Dividing up the Brain
Bird brains are wonderful structures. Indeed, “bird-brained” is an unusually poor insult, because the avian brain is one of the most complex seen in vertebrates. “Turtle-brained” might be a better insult in terms of neuroanatomy. That slight would be more hard-hitting if you meant to slander someone’s higher reasoning abilities. While bird brains are larger relative to body size than those of turtles, crocodiles, lizards and even some mammals, size is not the only thing that matters. Different parts of the brain are responsible for different functions. Deciphering the proportions of different parts of the brains of extinct animals can therefor give us insight into their biology.
Vertebrate brains can be divided into three major sections: the cerebrum, cerebellum and medulla oblongata. In our Paraptenodytes study, we were able to identify all three from the scans of the nicely preserved fossil. The cerebrum is where most of the “higher” functions of the brain like memory, communication and reasoning are conducted. This part of the brain is split in two hemispheres in birds, just like in humans. At the very front of the cerebrum, the olfactory bulbs connect to the brain. These structures are involved in the sense of smell, and in penguins are quite small. That makes sense, because penguins mostly locate their food using sight, not smell. In birds like albatrosses and turkey vultures that locate food via scent, the olfactory bulbs are much larger. Next up is the cerebellum. Some of the important functions of this part of the brain are controlling movement and balance. One of the projections of the cerebellum is the flocullar lobe, which is associated with stabilizing vision during rapid movements of the head. This structure tends to be enlarged in diving birds, possibly in relation to the twisting and turning underwater acrobatics that are invoked to catch prey. Paraptenodytes is no exception to this pattern, and has a strongly developed floccular lobe. In front of the cerebellum lie the optic lobes, which as you might expect are associated with visual acuity. These are well developed in most birds, with some real impressive size in owls and a great reduction in nocturnal birds like kiwi. Below the optic lobes there is a space for the pituitary gland which also shows up in endocasts. This gland secrete hormones that control growth rates and other metabolic functions. At the rear, lower portion of the brain is the medulla oblongata. This region is associated with many of the important life functions we take for granted, like regulating heart rate and breathing. This region is also where many of the cranial nerves leave the brain and branch out to the parts of the body they innervate. Most of these nerves can be identified in the endocast, because they form small openings called foramina in the places where they leave the protective bony braincase.
So, at the end of the day we can learn much more about the brain than just its raw size. One important caveat to all of this is that there is even more subdivision of duties in the brain. Different layers of brain cells are associated with different tasks as well – navigation, memory, vocalization and problem solving parts of the brain are all layered inside the cerebrum of a living bird, but we will only ever be able to decipher the total cerebrum size from an endocast, not the depth of each layer. While this is a real limitation, the data that paleontologists are gaining from even the volumes of major structures is greatly expanding the limits of our knowledge about extinct animals.