The bird brain has been an enigma for years. We now know that avian neurobiology got off on the wrong foot in the late 1800s, making some incorrect assumptions that remained basically unchanged for about 100 years.
Led by Germany’s Ludwig Edinger, early biologists were greatly influenced by Darwin’s theory of evolution and Aristotle’s Scala Naturae. They expected each vertebrate group to retain parts of the primitive brain and add more complex structures. Brain complexity would increase in a step-wise, linear fashion from the primitive fish brain to amphibians, reptiles, birds, and mammals, peaking with primates and humans. They were surprised to find the bird brain so different from the mammalian brain.
The structured cortex in the mammalian cerebrum (now called pallium) was laminar, arranged in six layers with nerve cells (neurons) and fibers running in the same direction. The pallium of the bird brain lacked structure, consisting of clusters of neurons (called nuclei) that were randomly arranged. Both bird and mammal brains had cell masses below the pallia called basal ganglia, with a variety of functions including instinctive behavior.
Now comes the early anatomists’ biggest error. They said that the brain cells of the avian pallium were actually hypertrophied basal ganglia that were pushed up and into the pallium region. If the pallium could only function with instinctive behavior, the brain could never perform cognitive functions. Ouch!
A new consensus
For the last 50 years or so, avian neuroanatomists have discovered that the work of Edinger and colleagues contained many errors. After years of communicating about these inaccuracies, the Avian Brain Nomenclature Forum gathered at Duke University in 2002. This was a massive undertaking that resulted in a consensus on the understanding of avian neuroanatomy.
One of the major results of the forum was the correction of Edinger’s views on the avian pallium. It was not filled with basal ganglia cells but with brain cells (neurons) that provided cognitive function like the neurons in the mammalian neocortex. The bottom line: Birds and mammals have similar brain cell neurons but different brain structure and arrangement. What’s going on?
Remember when Edinger and his colleagues were stymied because the avian brain looked so different from the mammalian brain? They expected direct, linear, and stepwise changes between the vertebrate groups, as opposed to more dramatic events like the splitting of an evolutionary branch or where intermediate stages might form. Well, evolution threw them a curveball.
Early vertebrate evolution was fairly straightforward. Fish lived in the water; amphibians ventured onto land but returned to water to lay their eggs. Reptiles, birds, and mammals wanted to make a permanent break from water, but where would they lay their eggs?
Rather than give rise directly to reptiles, amphibians gave rise to an intermediate group called “stem amniotes” that solved the egg problem by creating a “land egg.” Think turtle or chicken eggs. The embryos developed membranes that surrounded them and aided in development, and the mother secreted a shell to keep it all together. The innermost membrane (amnion) surrounding the embryo is fluid-filled and protects the embryo. Some mammals (monotremes) feature the land egg, but most mammals utilize placentas with live birth. This includes marsupials and all other mammals, but their methods are very different. All are called amniotes.
The now-extinct stem amniotes were the curveball. Rather than simply giving rise to reptiles, the amniotes made a major split: One branch went to reptiles and birds (sauropsids) and the other branch to mammals (synapsids).
Ever since the mammal branch separated from the reptile/bird branch, about 315 million years ago, birds and mammals have evolved independently, each responding to its own natural-selection pressures leading to their current differences. Not surprisingly, they are quite different.
Different path, similar functions
We now know that the brain cells (neurons) in the avian pallium are nearly identical to the neurons in the mammalian neocortex, and they go back to their last common ancestor, the stem amniotes. The avian pallium and mammalian neocortex are the centers of cognitive function and learning. By following different evolutionary pathways, leading to different brain anatomy, both birds and mammals developed similar cognitive brain function, an example of convergent evolution.
New studies show that birds have more brain cells (neurons) in their pallia than mammals do in theirs. The most intelligent bird groups, songbirds and parrots, have twice as many neurons as primate brains of the same mass. Corvids and large-brained parrots have even more. It is suggested that the large numbers of neurons concentrated in high densities in the pallium substantially contribute to the neural basis of avian intelligence.
After a century of being misunderstood and kept in the dark, birds are finally being recognized for having brains with significant cognitive function that rivals or surpasses some primates. Never doubt the attributes and abilities of those amazing birds.
This article was first published in the “Amazing Birds” column in the July/August 2020 issue of BirdWatching magazine.
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