Development of a sexually dimorphic neuromuscular system involved in green anole courtship behavior.

During courtship and aggression, adult male green anoles (Anolis carolinensis) extend a large red throat fan called a dewlap. Adult females have a very small dewlap; they rarely use it during agonistic encounters and never during courtship. The motoneurons, nerve, muscle fibers, neuromuscular junctions, and cartilage pieces responsible for dewlap extension are all larger in adult males than females, parallel to the behavioral dimorphisms. However, the general developmental pattern of these structures, and when they become sexually dimorphic, was largely unknown. In the present study, we examined the length of the 2nd ceratobranchial cartilages (which unfold the dewlap), muscle fiber size, nerve cross-sectional area, and motoneuron soma size, as well as measures of body size (snout-vent length and body weight) at post-hatching days 1, 30, 60, 75, and 90. All measures other than motoneuron soma size increased over time. However, only cartilage length and muscle fiber size became sexually dimorphic in the first three months after hatching. The cartilage was greater in males than females by d60, and muscle fiber size differentiated by d75. The results indicate that peripheral structures involved in dewlap extension differentiate prior to those in the brain and thus, because males and females in this age range are identical in size, dewlap (cartilage) size could be a relatively specific factor contributing to the sexual differentiation of the rest of the neuromuscular system.