Why Do Mammals Have But Seven Cervical Vertebrae?
One of the great questions of comparative vertebrate anatomy is the constant number of mammalian cervical vertebrae. While birds, amphibians, and reptiles have large variations in this number, in mammals it is fixed at seven. The long neck of the swan is composed of from 22 to 25 cervical vertebrae, while ducks have 16. In contrast, the long necks of the giraffe and camel have seven cervical vertebrae, the same number as the shorter necked humans, mice, Dugong, and whales. The only mammals that depart from this rule are those with spectacularly low metabolic rates–the manatees and the sloths. (This low metabolic rate may be an important feature as to how they can get around the seven-neck vertebrae rule.) Frietson Galis of the University of Leiden has proposed a hypothesis (Galis, 1999) as to why mammals have this constant number and other vertebrates do not. Her proposal is that, in addition to specifying the anterior-posterior position of the parts of the nervous system and skeleton
