With such economies of scale in mind, relative brain size can be used as a measure of intelligence, which is otherwise difficult to quantify in animals. Larger relative brain mass often correlates with many indicators of better cognitive performance, such as the ability to learn, tool use, or complex social behavior. In extinct species, brain size often turns out to be the only variable predicting intelligence, because behavior rarely leaves fossil traces. Therefore, much effort has been devoted to studying the evolution of brain size, particularly in primates.
Here’s where things get complicated, but also intriguing: It turns out that the brain-body size curve doesn’t show the same slope in all vertebrates. A changed curve indicates evolutionary or developmental adaptations to new environmental conditions, locomotion strategies or diets. In groups of animals where the curve rises only moderately, brain mass also increases slowly with increasing body size, while it increases more rapidly with a steep incline. Due to this connection, aspects of brain evolution can easily be overlooked if one assumes a uniform slope of the curve for all vertebrates. An example is provided by the extinct dodo: often denigrated as dimwitted, it was actually a flightless relative of the dove, whose brain-body scaling curve is flat. The small dodog brain thus does not reflect a species of very low brains, but rather a slightly larger pigeon.
Birds evolved from the theropod dinosaur group. Therefore, to understand how the bird brain evolved, we need to study fossils closely. Unfortunately, neuronal tissue is usually broken down quickly and therefore hardly fossilizes. Still, fossils can give us clues about the brain size of long-extinct species. The brain lies in a protected cavity in the head, the cranium. This makes it very easy to determine the brain volume in a modern-day bird: the organ is removed from the skull, measured and preserved in a container for posterity. Instead, you can fill the cavity with small lead shot and then determine their weight (see »Natural and virtual skull casts«).
Things get more complicated with fossils. For nearly two centuries, the size and shape of the brains of extinct species could only be deduced when sediments, such as silt or mud, filled in an empty skull and then petrified. Every once in a while, a fossil skull will shatter—or be opened up by an inquisitive researcher—revealing a cranial spout like a cracked walnut, revealing the kernel. As long as paleontologists depended on such serendipitous discoveries, they learned little about the brains of dinosaurs and other extinct creatures. Natural skull casts are rare, and no museum curator would allow a paleontologist to study the skull of an ancient animal like Archeopteryx to break up
Since the 1980s, new technical possibilities have opened up to study prehistoric brains non-destructively: Computer tomography (CT) is used to record the boundaries of a fossil brain skull, which was usually penetrated by sediment during the fossilization process. I got to know this virtual spout technique as a student 15 years ago. I still remember well how we paleontologists would always ask a friendly x-ray assistant at a New York hospital to run a dinosaur skull through the medical computer tomograph at night, and then proudly hold a DVD with the fresh scan images in our hands, labeled with a felt-tip pen. Once we showed up with the skull of an extinct penguin, but were promptly ushered out of the CT room to make way for a traffic accident victim. An hour later the emergency was taken care of and we were able to have the skull scanned.
Computer tomographs provide great images of the skulls of long-extinct species. Initially, however, only rough images were available: due to the low resolution, the fossil brains looked as if they had been assembled from Lego blocks. The fact that the boundaries between bone and rock appeared so blurred was due to the low dose of X-rays. Medical tomographs work with low energy to protect the patients. The low-energy radiation can be used to image human bones and organs without damaging them, but it cannot penetrate solid rock without distortion. Nowadays, paleontologists often use industrial micro-CT devices, which are otherwise used to look for cracks in the material of machines, for example. Its stronger X-rays would be deadly to humans, but it’s excellent for producing sharp, high-resolution images of fossils.