Last week we had a short field note talking about brain size and how it's not always a good indicator of animal intelligence. Then in today's podcast - Episode 21 - we spoke more in depth about the difficulties of quantifying intelligence, especially in other organisms. At the end of the technical section, I mentioned relative brain to body size. So that's the goal of this note: to explain the concept of relative brain size.
Depending on which biologist you talk to, some will prefer absolute brain size and others will prefer relative brain size as an indicator of the animals' intelligence. Note that I said an indicator. This is because neither concept is perfect and there are exceptions to both.
On the ratio, it's usually written as EQ, which stands for Encephalization Quotient. The idea is that, using some specific equations, it creates a prediction for a standard brain size when compared to the body size of a number of species. Then, a specific species is tested against this line. If the brain is larger, the animal is believed to be more intelligent, whereas if the brain is smaller than the predicted size for the body, the animal is not as intelligent. If the brain is the same size as predicted, then the intelligence is judged to be average.
Since it is an equation, animals are given a number. If you have a number of 1, you're considered average. However, if you receive a number below 1, you are considered less intelligent, whereas if you are given a number greater than 1, you're considered more intelligent. If we make the cat the average and give it a EQ number of 1, some more intelligent creatures are elephants (EQ=1.3), Ravens (EQ=2.5), Chimpanzee (EQ=2.5), Bottlenose Dolphin (EQ=5.3) and Human (EQ=8). This, on the surface, seems like it might be on to something, but let's explore some of the limitations of this method.
Since it takes body weight and size into account, an obese organism and an underweight organism would have a similar brain size but a significantly different body size. For this reason, this method does not work well to calculate an individual's intelligence and needs to be used to calculate groups or species on a whole. The equation also only seems to be plausible for mammals. Using it for other groups like birds or reptiles seems to be less accurate, and when using it on insects it seems to fall apart completely.
Another theory is that the size of the neocortex and the density of neurons are better indicators of intelligence than the EQ equation. There seem to be some studies that support this, but the entire field shows how difficult it is to pin the answer on one factor.