Dolphins' big brains evolved in spurts

18:53 29 October 04

NewScientist.com news service

Dolphins have evolved surprisingly big brains over the last 47 million years, according to the largest fossil study ever done on the animals. The growth - which occurred in two spurts - may shed light on how humans became so brainy.
Dolphins are famously bright, performing mental feats few other animals can, such as recognising themselves in mirrors. That intelligence is probably due to their exceptionally large brains - some dolphin species boast brain-to-body mass ratios second only to humans. But how they evolved such big brains has been a mystery.
Now, a trio of researchers led by biologist Lori Marino at Emory University in Atlanta, Georgia, US, has tracked how dolphins evolved their big brains using the fossil record. After four years of scouring museum collections, the team turned up 66 fossilised skulls of dolphin ancestors - adding to only five studied previously.
They probed the specimens' brain sizes with computed tomography (CT) scans and estimated the animals' body masses by analysing the size of bones around the base of the skulls.
They studied the fossilised skulls - dating back 47 million years - along with 144 modern specimens, and found each creature’s encephalisation quotient (EQ). This measurement relates a specimen's brain mass to that of an average animal of similar size, so if an animal's EQ is less than 1, it has a smaller than average brain, while if it is greater than 1, it has a relatively large one. Humans are the brainiest of all creatures, with an EQ of 7.