Sexual dimorphism in neuromuscular junction size on a muscle used in courtship by green anole lizards

Sculpting reproductive circuits: relationships among hormones, morphology and behavior in anole lizards

Morphology parallels function on a variety of levels in reproductive circuits in anole lizards, as in many vertebrate groups. For example, across species within the anole genus the muscle fibers regulating extension of a throat fan used in courtship are larger in males than females. Endocrine factors controlling behavior and morphology have been studied in detail in one species, the green anole (Anolis carolinensis). This review briefly describes the results that have been obtained and highlights key areas for future investigation that will provide insights on mechanisms from a comparative perspective.

Relationships among hormones, brain and motivated behaviors in lizards

Lizards provide a rich opportunity for investigating the mechanisms associated with arousal and the display of motivated behaviors. They exhibit diverse mating strategies and modes of conspecific communication. This review focuses on anole lizards, of which green anoles (Anolis carolinensis) have been most extensively studied. Research from other species is discussed in that context. By considering mechanisms collectively, we can begin to piece together neural and endocrine factors mediating the stimulation of sexual and aggressive behaviors in this group of vertebrates.

Hormones, sexual signals, and performance of green anole lizards (Anolis carolinensis)

The evolutionary processes that result in reliable links between male signals and fighting capacity have received a great deal of attention, but the proximate mechanisms underlying such connections remain understudied. We studied a large sample of male green anole lizards (Anolis carolinensis) to determine whether testosterone or corticosterone predicted dewlap size and/or bite-force capacity, as dewlap size is known to be a reliable predictor of bite-force capacity in territorial males. We also examined whether these relationships were consistent between previously described body size classes ("lightweights" and "heavyweights"). Heavyweights had 50% higher testosterone concentrations than lightweights during the breeding season, suggesting a mechanism for the disproportionately larger heads and dewlaps and higher bite-forces of heavyweights. Plasma testosterone concentrations were positively correlated with dewlap size and bite-force performance in lightweights (but not heavyweights) but only because of mutual intercorrelation of all three variables with body size. We suggest two possibilities for the relationship between testosterone levels and body size: (1) testosterone promotes growth in this species or (2) smaller sexually mature males are unable to compete with larger males such that the benefits of elevated testosterone do not outweigh the costs. Corticosterone levels did not differ between the male morphs, and lightweights, but not heavyweights, showed an inverse relationship between testosterone levels and corticosterone levels. Our results suggest that testosterone is important for traits related to dominance in adult male green anoles and may influence the ability to compete with rivals via fighting ability or through the use of signals.

Androgen dependent seasonal changes in muscle fiber type in the dewlap neuromuscular system of green anoles

Green anoles (Anolis carolinensis) possess two sexually dimorphic neuromuscular systems involved in reproductive behaviors. One controls extension of a red throat fan (dewlap), which males employ during courtship, and the other controls intromission of copulatory organs (hemipenes). Although seasonal changes in circulating androgens mediate both courtship and copulatory behaviors, testosterone has differential effects on the underlying neuromuscular morphology. The present experiments were designed to test whether changes in muscle fiber type correspond to seasonal and androgenic regulation of reproductive behaviors in gonadally intact males (Experiment 1) or castrated males treated with either testosterone propionate or vehicle (Experiment 2). Gonadally intact males housed in breeding environmental conditions had a higher percentage of fast oxidative glycolytic fibers in the dewlap muscle than non-breeding males, but no effect of season on copulatory fibers was detected. Interestingly, testosterone treatment increased the percentage of fast oxidative glycolytic dewlap fibers independent of season, suggesting that routine changes in this hormone may mediate fiber type in gonadally intact males. In contrast, testosterone manipulation had little to no effect on copulatory muscle fiber type, demonstrating that a change in this feature is not the primary mediator for seasonal changes in male copulatory behaviors.

Current research on the behavioral neuroendocrinology of reptiles

Selected reptilian species have been the targets of investigations in behavioral neuroendocrinology for many years. Reptiles offer a particularly powerful set of traits that facilitate comparisons at multiple levels, including those within and between individuals of a particular species, between different environmental and social contexts, as well as across species. These types of studies, particularly as they are considered within the framework of results from other vertebrates, will enhance our understanding of the genetic and hormonal influences regulating changes in the structure and function of the nervous system. Work on the hormonal and environmental factors influencing courtship and copulatory behaviors in green anoles, including the development and maintenance of the neuromuscular structures critical for their display, is highlighted. Some very recent work on other model systems is also discussed to provide a context for suggested future research directions.

