Most vertebrates lay eggs which then develop outside of the mother's body. Almost all living mammals retain their eggs within the body, and nutrition flows from the mother to the embryo. An intermediate condition is called by the 50-cent word, ovoviviparity, which means that eggs are retained within the body, and the young are born alive. The difference between that and the situation in mammals is that the yolk of the egg sustains the embryo rather than food being transferred from the mother.
A number of Chihuahuan Desert reptiles give live birth, including the
rattlesnakes and the Short-horned Lizard. There may be several reasons why these
reptiles have evolved this trait, including protection of the clutch (egg poachers are
much more likely to dig up and eat eggs than face an angry mother-to-be rattlesnake).
But since temperature affects embryonic development, some reptiles speed up the process
by specialized behavior, such as basking in the sun to increase temperature.
Ovoviviparity may not be quite as efficient as the mammalian way, but I don't
anticipate running out of rattlesnakes anytime soon.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.