BY DR. JOSE PALU-AY DACUDAO
(Part 2)
GOING back to the question why did dinosaurs never evolve to be as intelligent as mammals?
In my opinion it is because dinosaurs do not possess a cerebral cortex.
Both reptiles and mammals possess deep gray matter composed of neuronal cell bodies. (A neuron is made up of a neuronal cell body and processes that project from it – axon and dendrites.) But mammals also possess neurons that cover the brain, called the neo-cortex or neo-pallium.
The brains of mammalian fetuses start out with the same set of deeply located neurons as reptiles. However, in mammalian ontogeny, some of the stem cells of the neurons from the deep gray matter located around our ventricular system migrate to the outside to cover the cerebral hemispheres. They multiply and become the cerebral neo-cortex (also called neo-pallium), or in layperson’s terminology, new-covering.
So, mammals have evolved a second set of neurons, or from a layperson’s perspective a second set of brains.
In brief, we possess both a ‘reptilian brain’ (as popular press calls the neuronal nuclei of the deep gray matter) and a neo-cortex. Thus, we have a whole lot more neurons than reptiles. Two ‘brains’ are better than one.
Brains aside, birds have advantages over mammals.
Most obvious is their mostly unidirectional breathing. A bird typically possesses a posterior air sac which connects to the lungs and air spaces in the bones. When a bird inhales, only around 25% of the air goes directly into the lungs. This is because a bird’s trachea (windpipe) splits into multiple bronchi.
Two bronchi enter the lungs, one for each lung. This accounts for the 25% of inhaled air that goes into the two lungs.
However, other bronchi do not enter the lungs but bypass them. These tubes connect to a posterior air sac and air spaces within the bones. This accounts for the other 75% of inhaled air. When the bird exhales, the now unoxygenated 25% of the air that went straight into the lungs come out. However, the remaining 75% of the still oxygenated air in the posterior sac and the bones go via other bronchi into the lungs.
Thus, amazingly enough, oxygenated air goes into the lungs whether a bird is inhaling or exhaling. A bird’s lung is continuously oxygenated. This same condition can be found in crocodilians, and also in some lizards. Dinosaurs and crocodilians are close relatives, belonging to the same clade Archosauria.
Evidence of air sacs in the bones of non-avian dinosaurs indicate they had this kind of respiratory system too. In contrast, the mammalian respiratory system acts as bellows, and results in tidal breathing. Oxygenated air goes in the lungs upon inhalation. Upon exhalation, unoxygenated air goes out. When we breathe out, no oxygenated air enters our lungs.
As an additional bonus, the air spaces in a bird’s bones significantly decreases the bones’ weight, making flying easier. With such an efficient respiratory system and light bones, it’s no wonder that birds dominate the skies until now.
Another advantage of birds over mammals is that they excrete their nitrogenous wastes as solid uric acid (uricotelic). Mammals on the other hand excrete their nitrogenous wastes as urea dissolved in water. Consequently, birds (and also the likewise uricotelic reptiles) waste less water than mammals. (To be continued)/PN
