The complex link between brain size and intelligence

by Marc Wilson / 26 October, 2018
Dolphins offer evidence that brain size matters; the kiwi, below, is relatively lacking in grey matter. Photo/Getty Images

Dolphins offer evidence that brain size matters; the kiwi, below, is relatively lacking in grey matter. Photo/Getty Images

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Brain size may have a bearing on intelligence, but there is always a trade-off.

Dumb bunny, stupid cow, bird brain. We do love to associate lack of intellect with other animals. I’m not even going to start on how these terms are more likely to be applied to women than men.

It seems reasonable to suggest that we use these descriptors because we perceive animals to be less intelligent than we homo sapiens. One way in which we differ from non-human animals is the size of our brains.

Human brains vary in size. A male brain averages just over 1250 cubic centimetres, while a female brain averages just under 1150cu cm. But they can range in volume from just under 1000 to almost 1400cu cm. An adult brain weighs about 1.25kg.

It’s not entirely silly to obsess about brain size as a marker of intelligence. In humans, there is a very small correlation between the size of one’s brain and one’s score on an intelligence test. This is quite a contentious discussion, and it’s more complicated than just “bigger brain equals less stupid”.

We probably don’t want to assume brain size is the be-all of smarts for another reason. Obviously, we don’t have the biggest brains around. Sperm whale brains weigh in around 8kg; dolphin brains are also bigger than humans at about 1.7kg.

Dolphins are a good example to use, because dolphin brain size has been associated with tool use. If “Flipper” has a bigger brain, she or he is better at using tools to solve problems. The same is true of chimpanzees, and tool-using birds such as crows.

There is good reason to think that bigger brains are an adaptation – in general, the bigger a brain is, the more energy it requires to keep it running, so there must be a benefit to having a bigger one.

It’s hard to test this on humans because of the necessary timescale (and the ethical issues), but it has been done with guppies – small fish; quick reproductive cycle; fewer ethical issues. If you selectively breed for bigger brains in guppies, you end up with big-brained guppies who are better at intelligence tests. I’m not kidding – you can test fish using visual tasks to look at relative intelligence. Guppies can count.

The downside is that bigger brain means smaller gut and lower fertility. It’s a trade-off – if you need to divert energy to the brain, you need to take it from somewhere else, and this is common across species. Bigger brains mean lower fertility (and a host of other relative weaknesses).

Environmental challenges play a part here, with implications more subtle than just overall brain size. Bats that use smell to track prey have larger olfactory brain centres than those that use sound, but their auditory centres are smaller.

Brain weight relative to body weight is another way of looking at this. Elephant brains average about 5kg, but they’re also very heavy animals, so the ratio of their brain to body-weight is about 1:600. Wikipedia tells us that the bony-eared assfish has the smallest known brain- to-body-mass ratio among animals with a spine. The ratio for humans is about 1:40, and for small birds about 1:10.

So, back to bird brains. According to University of Auckland senior lecturer Fabiana Kubke, our national bird, the kiwi, weighs about 2kg, of which a minuscule 10g is brain. This doesn’t seem a good sign for intelligence, but as with all things, this is relative – it is estimated that long-extinct moas weighed up to 100kg, but their brains were a shade over twice that of a kiwi at 25g. Kiwis win.

This article was first published in the October 13, 2018 issue of the New Zealand Listener.

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