratio is actually a bit higher than that of its close relative the coati, which has an average group size of eight.
The bears are even more problematic. Few carnivores are less social; but North American black bears (NR 2.4), sun bears (2.7), and giant pandas (2.4) score even higher than lions. That’s an awful lot of social intelligence going to waste. As with orangutans, you could always invoke the hypothetical social ancestors. But you also have to wonder what else might be going on in a bear’s forebrain, or a raccoon’s.
Dieter Kruska’s article on evolutionary trends in brain size among mammals provides a clue. Kruska reports that one part of the raccoon’s neocortex is enlarged in comparison with those of similar-sized mammals: the part that receives sensory input from the forepaw. Raccoons are processing the feel of the world; they’re not keeping social scores, much less proving theorems. (What the bears are doing I can only conjecture. Maybe they have richly detailed mental maps of food sources, caching sites, places to hibernate.)
We shouldn’t get too mammal-centric here. I should point out that some birds-corvids and parrots for the most part-are paragons of social intelligence. And they manage just fine without a neocortex. Evolution has produced a whole different brain structure in birds, but it seems to allow for comparably complex social interactions.
I don’t think any of this necessarily invalidates the idea of social behavior as a driver of brain evolution. However, it’s always good to be wary of single-factor explanations. There’s likely to be more than one pathway to increased brain size.