Sexual dimorphisms in a copulatory neuromuscular system in the green anole lizard

Abstract

Sexual dimorphisms in neuromuscular systems have been investigated in several vertebrate groups, but data on reptiles are limited. The present studies were designed to establish the copulatory neuromuscular system of the green anole lizard (Anolis carolinensis) as an appropriate model. Like mammals, male reptiles have copulatory organs. However, each individual has two "hemipenes" that are controlled by bilateral sets of muscles. First, the anatomy of the hemipenes and associated muscles was described in males and the same anatomical region examined in females. Second, spinal motoneurons innervating one of these muscles, the transversus penis (TPN), were localized by using the retrograde tracer biocytin. They were detected in the last trunk and first sacral segments (T17-S1). Third, motoneuron number and soma size were assessed in Nissl-stained sections of spinal cord segments T17-S1 of breeding males and females. Male-biased sexual dimorphisms were detected in both measures, but the motoneurons innervating a tail muscle, the caudifemoralis (CF), are also located in the same region of cord. Therefore, in the last study, the CF was injected with biocytin in both sexes to eliminate its motoneurons from the analysis and gain a more accurate representation of the TPN motoneuron pool. An equivalent number of CF motoneurons were labeled in both sexes, and the results from the previous study were replicated. Thus, similar to other vertebrate models, parallels between morphology and function exist in the green anole copulatory system. Future investigations will broaden the comparative perspective on mechanisms regulating sexual dimorphisms relating to reproductive behaviors in vertebrates.

Relationships among reproductive morphology, behavior, and testosterone in a natural population of green anole lizards

“…Comparisons of male and female copulatory morphology included only renal sex segments and CF fiber size, as females lack hemipenes and the muscles that control their movement [34]. The size of the renal sex segment, and androgen-stimulated organ, was greater in males than females (F 1,34 = 82.4, P b 0.001), but fibers of the control CF muscle were equivalent in size in the two sexes (F 1,34 = 0.00, P = 0.992).…”

Courtship and copulation in the adult male green anole: Effects of season, hormone and female contact on reproductive behavior and morphology

“…They retract the hemipene by contracting the retractor penis magnus muscle (RPM) which attaches to the caudal end of the organ as it lies inside the tail. The motoneurons innervating the muscles controlling penis movement are located in the last trunk and first sacral segments (T17-S1) of the spinal cord [40,75,87].…”

Fiber Type Composition of the Muscle Responsible for Throat Fan Extension in Green Anole Lizards

“…Perhaps the most extreme dimorphisms exist in mammals and lizards in which the motor neurons and/or muscles associated with penile movements are present in males and virtually absent in females [Hayes, 1965;Breedlove and Arnold, 1980;Ruiz and Wade, 2002]. In certain species of teleost fish, frogs, and birds, males produce courtship sounds unique to that sex and, accordingly, the neural and muscular structures associated with sound production are larger or more numerous in the males of these species [sonic fish : Fine et al, 1984;Bass and Marchaterre, 1989;Bass, 1990;Bass and Baker, 1990; frogs: Sassoon and Kelley, 1986;Kelley et al, 1988;McClelland and Wilczynski, 1989;Boyd et al, 1999;birds: Nottebohm and Arnold, 1976;Wade and Buhlman, 2000;Schultz et al, 2001].…”