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 endemic to México and inhabit medium and high-elevation sites in the states of Oaxaca, Guerrero, and Puebla. We compiled information on distribution and reproductive cycle for these two species from the literature and from an examination of specimens in systematic collections. Our study confirms a viviparous mode of reproduction for both species and suggests that their reproductive cycles may be different from that observed in other viviparous Phrynosoma. Specifically, embryonic development occurs during the late fall and winter 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 distribution of reproductive modes in this genus suggests that two high–elevation clades of horned lizards have independently evolved viviparity from an oviparous ancestor, yet each of these clades has evolved a distinct reproductive cycle.