the female lizard is receptive, she always seems uninterested and ignores the presence of the courting male. The male A. costata accompanies his receptive mate for only 9 hours per day for 2–5 days, at a distance of 0.5 m or less, and practically all copulations take place during that time. In Aspidoscelis costatus, accompaniment increases the chance of successful copulation. Accompanied females copulated 6.7 times more with consensual, accompanying males. Opportunist copulations were also observed, however it is insignificant compared to accompanying copulation partners. During copulation, males will only copulate with their accompanying partner, and successive copulations are common to maximize sperm transfer. Copulation When time comes for mating, the male A. costatus would mount the female, throw his front leg on his mate's torso, straddle her and encircling her body with his hind legs, and scramble to try to shove his tail under his partner's tail, while the female at times remains motionless, and at times appears to be trying to buck him off. Mating is performed by cloacal (see Physical Description) copulation, commonly referred to as cloacal kiss or cloacal apposition. Although everted hemipenis could hardly be seen by the observers, the pair of everted hemipenis are protruding out of the male cloaca, and one of them is intromitted (inserted) into the female's cloaca, an act called hemipenis intromission. Aspidoscelis costatus males do not have the pair of anal spurs (also called pelvic spurs) to position the females,[9] but simply maneuvers to circle his long tail under that of his female partner to get into a hammerlock, ventral-side to ventral-side, so that the male and female can press their cloacas together. After hemipenis intromission is achieved, and doughnut posture is adopted, the mating pair is tangled motionless much like wrestlers in a hammerlock. They remain in this position for 5~10 minutes, during which time ejaculation takes place. Sperm ejaculated is actually stored in a “sperm storage structure” (the seminal receptacle) in the female cloaca. The male then rapidly dismounts from the female. Mate guarding Mate guarding can be costly for males due to the loss of energy from less food intake and increased aggressive behavior. However, the behavior persists, likely due to the gain in fitness surpassing the survival cost. During the allotted time where males follow females, they repeatedly repel approaching males, reducing the possibility of extrapair copulations, and increasing aggressive behavior and risk of injury.