"Bird-brained" is a bum rap

2011-02-08T15:16:00.000-08:00

Two hundred and fifty million years ago, all vertebrates had to lay their eggs in water or the embryos dried up after they started to develop. Then one lineage of four-legged vertebrates made a major evolutionary breakthrough. These animals acquired an amnion - a special sac that protects the embryo and permits a full life cycle away from water. These ancestral stem amniotes gave rise to mammals and birds.

Most mammal descendants remained quadrupeds that use all appendages for walking. Birds and many dinosaurs became bipedal, with pedestrian bodies is balanced on the rear appendages.¹ As front legs were converted into wings, manipulative skills were largely relegated to the beak at the end of an elongate flexible neck, as seen to the right in a primitive Archaeopteryx from the Late Jurassic, 147 million years ago.²

As the body plans of birds and mammals adapted to contrasting lifestyles, brain anatomies diverged as well.

In evolution and in embryonic development, the brain arises from swellings at the front end of the nerve tube. The front swelling has an upper part called the pallium and a lower part called basal ganglia. The pallium produces the olfactory bulbs and cerebrum that contain the highest executive centers.

In mammals, nerve cells in the cerebrum multiplied and stacked six-deep, like layers in plywood that rests upon a thick whitish mass of nerve processes. When more and more cells were added, the cerebral hemispheres grew to bury the basal ganglia and to overlap the midbrain. As the six-layered cerebral cortex expanded, it wrinkled and also infolded in many species. To protect the delicate brain and also to support teeth and chewing muscles of mammals, natural selection favored a thick skull that is like an armored brain vault.

Over hundreds of millions of years, birds evolved to become manuverable flying machines. To that end, they acquired a short compact body that enclosed major organ systems and contained all large muscles, as beautifully described in Gary Kaiser's recent book, The Inner Bird.¹ Flying well in three dimensions requires large wide-angle eyes and integration of the snowstorm of visual information as well as precise motor coordination and behavioral complexity. Thus early on, bird brains grew steadily larger, as revealed by reconstruction of the brain of Archaeopteryx, based on a CT scan of the fossil skull.³

The Archaeopteryx brain is about three times bigger than that of a modern reptile of the same body size. The enlarged optic lobes (ol), olfactory tract (ot), and well-developed inner ear likely reflect the importance of vision, olfaction, sounds, and spatial (3D) sensory perception. The expanded cerebellum (cb) and cerebrum (c) suggest improved fine control of movement and increased cognitive sophistication.

Natural selection bio-engineered birds for flight. A massive head with a thick skull, pendulous and swaying at the tip of an elongate neck, would be an aeronautical liability. Expansion of the bird pallium had to be balanced against selective pressure to avoid a big fat head.

During the Mesozoic Era, there were several prominent groups of flying dinosaurs including birds. Fossil evidence shows that forebrain of the bird lineage grew steadily. Except for birds, flying dinosaurs disappeared during the mass extinctions at the "K-T boundary", 65 million years ago. Using fossil skulls, virtual reconstructions of those ancient brains from that time has been accomplished with computer modelling. Bird brains went through a growth spurt in the Mesozoic and have continued to increase in size ever since. It may be that the better bird brain was the critical adaptation that permitted birds to survive the K-T boundary^3a.

The brains of modern birds and modern mammals are shown in the illustration to the right.⁴

In birds (middle), both cerebellum and cerebrum are large. During evolution and during embryonic development, the sides of the bird pallium thicken and the lobes of the midbrain (orange in illustration) bulge out and down.
In mammals (top), most of the growth is in the top-most "hyper-pallium" to yield the six-layered cerebral cortex, a unique mammalian acquisition.

Evolving in parallel, both birds and mammals acquired better brains but brains with different architectures.

As pallial neurons were added, selective pressures required a topography that kept the brain compact. Bird pallial neurons are concentrated in patches or clusters that are surrounded by a matrix of nerve axons coated with whitish insulating myelin. These clusters, called brain nuclei, are pressed close together. Thus the present-day avian pallium is somewhat like a mass of garlic cloves, each surrounded by a skim of white matter. The outermost surface of the this mass of clove-like nuclei is smooth as shown in the figure below.⁵

All brains are fragile. But weight constraints preclude a massive protective skull. Birds frequently collide with windows and fall to the ground. Some die, but quite often the stunned bird recovers in a few minutes.

Why is there relatively little damage?

Two explanations are common:
First, birds are protected by their "pillow" of head feathers that softens impacts.
Second, although bird skulls are light and thin, the bone is exceptionally dense and tough⁶. To use a broad analogy, bird skulls are like Kevlar while mammal skulls are like "Old Ironsides". Even when birds are killed by collisions with windows, actual fractures of skull bones are rare⁷.

