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 鈥� which occurred in two spurts 鈥� 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 鈥� 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 鈥� adding to only five studied previously.
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They probed the specimens鈥� brain sizes with computed tomography (CT) scans and estimated the animals鈥� body masses by analysing the size of bones around the base of the skulls.
They studied the fossilised skulls 鈥� dating back 47 million years 鈥� along with 144 modern specimens, and found each creature鈥檚 encephalisation quotient (EQ). This measurement relates a specimen鈥檚 brain mass to that of an average animal of similar size, so if an animal鈥檚 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.
Echolocation
Dolphins鈥� earliest ancestors were land-based hoofed creatures that became aquatic, losing their hind legs and gaining flippers over time. About 35 million years ago, those flippered creatures were about the size of small baleen whales 鈥� about 9 metres long 鈥� had sharp teeth and an EQ of about 0.5.
But around that time, that group mysteriously died off, spawning a new group called the odontocetes, or toothed whales, which include today鈥檚 dolphins, porpoises, belugas and narwhals.
The new research shows these creatures were smaller, with smaller teeth, but 鈥� crucially 鈥� relatively bigger brains. Their EQs had jumped to 2.5 鈥� a phenomenon Marino suspects is related to their development of echolocation 鈥� the use of sound waves to locate objects 鈥� and their complex social lives.
The research shows that about eight out of 67 odontocete species 鈥� including dolphins 鈥� went through a second boost in brainpower about 15 million years ago, attaining EQs of 4 and 5, although the reasons for this leap remain unclear. And some species鈥� brains also got smaller.
Myth busting
鈥淭here鈥檚 a myth that says that since life began, there鈥檚 been this increase in brain size,鈥� Marino told New 杏吧原创. 鈥淏ut brains are metabolically very expensive, so you have to have a really good reason to have a big one.鈥�
Another myth is that only a single big-brained species can exist at one place and time, she adds. But the new work shows that for 15 million years, 鈥渁 lot of dolphin and whale species have been hanging out together in the ocean鈥�.
Only patchy 鈥� if any 鈥� historical brain size records exist for most animals, but researchers know that in the five million years of human history, EQs have risen from about 2.5 to 7.
鈥淭here鈥檚 a lot of controversy about that particular pattern 鈥� whether it was gradual or whether there was a big jump,鈥� says Marino. 鈥淲e don鈥檛 even really know why we developed big brains. That鈥檚 the $6 million question 鈥� what provided the selection force for increasing brain size in humans as well as in dolphins?鈥�
Patrick Hof, a neurobiologist at Mount Sinai School of Medicine in New York City, US, speculates on how dolphins got their 鈥渕onstrous鈥� brains.
鈥淚t probably reflects the computational strategies the animals are using to conduct their daily tasks,鈥� Hof told New 杏吧原创. He says the pressures and temperatures the animals face while diving could also have played a role.
Journal reference: The Anatomical Record (DOI: 10.1002/ar.a.20128)