So a female that retained her eggs, he reasoned, could ensure high incubation temperatures and proper egg development by basking, giving her a selective advantage. And the bulk of the Northerns' range is in Can-ada, usually on the edge of moist conifer forests and up to 10,000 feet above sea level. Southerns generally live near dry oak woodlands of lower elevations and occupy the western half of California into Baja, excluding the San Joaquin Valley. But the cold climate hypothesis is based on today's population distributions and, though the correlation is strong, it alone cannot explain the origin of viviparity. So researchers like Sheen continue to collect more details.
Herpetologists have other information about reptiles to work with.
Embryos developing within a viviparous female are kept warmer and develop more quickly than eggs do in soil. Sheen aims to determine how such factors may help or hinder alligator lizard survival. To do this, she's comparing the body temperatures of both species. Sheen is also calculating their accelerations and observing how they move. Alligator lizards bolt for cover when threatened, and the way they curve their bodies to increase stride and accelerate is key to their survival.