Two additional explanations deserve further consideration.
Third, since the skull bones are "pneumatized" with internal air pockets⁸, the skull might function like a helmet of bubble-wrap or styrofoam. In a fascinating older paper that often goes unnoticed, JG and DL Harrison further suggested that air sacs also function like "an inflatable air-suit" that buffers the brain from surges in blood pressure and thus prevents blacking out when birds dive sharply.⁹
Fourth, avian neuro-architecture might inherently confer shock resistance. The globular centers packed together in the bird pallium might be less prone to ripping and tearing than the six-layered mammal pallium.

Further research might elucidate the exquisite physiological adaptations of the bird body plan and also (we hope) might facilitate the invention of more humane protocols for poultry slaughter¹⁰

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Natural selection has produced an elegant design for bird brains. Why don't these brains get more respect? The answer is ignorance reinforced by historical scientific bias.

Modern brain terminology has ancient roots. Mostly working from animal dissections, Erasistratus of Alexandria (304 BC – 250 BC) described and named the cerebrum and cerebellum. Renaissance physicians like Vesalius broke open the skulls of felons after their executions and pondered the role of each brain component. In due course, higher mental processes were linked to the cerebral hemispheres while instinct, reflex and vegetative functions were assigned to the underlying basal ganglia, including the striatum.

Early avian anatomists dissected birds and named the brain structures by reference to mammal brains. Finding no wrinkled cerebral cortex, the anatomists surmised that the bird pallium was puny compared to mammals. Since they concluded that basal ganglia are the predominant centers of bird brains, dogma suggested that birds must lack complex cognition.

The explosion of genetics in the last fifty years enriched not only our understanding of biological inheritance but also of embryology and physiology. Cell populations can now be tracked as body parts mature and thus it is easier to identify homologies among disparate animals. Markers of gene expression allow mapping of activity in time and space and thus permit diagnoses of exact physiological roles of individual cells or groups of cells. Simultaneously, new techniques in cell biology allowed nerve cell transmitters and receptors to be precisely mapped.

Molecular and cell biology have revolutionized bird neuroanatomy. Tracers of gene expression revealed that large parts of a bird's brain previously thought to be basal ganglia are in fact descended from the embryonic pallium.

In addition, sophisticated new techniques of microscopy allowed tracing of the nerve cell processes than connect parts of the brain. The consequence of these techniques is a drastically revised view of bird brain capacity.

The new scientific results reveal that the bird pallium is not tiny. "As in mammals, the adult avian pallium comprises about 75% of the telecephalon" (forebrain).⁵

The long-established bird brain nomenclature, built up over centuries by meticulous dissections and by repeatedly peering at pickled colored tissue slices, has become not only inadequate but actually quite misleading.

Names matter. In science as in politics, names color logic and bias conclusions.

In 2002, the Avian Brain Nomenclature Group assembled at Duke University to revise bird brain terminology. The cartoons below, drawn by Zina Deretsky for the National Science Foundation¹¹, summarize the conclusions. Notice particularly the small size of the green-colored bird pallium in a classic depiction of the songbird (top left) and the much larger size in modern view (bottom left).

This new nomenclature facilitates understanding of How birds think. When bird brains are compared with those of mammals, both differences and similarities require intensive study. Thanks to correct identification of homologies, we can use results from mammalian research to draw inferences about bird brain mechanisms and vice versa.

Later posts on this blog will review recent research on bird brains and behavior proffer tentative insights into bird minds.

References:

1. Kaiser GW 2007. The Inner Bird - Anatomy and Evolution. U British Columbia Press, Vancouver.
2. Witmer LM 2004. Inside the oldest bird brain. Nature 430:619-620.
3. Alonso PD, Milner AC, Ketcham RA, Cookson MJ, Rowe TB 2004. The avian brain and the inner ear of Archaeopteryx. Nature 430:666-669.

3a. Milner AC, Walsh AS 2009. Avian brain evolution:new data from Palaeogene birds (Lower Eocene) from England. Zool J Linnean Soc 155:198-219.

4. Northcutt RG, 2011. Evolving large and compact brains. Science 332:926-927
5. Jarvis ED, Gunturkun O, (25 other authors) Reiner A, Butler AB, 2005. Avian brains and a new understanding of vertebrate brain evolution. Nature Neuroscience 6:151-159.
6. Dumont ER, 2010. Bone density and the lightweight skeletons of birds. Proc Roy Soc B 277:2193-2198.
7. Veltri CJ, Klem D(Jr) 2005. Comparison of fatal bird injuries from collisions with towers and windows. J Field Orinothol. 72:127-133.
8. Hogg DA, 1990. The development and pneumatisation in the skull of the domestic fowl (Gallus gallus domesticus). J. Anat. 169:139-141.

9. Harrison JG, Harrison DL,1949. Some developmental peculiarities in the skulls of birds and bats. Bull Brit Orinthol Club 69:61-70.
10. Erasmus MA, Turner PV, Nykamp SG, Widowski TM, 2010. Brain and skull lesions resulting from use of percussive bolt, cervical dislocation by stretching, cervical dislocation by crushing and blunt trauma in turkeys.167: Vet Rec 850-858.
11. This color plate uses new nomenclature from reference 4.

Revised May 23 & September 30, 2011.