Plasma steroid hormones concentrations and their relevance in the reproductive cycle of female Mountain Horned Lizard, Phrynosoma orbiculare

Reproduction is a fundamental trait in the life history of a species; therefore, it is of great interest at all levels within a species, including the individual, ecological and conservational scope. Hormonal studies in reproduction have become relevant because of their implications on the regulation of reproductive physiology. Herein we examined hormonal profiles of progesterone (P4), estradiol (E2) and testosterone (T) of adult female Mountain Horned Lizards, Phrynosoma orbiculare, from the Parque de la Ciencia Sierra Morelos, State of Mexico, Mexico. Hormonal concentrations were measured in the plasma of adult females monthly throughout a year and linked to the reproductive cycle of the species and environmental factors [mean monthly temperature (T°), total monthly rainfall, and daylight hours representing photoperiod]. We used direct ELISA technique to assess hormonal concentrations. We report that P4 is linked to pregnancy period, as maximum values of P4 are observed during the early gestational period and decrease to minimum levels during the late pregnancy phase and near the time of birth. An increase in E2 concentration occurs after pregnancy and parallels vitellogenesis in this species. Testosterone concentrations are at maximum levels during breeding and abruptly decrease to a minimum during late pregnancy. The correlation between P4 and T is positive while the relationship between P4 and E2 is negative, but no significant correlation between E2 and T is observed. Temperature and photoperiod have a negative correlation with P4 and a positive correlation with E2, while rainfall is not supported as an important environmental factor in the regulation of these hormones. Also, T does not seem to be affected by environmental factors revised in this study. A seasonal annual steroidogenic pattern is observable in female P. orbiculare that parallels its reproductive cycle characteristics and suggests an endogenous control of reproduction. However, it appears that certain environmental conditions, such as temperature and photoperiod, may influence the reproductive rhythmicity observed in female P. orbiculare.