Elevation, oxygen, and the origins of viviparity.

Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.