With this economy of scale in mind, relative brain size can be used as a measure of intelligence that is otherwise difficult to quantify in animals. A greater relative mass of the brain often correlates with many indicators of better cognitive performance, such as learning ability, tool use, or complex social behavior. In extinct species, brain size often turns out to be the only variable in predicting intelligence, as behavior rarely leaves traces of fossils. Hence, a great deal of effort has been devoted to studying the evolution of brain size, especially in primates.
This is where things get complicated, but they’re also intriguing: it turns out that the brain-body size curve does not show the same slope for all vertebrates. The altered curve indicates evolutionary or developmental adaptations to new environmental conditions, locomotion strategy, or diet. In groups of animals where the curve only rises moderately, brain weight also increases slowly with increasing body size, while it increases faster with a steep slope. Due to this connection, aspects of the brain’s evolution can be easily overlooked if a uniform slope of the curve is assumed for all vertebrates. An example is the extinct dodo: often vilified as blunt, it was in fact a flightless relative of a pigeon whose brain-body scaling curve is flat. The brain of a small dog, therefore, does not reflect a very low-brained species, but rather a slightly larger pigeon.
Birds evolved from the theropod dinosaur group. Therefore, to understand how the bird’s brain evolved, we need to carefully examine the fossils. Unfortunately, neuronal tissue is usually rapidly degraded and therefore hardly fossilized. Still, the fossils can provide us with clues about the brain size of long-extinct species. The brain is located in the protected cavity of the head, the skull. This makes it easier to determine the brain volume of a modern bird: the organ is removed from the skull, measured and stored in a container for posterity. Alternatively, you can also fill the cavity with a small lead shot and then determine its weight (see “Natural and Virtual Skull Castings”).
Things get complicated with the fossils. For nearly two centuries, the size and shape of the brains of extinct species could only be deduced when sediment such as silt or mud had filled the empty skull and then petrified. Every now and then a fossilized skull cracks – or is opened by an inquisitive researcher – revealing a cranial spout like a cracked walnut, revealing the nucleus. As long as paleontologists relied on such random discoveries, little has been learned about the brains of dinosaurs and other extinct creatures. Natural skull casts are rare, and no museum curator would allow a palaeontologist to examine the skulls of an ancient animal such as Archaeopterix break off
Since the 1980s, new technical possibilities have opened up to study prehistoric brains in a non-destructive way: computed tomography (CT) is used to record the boundaries of the fossilized brain skull into which sediments usually penetrated during the fossilization process. I learned about this virtual spout technique as a student 15 years ago. I still remember well how we palaeontologists asked a friend’s X-ray assistant at a New York City hospital to run a dinosaur’s skull through a medical CT scanner at night, and then proudly held a DVD of fresh scanned images, signed with a marker pen. We once appeared with the skull of an extinct penguin, but were immediately led out of the CT room to make room for a road accident victim. An ambulance was taken an hour later and we managed to scan the skull.
Computer tomographs provide great images of the skulls of long-extinct species. Initially, however, only approximate images were available: due to the low resolution, the fossilized brains looked as if they had been assembled from Lego bricks. The fact that the boundaries between bone and rock seemed so blurry was due to the low dose of X-rays. Medical tomographs work with low energy to protect patients. 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 microtomography devices, which are used, for example, to look for cracks in the material of machines. Its stronger X-rays would be fatal to humans, but it’s great for creating sharp, high-resolution fossil images.