Comparative Neuroanatomy Puzzle 2

Submitted by jan on Thu, 09/05/2019 - 18:56
Various Mammalian Brains

Here are images of brains of a number of different mammals, shown to scale. It is relatively obvious that the human brain is large, particularly compared to that of cats and dogs and mice. We humans tend to be oh so proud of our large brains. But is that a fair comparison? Cats are smaller than humans in every respect. Isn't it to be expected that they have smaller brains because they are just smaller overall? 

If you look at the chart below  which plots body weight against brain weight, you can clearly see a strong correlation: larger animals have larger brains. And when you inspect the plot you can see that the brain of elephants is quite a lot larger than the brain of (wo)man. But if an elephant brain is twice as large as that of a human, does that mean the elephant is twice as smart? Why should brains get larger with larger bodies? Can you think of good reasons why a bigger body needs a bigger bran? Can you think of other reasons why a larger body might not necessarily need a bigger brain? And are there downsides, or costs related to big brains?

Brain weight vs body weight

Another thing you will see on this graph is that, while human brains are large, they are not the largest in the animal kingdom, and whales and dolphins feature prominently among mammalian species that often have larger brains than humans. Again, does that mean they are smarter? Note that the brains of the two whales plotted here are below the black diagonal, that of (wo)man is much above the diagonal, but so is that of the dolphin. What do you think the diagonal shows? Why might it be relevant?

Some people might argue that the fact that both dolphins and humans have points much above the diagonal does mean that they are particularly brainy species. But you may find it interesting to learn that histological studies of dolphin brains show that dolphin brains contain many more glial cells, and relatively fewer neurons, per cubic mm, than human brains. Given that glial cells provide important metabolic support for neurons, why might dolphins need so many glia? What can dolphins do that you can't? And would a brain with more glia be smarter than a brain of the same size with fewer glia?

Discuss you answers with the TAs.