Big bodies, not-so-big brains: Study upends link between size and intelligence in nature

For centuries, scientists have assumed a simple rule: Bigger bodies mean bigger brains. But new research has thrown a wrench into this theory, suggesting these organs can get big only to a certain extent.

This, however, does not apply to humans. Our brains have evolved 23 times faster than other mammals, resulting in big brain sizes, the study published in journal Nature Ecology & Evolution states.

Chris Venditti, the lead author of the study from the University of Reading, the United Kingdom, explained that the relationship between the brain and the body was assumed to be linear — meaning that brain size gets proportionally bigger the larger an animal is. The study, however, suggested there might be a physical limit to how large brains can become.

“The brain is a fascinating organ — variation in its size has fundamental implications for intelligence, behaviour, sociality and ultimately, humanity,” Joanna Baker from the University of Reading and the co-author of the study, told Down To Earth.

However, she added that, despite decades of study, there is a lot we don’t understand about how brain size has evolved over time.

Baker and colleagues collected and analysed the dataset of brain and body sizes from around 1,500 species to find concrete answers on brain size evolution. They also focused on detecting any rapid increases or reductions in the rate at which the relative brain size has evolved.

Their analysis showed that the brain and body size can be described as having a curved relationship. This, according to Baker, means that while brains get bigger with body size, there is a limit to how big they can get. “The largest mammals have smaller brains than expected for their body size,” she added.

The researchers do not know the reason, but they think big brains beyond a certain size could prove too costly for animals to maintain.

The brain size of the largest mammals changes about 44 per cent less per unit of body mass than that of the smallest mammals.

Further, the study finds that primates, rodents and carnivores showed a more pronounced rapid change in brain size.

The relative brain sizes of these three groups, including humans, tend to increase over time. “We discovered a phenomenal acceleration in the rate of evolution along the branch leading to our species — associated with a massive increase in relative brain size,” Baker noted.

The researchers do not yet know the reason behind this trend. But Baker thinks these animals exhibit enormous diversity in behaviour and ecology — perhaps more so than other groups.

“We also see more social behaviours amongst these groups — some of the exemplars of sociality fall within these clades (for instance, humans, meerkats and mole rats),” she highlighted.

It remains to be seen whether the large brains of rodents, carnivores and primates are driven by common causes such as diet, life history, or behaviour, or nuanced patterns within each group, she added.

Bats showed the opposite trend. Their brain size has reduced since they first evolved, showing a very low rate of relative brain mass evolution, roughly 2.5 times lower than other mammals.

The researchers explain that this could be driven by challenges posed by flight. Other factors such as diet or hibernation also need to be investigated.

Going forward, the team hopes to understand why brain sizes have a limit. “We tested a few potential reasons — perhaps an energetic constraint? Or a restriction on neuronal capacity? But so far, we have drawn a blank. Unravelling this mystery is vital for future research in brain evolution,” Baker explained.