Bigger animals don’t always have the biggest brains relative to body size, finds latest study

Reading: Scientists have long believed that, generally speaking, the bigger an animal is, the bigger its brain. But our recent study challenges the nature of that linear view and reveals new insights about how brain size and body size have evolved together.

There is astounding diversity in the shape, size and internal structure of the brain among different animals. While the size of the brain usually correlates with cognitive abilities, its variation is strongly linked to the size of the animal itself.

As an animal gets larger, its brain size increases as well, though not in direct proportion – it is not a one-to-one relationship. This phenomenon is called allometric scaling.

Body size in mammals ranges from tiny bumblebee bats weighing in at less than two grams right up to the largest animal that has ever lived, the several-tonne blue whale. So we need to understand how body size and brain size have evolved together to say anything meaningful about how variation in brain size developed.

This is what our recent study tried to do. We applied powerful computational analysis to a dataset spanning over 1,500 species of mammal and their brain sizes. We asked how has the size of brains evolved in relation to the size of the animal?

Our results revealed a surprisingly simple pattern of evolutionary change between brain and body size. We found that the relationship between brain and body size is not linear, as previously assumed, but curved. It plateaus once body size reaches a certain threshold.

Previously, researchers had scratched their heads over the great variability in the brain and body size relationship. For example, the relationship appeared much steeper in some groups of animals than others (such as elephants and their cousins in purple compared with primates in pink).

The relationship between brain and body size is less strong among species of the same family (such as the ape family Hominidae, or the dog and wolf family Canidae) than it is among species of the same order (for example primates or carnivores).

We took another look at the data to investigate why brain size diminishes in some big animals. In our paper, we demonstrated – to our surprise – that this constraint is not explained by the high energy cost of maintaining a large brain. (About 20% of all energetic expenditure in humans is dedicated to the brain.) Nor is it linked to neuron density, which may allow enhanced processing power without the need for a bigger brain.

So, the processes behind this aspect of our findings remain a mystery – for now.

However, our study allowed us to which identify mammal species are outliers to the brain-body size relationship. To do this, we used an analysis method that identified species and lineages in which there have been intensely rapid changes in brain size.

Among these rapidly evolving lineages is our own species, Homo sapiens. We found that our large brains developed at a rate of evolution more than 20 times faster than the average rate of all other mammal species, resulting in the massive brains that characterise humanity today.