"Now that there are strong grounds to dispute Descartes' contention that animals lack the capacity to think, we have to ask just how animals do think." (emphasis added) Terrace1
How birds think asks what we know and what we don't about brain mechanisms that mediate behavior and physiology in birds. Our approach will emphasize comparative biology and attempt to exploit the deep commonalities between bird and mammal brains.
When pioneer anatomists cracked open the skulls of birds and mammals, the brains looked rather similar. Both have the major divisions of the ancient vertebrate brain-plan. In the drawing, the cerebrum (brown region) and cerebellum (gray) are prominent in mammals (top) and birds (middle).2
With the birth of neurophysiology and pharmacology in the early 20th century, scientists showed that birds, mammals, and other vertebrates share in common the same tools for intercellular signaling:
- a set of chemical neurotransmitters and neuropeptides, and
- a library of membrane-bound protein receptors specific for each neurotransmitter.
Discovering common origins between cell masses of bird and mammal brains is something like trying to understand continental drift during geologic time on the earth. Masses of cells, like continents or parts of continents, moved gradually over evolutionary time. "Nerve cell drifts" in birds and mammals followed different plans in three dimensions.
Nerve cells in the mammal cerebral neocortex are stacked like layers in plywood (left, above) while those in the bird cerebrum are clumped in nuclei, crowded together, rather like cloves in a large garlic bulb (right, above). The diagrams have been borrowed from a paper of Clifton Ragsdale and colleagues.3
How should we relate layers to clumps? What nerve cells in birds correspond to what cells in mammals? How did ancestral nerve cell types get scrambled differently as the brains of birds and of mammals evolved in parallel? Reconciling the discrepancies proved daunting.
In the absence of ancient mummified brains, scientists looked carefully at the development of brains in embryos. The observations were subject to many interpretations. Over decades, various theories were advanced but none was universally accepted.
A common wiring legacy
In October 2012, clarity emerged. A seminal paper from Clifton Ragsdale's laboratory at the University of Chicago tracked markers showing gene expression as nerve cells develop in birds, mammals, and turtles.3 The results were a Rosetta Stone for brains.
The same nerve cell types, each identified by a unique gene expression profile, are found in both birds and mammals. Each precisely identified cell lineage could be reliably mapped in birds and in mammals. As embryos develop, identifiable neuron types could be tracked through each stage.
The results of comparative mapping were startling. A stable pattern of cell-to-cell contacts (synapses) has endured through evolutionary time, in spite of the selective pressures that yielded different overall geography and often different cellular geometry in bird and mammal brains.
The Ragsdale results reveal the existence of an underlying wiring legacy - a shared heritage of nerve cell circuits shared by birds and mammals.
Put another way, the input and output characteristics for specific lineages of nerve cells are stable; they have not changed even though the cells came to migrate in different ways during embryonic development and the adult cells have assumed different shapes in birds and mammals.
. One of Ragsdale's examples is a chain of four nerve cells. The input for the pathway comes from a neuron (dTh) in the thalamus, a brain region that lies the beneath the cerebrum; next there are two cells (green and blue) within the cerebrum, and finally the output (red) runs to the brainstem (Bst).
The neuron from the thalamus (black in the diagram) inputs to a cerebral nerve cell (green) that connects to a second cerebral cell (blue). These cells play similar roles in both bird and mammal brains. But since the anatomy and geography of the cerebra have diverged, green cells are found in layer 4 of the modern mammal cerebral neocortex whereas they are packed within a nucleus of the cerebral entopallium in birds. Blue cells are in layer 2/3 of mammals and in the nidopallium of birds3.
When biologists try to map a brain function between mammals and birds, it can be confusing. Careful physiological, cellular, and developmental research on living creatures is required.
Consider for example high-level cognitive functions, like paying attention, planning, multi-tasking, integrating converging sensory information, and monitoring of actions.
The "executive crossroads" for high-level cognitive functions in mammal brains is the prefrontal cortex (PFC) which lies at the front of the cerebrum. The equivalent center for convergence and coordination in bird brains, called the nidopallium caudolaterale (NCL), is at the rear of the cerebrum.
4, shows brains of a human and a pigeon, viewed from the left side. The executive center of the human brain (PFC) is at the front while that of the pigeon (NCL) is at the back of the cerebrum.
Deciphering the bird brain is intrinsically exciting because it helps us understand and better appreciate bird behaviors that so many people find entrancing.
Perhaps even more interesting is the possibility that a better understanding of commonalities and contrasts between brains might offer new perspectives on the connection between anatomy and mentality.
In How birds think, we will draw upon field observations, laboratory experiments, and comparative biology to explore function, development, and evolution of bird brains.
1. HS Terrace, 1987. Thoughts without words. In Mindwaves: Thoughts on intelligence, identity, consciousness, ed. C. Blakemore & S. Greenfield, Oxford Press.
2. RG Northcutt RG, 2011. Evolving large and compact brains. Science 332:926-927.
3. J Dugas-Ford, JJ Rowell & CW Radsdale, 2012. Cell-type homologies and the origins of the neocortex. Proc Nat Acad Sci US 109:16974-9, doi:10.1073/pnas.1204773109.
4. O Güntürkün, 2012. The convergent evolution of neural substrates for cognition. Psychol. Res. 2:212-219.