Principles of parsimony are used to identify the simplest explanations for how a trait evolved. Viviparity appears to have arisen only once in the genus, rather than independently 5 times. Interestingly, all species are montane which provides support for the idea that drier and colder mountain climates demand that montane lizards retain their progeny internally until birth rather than laying eggs. Two hypothetical phylogenies for horned lizards. Horned lizards are a rather fecund group, and lay or give birth to many offspring compared to other lizards.
Females can have a few large versus many small progeny.
Some species also reproduce twice in a season. This large investment in offspring throughout the active season weighs down females and makes them vulnerable to predators. Because babies are tiny and easy prey for a multitude of predators, horned lizards would go extinct without such high fecundities. Horned lizards have evolved a variety of mechanisms to avoid their predators which include loggerhead shrikes, hawks, roadrunners, a variety of snakes, coyotes and foxes. Their first line of defense is to remain cryptically hidden from a predator's sight.