Snakes: Nurturing the young
Possibly the most advanced snakes are the pit vipers (Family Cotalidae) which include the rattle snakes (Crotalus) of the southwestern regions of the United States. These animals invest heavily in parental care and like some amphibians retain their eggs inside their body. The shell is reduced to a thin membrane so that the embryos, as they lie inside the oviduct, not only feed on their yolk but draw sustenance from their mother's blood diffusing from the walls of the oviduct pressed against them. Such a system for nourishing of its young is functionally analogous to the placenta used by mammals. The mother snake will also safeguard her young after they have hatched, warning intruders with sound of the vibrating rattle at the end of the tail. Each time a rattle snake sheds its skin a special, hollow scale remains and accumulates at the end of their tail. Up to twenty scales may accumulate.