Reproduction About one third of whiptail lizards are parthenogenic, which is asexual reproduction by development from an ovum without fertilization, essentially cloning, but with strategies to create diversities. This is not the case with Western Mexico Whiptail lizard, which is a bisexual species. They have sexual and dioecious (having the male and female reproductive organs in separate individuals) reproduction. Aspidoscelis costatus males, like the majority of other lizards, reproduce through cloaca, which is an orifice, sort of semicircular-shaped, located in the ventral side (underside) of the tail base of the lizard, just distal to its hindlegs. Digestive wastes like urine and feces are eliminated through cloaca. And the male reproductive organs, called hemipenes, which come in a bilateral pair, are held inverted within the cloaca when not in use. During copulation, the pair of hemipenes becomes evert, and sperm are ejaculated. Usually only one hemipenis is used at a time. Female also use cloaca for elimination of digestive waste and for reproduction. For female lizards, the cloaca is the external genital opening for the genital canal, equivalent to vagina in mammals. Copulation is performed through cloacal kiss, in which the male and female press their cloacas together as the male discharges sperm. Unlike Cnemidophorus males, Aspidoscelis costatus males do not have anal spurs, or pelvic spurs, which are vestigal limbs used by males to stimulate and position females during mating. Mating Female reproductive cycle was markedly seasonal, recorded from April (end of the dry season) through August (middle of the rainy season). And this is in regardless of climate. Same timing of female reproduction period is reported for populations inhabiting different climate zones, including semi-humid warm climate to temperate-zone, to higher-elevation. Reproduction is associated with day length rather than with temperature. Aspidoscelis costata lizards exhibit associated reproductive cycles (also called synchronic reproductive cycles), in which males and females reach the maximum sperm production, mating, and ovulation activities during the same period. However, male A. costatus reaches reproduction maturity at a smaller SVL length than female (about 2 inches for male versus 2.5 inches for female). Female/male interactions Accompaniment During mating season, a male whiptail appear to be continuously sexually active throughout the season. A male A. costatus would starts escorting a female whiptail around, guarding her, a behavior that is called accompaniment. However, only some females will be receptive (female receptivity is probably related to ovulation in females). Even if