Brain nuclei in actively courting red-sided garter snakes: A paradigm of neural trimorphism

► Brain morphology varies among three behavioral phenotypes of red-sided garter snakes. ► Males direct courtship behavior toward a sub-population of males called she-males. ► She-males produce a female-like pheromone and have higher T levels than normal males. ► Volume of the AHPOA in she-males is significantly larger than the male or female. ► Cell size and cell density of the AHPOA is also greater in she-males.