Parthenogenesis.
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   The  whiptail  lizard  is  an  all-female  species,  and reproduces by
   parthenogenesis, the development of an unfertilized egg.
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   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.
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   The ovaries grow as estradiol levels rise, and the lizard behaves like
   a female.
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   After  ovulation,  estradiol levels drop and progesterone levels rise;
   eliciting male behavior.
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   Many   aphid  species  reproduce  by  parthenogenesis  without  sexual
   behavior.
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