This is something new to me: “Mammals breathe in and out, birds breathe straight through.”
Straight through what? Birds have lungs; their lungs have a certain capacity. How can they NOT breathe in and out?
Please enlighten me.
This also bypasses the normal mammalian requirement to breathe out carbon dioxide first, before the next intake of oxygen. Human beings breathe about 12 times a minute, whereas small birds can breathe up to about 250 times a minute. This is a perfect system for birds, which use up energy very quickly and so have a high metabolic rate.
A leading evolutionary expert on birds, Dr Alan Feduccia, University of North Carolina, didn’t even attempt to solve this major problem in his book on the evolution of birds.4 John Ruben, an evolutionary respiratory physiology expert at Oregon State University, said a dinosaur’s ‘bellowslike lungs could not have evolved into the high-performance lungs of modern birds.’5 This would apply to the lungs of any reptile, because any hypothetical intermediate forms would not be functional—the earliest stages would have to have a diaphragmatic hernia,1 i.e. a hole in the membranous muscle powering respiration, and natural selection would work against an animal with such a harmful condition.
1. Ruben, J. et al., Lung Structure and Ventilation in Theropod Dinosaurs and Early Birds, Science 278(5341):1267–1270, 14 November 1997.
2. Schmidt-Nielsen, K., How Birds Breathe, Scientific American, pp. 72–79, December 1971.
3. Engineers make much use of this principle of counter-current exchange, which is common in living organisms as well—see Scholander, P.F., The Wonderful Net, Scientific American, pp. 96–107, April 1957.
4. Feduccia, A., The Origin and Evolution of Birds, 2nd ed., Yale University Press, New Haven and London, 1999. However, this book shows that the usual dinosaur-to-bird dogma has many holes.
5. Rubens, J., quoted in Gibbons, A., Lung Fossils Suggest Dinos Breathed in Cold Blood, Science 278(5341):1229–1230, 14 November 1997.