Why do dolphins need big brains?
The human brain weighs about 1.5 kg (3.3 lb). Bottlenose dolphin brains weigh about 1.7 kg (3.7 lb). Does that mean they're smarter than us?
Big brains don't always make for smart animals; cow brains are bigger than those of most monkeys, but no one thinks cows are clever.
One approach that scientists use is to look at the brain size compared to the weight of the body:
In humans: the body is 50 times as heavy as our brain.
In dolphins the the body is 40 times as heavy as the brain, which is pretty close to humans
In apes it's 8:1.
In cats its' 5:1
With fish the brain weighs less than the spinal cord.
But in mice it's 40:1 (about the same as a dolphin)
However just because a brain is big compared to body size does not mean it packs in the most neurons.
In humans the average number of neocortical neurons in the cerebral cortex (the outer layer of the brain used for higher thought) is 19-23 billion. In bottlenose dolphins it's reportedly only 5.8 billon which is more than that of a gorilla, but only about half that of an African elephant.
Unlike the human brain (where we do the clever stuff with the prefrontal cortex) it may be that dolphin brains are wired differently to ours, and do their thinking using both the old and new cortex. The short answer is that we still don't know.
So what do dolphins use their big brains for?
Dolphins are highly social creatures and may use a lot of brain-power on socialisation. Dolphins may not use language like humans, but are still sophisticated communicators. Dolphins use a sort of sonar or echo location to and may use a fair amount of brain power for that. We are still learning how intelligent dolphins are.
Read also myths about the human brain.