Lauren Rizzo
2/11/04
Hayssen
On “Selective Factors Associated with the Origin of Fur and Feathers”
Many scientists have wondered about the evolution of fur and feathers. There have been many theories concerning the appearance of these epidermal coverings in the animal world. Most of these theories take into account that fur and feathers may have been a product of natural selection, but debate what factor the introduction of these features may be linked to. The authors of “Selective Factors Associated with the Origin of Fur and Feathers” discuss many of these theories, disproving each or explaining why that particular route of evolution would be unlikely. Finally seizing upon their own theory, the duo explain why it is most likely that fur and feather came into existence as a product of respiratory turbinates and Harderian glands which are a signal for hair or fur in today’s mammals.
The first theory tested was that high metabolic rates lead to a different bone structure as well as a different growth factor than low metabolic rates. Since mammals have the high metabolic rates and high recorded growth factors, the conclusion was drawn by many scientists that a high metabolic rate would be needed for fur. Scientists in the past had recorded extremely low growth rates for reptiles. The authors dispel this theory by claiming that measurements taken of reptilian growth rates were recorded erroneously, and that they are, in fact, on a level with mammalian rates, which would disprove this theory.
Another theory that was dispelled concerned feathers in birds. The scientists refer to commonly-held scientific beliefs that the Archaeopteryx lithographica (the first feathered bird) was endothermic and had developed plumage as a barrier against the sun. The authors point out that these features “need not necessarily signal the presence of endothermy or even an approach to it” (Ruben and Jones, 2000:590). The authors go on to make examples of today’s roadrunner which has low body temperatures and uses its less-feathered body parts to soak up solar radiation and keep warm (Ruben and Jones, 2000). Citing that the Archaeopteryx might have had similar behavior, they leave this theory.
Their own theory concerning fur and feather evolution involves the idea that respiratory turbinates made hair and feathers necessary. This theory would allow scientists to link endothermy and the appearance of hair on mammals. The theory states that the nose structures of today’s various mammals indicates that there is a special gland needed (the Harderian gland) which also secretes lipoidal products (Ruben and Jones, 2000). This gland exists today in animals with fur or feathers which require water-proofing to keep out the cold. The pelage and plumage would then have aided the animal in thermo-regulation. The authors cite that because later dinosaurs and the Archaeopteryx lacked the respiratory turbinates, it is far more likely that early birds were ectotherms, and this would have been a good reason for feathers to be introduced (Ruben and Jones, 2000). The authors conclude that this is indeed the reason for the feathers and fur that is seen on the majority of birds and mammals today.
This last theory seems particularly likely because there are so many factors involved and all seem to point towards the actual evolution of birds and mammals. Not only does the theory point to actual fossil records, but it also draws connections between the theory and extant species which follow the pattern set forth by this theory. Being sound in each aspect, it would appear that respiratory turbinates could indeed be the reason behind the evolution of fur and feathers.
Bibliography
Pough, Harvey F., Christine M. Janis, and John B. Heiser. Vertebrate Life. New Jersey: Prentice Hall, 2002.
Ruben, John A., and Terry D. Jones. “Selective Factors Associated with the Origin of Fur and Feathers.” American Zoologist. Feb. 2000: 40.