Green anole lizards are native to the southeastern US and display highly sexually dimorphic courtship behaviors. Males extend a bright red throat fan called a dewlap. Females have only a rudimentary dewlap, and while they use it in a limited fashion during aggressive encounters, females do not display the dewlap during reproduction. The neurons and muscles controlling this behavior are larger in males than in females. Similarly, the structures controlling male copulation are highly sexually dimorphic. The muscles regulating the movement of their bilateral penises (called hemipenes) are present only in males, and the spinal motoneurons innervating these muscles are larger and more numerous in males.
Male green anole extending his dewlap (left), and cross-sections through the portions of the throat (right) containing the smile-shaped dewlap muscle from a male (top) and female (bottom).
Photo by David Crews
Tracing from the two hemipene (H in panel C of the left figure) muscles (the TPN and RPM) with fluorescent markers showed us the locations of the corresponding motoneurons in spinal segments T17 and S1.
