Mammals are unique in that they're the only animals whose brains feature a neocortex — a recently evolved structure that's responsible for higher order functioning like conscious thought, sensory perception, spatial reasoning, and language.
But now, scientists working at the University of Chicago have confirmed that birds have their own version of the neocortex — one that may actually give them certain cognitive advantages over mammals. The discovery will have profound implications for our understanding of why birds appear to exhibit such high-levels of intelligence, reasoning, and self-awareness.
Both mammals and birds have a brain structure called the dorsal ventricular ridge (DVR) that originates from an embryonic region called the telencephalon. But that's where the similarity ends; the two regions go on to develop into very different shapes, with the standard mammalian neocortex comprised of six cortical layers, while the bird's DVR contains large clusters of neurons called nuclei.
For the past 50 years, scientists have debated the purpose of this mysterious structure. Because the neocortex was only seen in mammals, it was thought that the DVR was a kind of amygdala, the brain organ responsible for learning and emotional reactions. Not satisfied with that explanation, neuroscientist Harvey Karten speculated back in the 1960s that it may in fact be a kind of neocortex — but he lacked proof.
But this has changed. Neurobiologists Jennifer Dugas-Ford and Clifton Ragsdale have collaborated on a paper published in Proceedings of the National Academy of Sciences indicating that birds have neocortical-like functioning, but that it's done differently — the result of convergent evolution.
To determine the true functionality of the DVR, Dugas-Ford and Ragsdale's team studied the brains of two different bird species, chickens and zebra finches. Specifically, they were using recently discovered sets of molecular markers used to identify layers of the mammalian cortex (layer 4 is for input neurons, and layer 5 is for output neurons). Using these markers, the researchers looked to see if these same genes were expressed in the DVR nuclei of birds.
And in fact, that's exactly what they found. The level 4 and 5 markers were expressed by distinct nuclei of the DVR, supporting the suggestion that the DVR nuclei is the avian equivalent of the mammalian neocortex. The discovery also confirms that different physical brain structures that are built with the same cell types can execute different functions.
Moreover, the scientists theorize that the DVR configuration may actually confer an advantage to the birds. Birds mayhave special nuclei that are involved in vocalization, and possibly other characteristics as well. Moving forward, the scientists are interested in exploring how the length of circuits and differently evolved processing centers can impact on animal cognition.
Images: Shutterstock/Ana de Sousa/Wang LiQiang.