We humans like having big brains. After all, our noggin has helped us become one of the most successful species of animal on this planet. So why haven't all animals developed big brains?
Scientists have pondered this question for decades. One of the most highly favoured theories suggests that the size of brains is limited because of its inherent metabolic cost: for instance, the human brain, accounting for only 2% of our body mass, takes up 20% of our total energy expenditure. This theory, the ‘expensive-tissue hypothesis’, suggests that the size of the brain is part of a trade-off between the advantages of greater cognitive ability and the energy demands that having a bigger brain entails.
A key prediction from this model is that developing a bigger brain should come at the expense of other tissues, most notably the gut, the other most energetically expensive tissue in our bodies. While a number of studies have shown that for many animals brain size and gut size are negatively correlated, no one has done experiments to test whether evolving a bigger brain affects the size of the gut. Now, a group of scientists at Uppsala University have taken a direct approach to address this question, recently published in Current Biology. Using the guppy, they determined the effects of artificial selection on brain size in order to put the theory to the test.
First, they bred multiple generations of guppies, and for each generation selected the individuals with the largest and those with the smallest brains, creating an ‘up’ selected and a ‘down’ selected strain of fish. They found that after only two generations, brain size within the ‘up’ selected group had already increased by 9%, showing that brain size is a trait that can rapidly change during evolution.
Next, they determined whether the fish with larger brains have greater cognitive abilities. As guppies have a crude sense of numbers, these fish can be put to the test using simple numerical learning tasks. The authors found that females with larger brains are indeed better at this task than their ‘down’ selected counterparts, showing that the increase in brain size gave them a cognitive advantage.
Furthermore, when the team weighed the guts of the fish from the two different groups, they found that the larger brained females had 8% smaller guts, while the larger brained males' guts were 20% smaller. This demonstrates that acquiring a bigger brain leads to a smaller gut, and shows that the increase in brain size is the result of a trade-off between cognition and metabolic cost.
Interestingly, when the authors looked at the offspring of larger brained animals, they saw that, although the offspring were of the same size, there were fewer of them per generation. This suggests that increases in brain size come at the expense of the reproductive process, as well as gut size.
This study has provided the first direct evidence of the expensive-tissue hypothesis by showing that evolving a bigger brain comes at the expense of gut size and reproductive performance. This gives us clues as to how our own brains might have evolved: the availability of more highly nutritious foods is thought to have allowed us to grow a bigger brain, with the reduction in gut size that that would cause.