For many researchers, one measure of intelligence is the ratio of brain size to body weight. But that would mean that the tiny mammal known as the shrew would be capable of great intellectual accomplishments, which it is not presently known for.

Why is this? A human brain weighs 1.3 to 1.5 kilos (2.9 to 3.3 pounds) – clearly smaller and lighter than a sperm whale’s (8.5 kilograms or 18.7 pounds) or an elephant’s (five kilograms or 11 pounds), but representing about 2% of body mass, which puts humans way ahead in the animal kingdom.

The shrew, on the other hand, has a relative brain weight of 4%, so why isn’t it smarter?

The answer is that the relationship between brain size and intelligence is more complicated than previously thought, shows a new study by researchers working with anthropologist Jeroen Smaers of University College London.

Their study, “Comparative analyses of evolutionary rates reveal different pathways to encephalization in bats, carnivorans, and primates,” was published this week in the Proceedings of the National Academy of Sciences.

The scientists analyzed data on brain and body size of hundreds of modern and extinct bats, carnivores and primates, and were able to identify trends that played out over millions of years.

As they evolved, bats’ brains decreased much more slowly than their bodies, which meant a higher relative brain weight. Adaptive advantages could have been the reason for this, the researchers say. Smaller bodies are easier to maneuver, while the bats retained enough cognitive operating efficiency to navigate and hunt in obscure surroundings.

With primates, on the other hand, brain sizes decreased a little faster than body sizes.

According to Jeroen Smaers, changes in body size often occur independently of changes in brain size and vice versa. This means that relative brain weight provides only limited information about intelligence. Nobody should be expecting intellectual feats from shrews  any time soon.