Which animal has the largest brain-to-body ratio?

The animal with the largest brain-to-body ratio is the shrew. The shrew's brain constitutes about 10% of its body weight, which is higher than any other known mammal. This high ratio is due to the shrew's small size and high metabolic rate, which require a lot of brain power to maintain.

Interestingly, humans do not have the largest brain-to-body ratio among animals. While humans do have relatively large brains, they are not the largest when compared to body size. For example, the brain of an elephant is much larger than a human's, but elephants have much larger bodies, so their brain-to-body ratio is lower than a shrew's.