Why larger animals don't always have proportionally bigger brains

In a new study an analysis of over 1,500 mammal species revealed that certain species, including humans, have evolved much larger brains relative to their body size at a faster rate than others. Interestingly, this rapid brain size evolution is seen in many intelligent species, while some species, like bats, exhibit constraints on brain size evolution

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 per cent 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.

The fact that humans are outliers to a mammal-wide pattern fits with our ability to use tools, build houses, make music and live in complex societies. But humans are not the only species to stray from the curve. We found many species throughout the mammalian tree of life which have rapidly increased their brain size (relative to their body size), including several species noted for their keen intelligence such as rats, bears, dolphins and elephants. But also others like wallabies, marmots and quolls.

Conversely, we observed a massive acceleration in rate towards smaller brain sizes when bats first evolved. But brain size has evolved minimally and slowly over the following 50 million years or so of their evolution. This could indicate some sort of constraint acting on brain size in bats, perhaps imposed by flight.

We found rapid bursts of change in the relative size of an animal's brain most frequently in carnivores, rodents and primates. In these three groups, these rapid changes almost always result in proportionally larger brains. This principle, known as the Marsh-Lartet rule, would suggest that relative brain size tends to increase over time in mammal evolution. But our models suggest that this is not a universal phenomenon observed across all species. The three mammal groups where the trend does apply have profoundly diverse behaviour and ecology even among close relatives (for example pandas and grizzly bears, or porcupines and guinea pigs). They include some of the most social animals such as humans, mole-rats and meerkats. Primate brain size has always been growing.

The earliest primates had brains about 0.5 per cent of their body size. Eventually, increasing brain size eventually culminated in our own large brains (about 2 per cent of our body size). Our data shows that sometimes it really is a simple explanation that can explain the complexities observed in nature. However, natural selection is inherently complex. Like any rule, the curved relationship between brain and body size has been repeatedly broken by animals who have evolved ahead of the curve.

(Joanna Baker, University of Reading and Robert Barton, Durham University)