Original Paper Brain Behav Evol 2001;58:362–369 Development of a Sexually Dimorphic Neuromuscular System Involved in Green Anole Courtship Behavior Erin L. O’Bryant a Juli Wade a, b Neuroscience Program and Departments of a Psychology and b Zoology, Michigan State University, East Lansing, Mich., USA Received: November 20, 2001 Returned for revision: January 14, 2002 Accepted after revision: February 12, 2002 Dr. Juli Wade Michigan State University, Department of Psychology 235 Psychology Research Bldg. East Lansing, MI 48824-1117 (USA) Tel. +1 517 432 8301, Fax +1 517 353 1652, E-Mail wadej@pilot.msu.edu ABC Fax + 41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2002 S. Karger AG, Basel Accessible online at: www.karger.com/journals/bbe Key Words Neuromuscular system W Sex difference W Development W Courtship W Lizard W Reptile Abstract 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 fi- bers, neuromuscular junctions, and cartilage pieces re- sponsible for dewlap extension are all larger in adult males than females, parallel to the behavioral dimor- phisms. However, the general developmental pattern of these structures, and when they become sexually dimor- phic, was largely unknown. In the present study, we examined the length of the 2nd ceratobranchial carti- lages (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 differ- entiated 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 con- tributing to the sexual differentiation of the rest of the neuromuscular system. Copyright © 2002 S. Karger AG, Basel Introduction Many vertebrate nervous systems contain anatomical differences between adult males and females that parallel, and presumably help create, reliable differences in sexual behaviors. Studies of the neuromuscular components in- volved in these behaviors indicate that these dimor- phisms are often created early in development. For exam- ple, in the spinal nucleus of the bulbocavernosus (SNB) system controlling penile function during copulation in rodents, male-biased sex differences in motoneurons as well as musculature [Cihak et al., 1970; Breedlove and Arnold, 1980] develop during the perinatal period [Breed- love and Arnold, 1983; Nordeen et al., 1985; Lee et al., 1989]. Similarly in the zebra finch, the volume of the motor nucleus (nXIIts) that innervates the vocal organ (syrinx) as well as syrinx weight and muscle fiber size are larger in adult males than females [Wade and Buhlman, 2000]. The sexual dimorphism in nXIIts volume appears to develop by day 53 after hatching [Bottjer et al., 1985] and the syrinx becomes heavier in untreated males than