Birds are some of the brainiest creatures on earth, while their direct ancestors - the dinosaurs - are often stereotyped as dull and doomed. But a new study published today in Nature challenges this notion: many dinosaurs were just as brainy as early birds, such as Archaeopteryx. With their small size and (proportionately) big brains, birds were probably dinosaurs that never grew up.
The expression “birdbrain” should be interpreted more as a compliment than an insult. Getting and staying airborne is a highly complex activity requiring finely-tuned coordination and sensory perception, which is partly why birds fill us with wonder and pilots are generally held in high esteem.
Birds accordingly have large brains with swollen cerebral hemispheres, more sophisticated than those of most creatures apart from some mammals. These big brains not only allow birds to stay aloft, but also to perform some impressive mental feats: Caledonian crows and many other birds make and use complex tools, and some resourceful blue tits learnt how to open the foil tops once used in milk bottles, then taught others across the UK this useful skill.
Dinosaurs, on the other hand, have long been caricatured as lumbering behemoths with tiny brains: children learn that Stegosaurus was as big as a car but had the brain the size of a walnut.
This is a bit of an exaggeration: nevertheless, the peach-sized brain in the elephant-sized Stegosaurus, or the apple-sized brain in the much larger Apatosaurus, were very unimpressive. Certainly, many dinosaurs, especially quadrapedal herbivores such as these, had relatively small, reptile-like brains.
But it is now increasingly clear that other dinosaurs were much brainier. We now know that birds (with their finely-tuned brains that help them stay aloft) evolved from bipedal carnivorous dinosaurs (theropods), so enlarged brains might be expected in some of these bird relatives. Indeed, the theropods most closely related to birds, such as the Velociraptors of Jurassic Park fame, “look” intelligent: small, agile, lightly-built predators with swollen braincases and large eyes.
Until now, though, it was generally thought even the most bird-like dinosaurs still had smaller brains than primitive birds such as Archaeopteryx, which in turn was less brainy than living birds.
Dinosaurs get a brain scan
The new study in Nature, by Amy Balanoff (palaeontologist at the American Museum of Natural History) and colleagues, has challenged this assumption.
To peer inside their skulls, they used three-dimensional CT scans similar to those used in hospitals and medical laboratories. Surprisingly, many theropod dinosaurs had brains proportionately as large as those of Archaeopteryx: indeed, some were even brainier, according to measurements.
Many of these bird-like dinosaurs had long stiff feathers on their arms and legs which they might have used for gliding or even flying. The presence of large brains is certainly consistent with some sort of aerial ability in these bird-like dinosaurs.
The new study adds to the mountain of evidence demonstrating that dinosaurs evolved into birds in a gradual series of changes.
Theropod dinosaurs exhibit many bird-like features, such as feathers, wishbones, and a three-toed foot. These dinosaur-bird similarities are even more impressive when we examine juvenile dinosaurs: their skulls, with typical “baby-face” features such as proportionately large eyes and brains, and small slender snouts, strongly resemble primitive birds such as Archaeopteryx.
Birds therefore evolved from a lineage of theropod dinosaurs which retained many juvenile traits (such as small size and baby-like skulls) into maturity. In this respect, human evolution might have followed a similar trajectory: we retain many traits of baby chimpanzees (our nearest living relatives) well into adulthood, such as our small jaws, flat faces, swollen brains and propensity for play.