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. Las especies endémicas de México Phrynosoma braconnieri y P. taurus habitan elevaciones intermedias y altas en los estados de Oaxaca, Guerrero y Puebla. En este trabajo revisamos ejemplares de colecciones y recopilamos información bibliográfica sobre la distribución y ciclos reproductivos de estas dos especies. Este estudio confirma la existencia de un modo de reproducción vivíparo en ambas especies, y sugiere que su ciclo reproductivo difiere del observado en otras especies vivíparas de Phrynosoma. En particular, en P. braconnieri y P. taurus el desarrollo embrionario se lleva a cabo durante el final del otoño y principios de invierno dándose el parto al principio de la primavera. En contraste, en otras especies vivíparas de Phrynosoma la gestación se realiza durante la primavera y el verano mientras que el parto ocurre en el verano tardío. La revisión de la distribución filogenética de los modos reproductivos en éste género, sugiere que dos clados de Phrynosoma que viven en elevaciones altas han adquirido independientemente un modo vivíparo a partir de un ancestro ovíparo. Sin embargo, cada uno de estos clados ha evolucionado un ciclo reproductivo particular.Abstract

Geographic range of Phrynosoma braconnieri and P. taurus in southern México. Black numbered circles represent collection localities for P. braconnieri, white lettered squares are localities for P. taurus except in cases of sympatry with P. braconnieri, where numerical indicators are used. Detailed collection sites are listed in Materials Examined

Reproductive cycles of Phrynosoma braconnieri and P. taurus. The typical cycles of an oviparous (with two clutches in one season) and a short-horned viviparous Phrynosoma are illustrated for comparison (modified from Goldberg, 1971b; Howard, 1974). For each case, we approximate the reproductive condition of vitellogenic (black bars), oviductal (hatched bars), and postparturient (white bars) females, and the estimated times of mating (M), egg deposition (D) and parturition (P). For P. braconnieri and P. taurus, data points gathered for this study are indicated by an “X” in the reproductive status bars

Reconstruction of the evolution of reproductive mode on two proposed phylogenies for the genus Phrynosoma. Oviparity is represented by black bars; viviparity by gray bars. Both phylogenies result in two separate evolutionary transitions to viviparity within the genus, from an egg-laying ancestor. (A) Phylogeny proposed by Montanucci (1987), based on morphological characters. (B) Composite phylogeny including relationships among most species proposed by Montanucci, with modification of the short-horned lizard clade proposed by Zamudio et al. (1997)