Abstract
There are marked differences between the physiology of birds and mammals. These reflect the evolutionary distance between the two classes with the last common ancestor estimated as existing 318 million years ago. There are analogous organ systems in birds and mammals. However, marked differences exist. For instance, in the avian gastro-intestinal tract, there is a crop at the lower end of the esophagus. This functions both to store feed and for microbial action. The avian immune system lacks lymph nodes and has a distinct organ producing B-lymphocytes, namely the bursa Fabricius. The important of spleen has been largely dismissed until recently. However, its importance in both innate and specific immunity is increasingly recognized. There is a major difference between birds and mammals is the female reproductive system as birds produce large yolk filled eggs. The precursors of the yolk are synthesized by the liver. Another difference is that there is a single ovary and oviduct in birds.
Highlights
The physiology of birds has attracted significant attention
Shifts in supposedly “domestication related” genes have been reported for yellow skin color (β-carotene dioxygenase 2) and thyroid-stimulating hormone receptor (TSHR) and wild-type allele (Rubin et al, 2010; Karlsson et al, 2015)
What is not clear is whether and, if so, the rate to which, lactic acid/lactate and other fatty acids are absorbed from the crop? Interestingly, there is evidence that the crop plays a role in calcium absorption in laying hens with reduced egg production and serum calcium concentrations following cropectomy (Stonerock et al, 1975)
Summary
The physiology of birds has attracted significant attention. A caveat is that much of the research on the physiology birds has been with domesticated birds, chickens and ducks. The present communication discusses examples from the following systems where birds differ from mammals: gastrointestinal tract and the crop and ceca, immune system and the bursa Fabricius and spleen and female reproduction. It is noted that the physiology of birds reflects impacts of their evolutionary history, effects of domestication and the sequentia of flight
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