However, the large brain is not apparently necessary to support this because some bats have an equally sophisticated echo-location system but one of the smallest mammalian brains. The large dolphin brain more likely explains their complex social system. Complexity of social relations in mammals is correlated with brain size. It is proposed that human language is an evolutionary offshoot of that process. Dolphin society may rival primate society in complexity.
Dolphins frequently act coopera tively and also appear to display altruistic behaviour.
For example, dolphin mothers frequently form swimming playpens for their calves, keeping the young safely at the centre of a circle. Dolphins can vocalise many sounds. The most conspicuous sounds are whistles. Dolphins are good mimics and can imitate whistling sounds presented to them. If presented with two separate whistling sounds they can repeat and combine them together.