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Controlling for Body Size

What do you think? When we "controlled for" body size by making all the animals' body sizes equal, what happened? It seems to me that the human and porpoise brains were slightly larger than the brains of the other animals and the brains of frogs were slightly smaller. It looks, in fact, that there may be something to this idea that "intelligence", or "behavioral complexity", is associated with somewhat larger brain size relative to body size. It may be difficult to eye-ball, so here are two graphs that may help us out in this discussion . . .

(image thanks to serendip.brynmawr.edu/~pgrobste/brainevolution/brainevol1.html)

(image thanks to weber.ucsd.edu/~jmoore/courses/allometry/allometry.html)

These graphs (based on data from Jerison, 1973) represent a method for studying relative or proportional features among members of a group of animals called "allometry". In this case, we are comparing brain weight/body weight ratios.

Look at the figure above. This shows data about relative brain size from a number of classes of vertebrates. Here the average weight of a brain in grams is plotted against the average weight of a body in kilograms. (look at chart of brain/body sizes) Within this plot are groups of convex polygons which represent the outer boundary of a set of data. Notice that there is a line running from the lower left to upper right through the two polygons . . . these lines represent the average, or best fit line, of all the data within that polygon.

What else do you see in this graph? The top surface, or outer limits, of the higher vertebrate polygon is set by animals that are usually thought to be most intelligent, i.e. humans, porpoises, andwhales. Yet, generally the primates, which as a group are considered to be the "highest" (or most complex) vertebrates, have brains that, for the most part, are only somewhat above average for mammals (for ex: look for the gorilla and chimpanzee).

What do we make of all this? The elephant and the blue whale seem to set the most outer limits of the high vertebrate polygon. Does this mean that elephants and blue whales are the smartest animals on earth? There doesn't seem to be a whole lot of evidence for that. So, what we need to do now is completely factor out body size. This is often done by calculating how far each point is from the average line in the polygon . . . these numbers are called "residuals". These residuals can be then used to calculate an index of brain size relative to what would be predicted for an animal of that body size. These indexes are called the "encephalization quotient" or EQ.

The encephalization quotient allows us to ask if the brain of a given species is bigger or smaller than would be expected. An "average" score is 1 and anything above that is assumed to indicate above average intelligence. Humans are most often at the top of this scale. Does this mean we are the most intelligent animals on the planet? How reliable do you think this method of allometry and EQ is for measuring intelligence? (What is intelligence anyway??????? Is it reasonable to compare the intelligence of a crocodile with that of a whale or that of a human? Go here for a discussion about intelligence)

So, the size of different brains in different animals IS different in a way that seems to relate to differences in behavior. I wonder whether, in addition to size, there are other differences in brains of different animals that might relate to differences in their behavior. Do you? If so, a good way to investigate that question is by making the brains of all animals the same size and then looking to see whether we see any other kinds of differences.