Sexually dimorphic areas in the brain of whiptail lizards.

The whiptail lizard species Cnemidophorus inornatus exhibits sexually dimorphic mating behaviors. We report that complementary sexual dimorphisms exist in two hypothalamic regions in male and female C. inornatus. The anterior hypothalamus-preoptic area (AH-POA), which is involved in male-typical mounting and intromission behaviors, is larger in males. The ventromedial hypothalamus (VMH), which controls female-typical receptivity, is larger in females. The all-female whiptail lizard Cnemidophorus uniparens, a direct descendant of C. inornatus, regularly and reliably displays both male-like mounting and female-like receptive (pseudosexual) behaviors. Rather than having a male-like AH-POA, however, the AH-POA as well as the VMH of C. uniparens resemble that of female C. inornatus. The biological basis of pseudosexual behavior in the parthenogen appears to be due to an unusual sensitivity to ovarian progesterone rather than to a difference in brain morphology.