Reproductive mode and female reproductive cycles of two endemic Mexican horned lizards (Phrynosoma taurus and Phrynosoma braconnieri)

Two species of horned lizards, Phrynosoma braconnieri and P. taurus, are en demic to Mexico and inhabit medium rind high-elevation sites in the states of Oaxaca, Guerrero, and Puebla. We compiled information on distribution an d reproductive cycle for these two species from the literature and from an examination of specimens in systematic collections. Our study confirms a vi viparous mode of reproduction for both species and suggests that their repr oductive cycles may be different from that observed in other viviparous Phr ynosoma. Specifically, embryonic development occurs during the late fall an d Hinter months, with a parturition date in early spring In contrast, other viviparous horned lizards gestate during the spring and summer months and give birth to their young in the late summer. Examining the phylogenetic di stribution of reproductive modes in this genus suggests that two high-eleva tion clades of horned lizards have independently evolved viviparity from an oviparous ancestor, yet each of these clades has evolved a distinct reprod uctive cycle.