I like this kind of question because it lets me talk about the way that a structure is related to its function. Let’s start with the brain size.

The size of the brain by itself doesn’t say a lot. A better predictor of intelligence is the ratio of brain size to body size. Say you were making a bunch of sandwiches and had to figure out whether you could spread the peanut butter pretty thick. Knowing the size of the PB jar is not enough, you have to know how many slices of bread you’re covering. That would be the ratio of PB to bread.

Bigger animals need bigger brains just to get the incoming information from a bigger body and control its movements. What we really want to know is whether the brain is larger than you would predict if you just knew the body size. Hippos have a pretty big brain, but their brain makes up roughly 1/2800 of their body weight. Our brain makes up about 1/40 of our body weight. Dolphins are close to us with a ratio of 1/50. This isn’t quite the whole story for a couple of reasons. First, there’s apparently a minimum size for a mammal’s brain. Then as body size increases, brain size increases more slowly. This means that the ratio of a mouse’s brain to its body is similar to ours, even though mice aren’t capable of the same kind of problem solving that dolphins are. Researchers use a mathematical equation to correct for this and get something called an encephilization quotient. “Cephala” means head. Let’s just call it an EQ. We’re way up there on the EQ sale with a score of 7.44, while dolphins are a 5.31. That’s still really high. Mice are down there a 0.5, but they’re still above the rabbits, which are at 0.4.

But there’s more. The part of a mammal’s brain that does the real thinking and reasoning is the outer layer. Now we have to talk about shape. Imagine that I challenged you to put an entire sheet of notebook paper between your thumb and forefinger. One edge of the paper has to touch each side. Pretty soon you’d probably realize that you could fold the paper like an accordion, something like this: WWWWW. Folding is how you fit something with a big surface area into a small container. The surface of our brain looks all folded up because we have a whole lot of that thinking surface layer packed into our skulls. Dolphin brains are wrinkly too. Check out this site for a comparison: compare brain size . The brain of a rabbit is pretty smooth. Sorry rabbits.

Of course, we also have evidence from dolphin behavior that they are quite intelligent. They can learn many tasks, understand many signals, and even solve problems. There is evidence that they recognize themselves in mirrors, which is an important test because it suggests self-awareness. Other animals just react to their mirror image as another animal at first, then ignore it.

It’s difficult to directly compare intelligence across species. Dolphins aren’t good at tying knots, but we can’t catch fish with our mouths. We can do algebra. They can echolocate. It’s sort of like trying to say whether a rugby player is a better athlete than a basketball player. It makes more sense to appreciate both for the skills they have.

Some people have suggested that dolphins are intelligent because they live in complex social groups. How do you think that living in a social group might require a big brain? Thanks for asking.