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.

Effects of testosterone on the development of neuromuscular systems and their target tissues involved in courtship and copulation in green anoles (Anolis carolinensis)

Male green anole lizards court females using a red throat fan (dewlap) and copulate by intromitting one of two penises (hemipenes). These structures begin sexually monomorphic, but by adulthood males have larger dewlaps, only males have hemipenes, and many of the neuromuscular components of both systems show male-biased dimorphisms. We hypothesized that testosterone (T), which increases in juvenile males but not females about a month after hatching, facilitates masculinization. To test this idea, on post-hatching day 30, gonadally intact females received either a blank or T implant, and males were either castrated or sham-castrated. At day 90, juveniles were euthanized and the length of the cartilage and cross-sectional areas of the muscle fibers and motoneurons required for dewlap extension were examined. We also measured the cross-sectional areas of the hemipenes and associated muscle fibers and motoneurons, and counted the motoneurons. T-treated females had longer cartilages and larger dewlap muscle fibers compared to those with blank implants. No effects on motoneurons were detected, and no females possessed hemipenes or associated musculature. In males, castration produced shorter dewlap cartilages and smaller hemipenes; other measures were not affected by treatment. These data indicate that components of the dewlap system differentiate relatively late in development, that T likely mediates the process, and that although components of the copulatory system are plastic in juvenile males, sexual differentiation of peripheral features is complete before day 30. The data also suggest that target structures (dewlap cartilage and hemipenes), compared to their neuromuscular effectors, are particularly sensitive to developmental T exposure.

The green anole (Anolis carolinensis): a reptilian model for laboratory studies of reproductive morphology and behavior

The green anol (Anolis carolinensis) is an excellent reptilian model for studying reproductive behavior and the neural and muscular morphology that supports it. This lizard has been the subject of behavioral and ecological study for more than 100 yr, and a rich literature exists on its natural history. Both courtship and copulatory behaviors reveal sex and seasonal differences, which allow for the study of mechanisms regulation naturally occurring variation in performance at multiple levels within a single animal model. Green anoles are readily obtained due to their abundance in the wild; once in the laboratory, they are easily maintained, bred, and reared. Background on the natural history and husbandry of this lizard is provided, and the authors' research program on the regulation of reproductive anatomy and behavior is reviewed, Discussion includes the similarities and differences in the mechanisms mediating both structure and function compared with more traditional animal models. This type of comparative research will make it possible to identify the fundamental principles governing reproductive biology, thus advancing both basic and applied knowledge.

Characterization of projections from a sexually dimorphic motor nucleus in the spinal cord of adult green anoles

Male green anoles possess two copulatory organs (hemipenes), which are independently controlled by bilateral muscles: the transversus penis (TPN) and retractor penis magnus (RPM). Adult females do not possess hemipenes or either of the two related muscles. Motoneurons projecting to the TPN lie in spinal segments trunk 17 and sacral 1 (T17-S1). Overall, motoneurons in this region are larger and more numerous in males than females. The present studies were designed to determine 1) whether motoneurons projecting to the RPM are located in the same sexually dimorphic nucleus, 2) other targets of T17-S1 motoneurons, and 3) the approximate proportion of motoneurons projecting to each muscle. In Study 1, unilateral injection of the retrograde tracer Fast Blue (FB) into RPMs and simultaneous unilateral injection of either Cholera Toxin-fluorescein (CT-FITC) or Diamidino Yellow into TPNs revealed that RPM and TPN motoneurons are indeed interdigitated in T17-S1. In Study 2, FB was used to characterize other targets of this nucleus in both males and females. In adult males, projections to four muscles accounted for 96% of the T17-S1 motoneurons: the TPN, RPM, caudifemoralis (CF), and cloacal sphincter (SC). In adult females, projections to the CF and SC comprised 70% of this nucleus. These data demonstrate that the T17-S1 nucleus is a mixed spinal nucleus that has projections to muscles present in both sexes, as well as those present only in males and specialized for male copulatory behavior.

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

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.