The whiptail lizard is an all-female species, and reproduces by parthenogenesis, the development of an unfertilized egg.
The lizard on top plays the role of a male; they switch sex roles every 2 or 3 weeks during the breeding season, mediated by hormones.
The ovaries grow as estradiol levels rise, and the lizard behaves like a female.
After ovulation, estradiol levels drop and progesterone levels rise; eliciting male behavior.
Many aphid species reproduce by parthenogenesis without sexual behavior.