Hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a unisexual, parthenogenetic lizard

Aromatase activity and regulation of sexual behaviors in the green anole lizard

Sexual behaviors in green anoles are regulated by steroid hormones. Androgens activate the display of masculine courtship and copulatory behaviors, and estradiol activates feminine receptivity. Testosterone can also facilitate receptivity in females. The present study was conducted to test the role of converting testosterone to estradiol (aromatization) in the regulation of sexual and aggressive behaviors. Adult males and females were gonadectomized and implanted with a Silastic capsule containing either testosterone propionate (T) or estradiol benzoate (E) or with an empty (blank, BL) capsule. T- and BL-treated animals were then given injections of either Fadrozole (FAD, an aromatase inhibitor) or saline (SAL). E-treated animals received saline injections. Each individual was then tested alternately with male and female stimulus animals. Overall, T stimulated masculine sexual behaviors and receptivity, but the androgen had little effect on the display of aggressive behaviors. The inhibition of aromatase activity by treatment with Fadrozole eliminated the effect of T on receptivity. In contrast, Fadrozole treatment had no effect on mounting behavior or the frequency of courtship bouts. The inhibition of aromatase activity did increase the number of dewlap extensions (the display of a red throat fan) during courtship. These results suggest that the metabolism of testosterone to estradiol is a mechanism through which androgens can facilitate receptivity, and that such aromatization of testosterone is not required for the display of masculine sexual behaviors. In addition, females performed courtship and mounting behaviors less frequently than males, suggesting that an organizational component to steroid hormone regulation of sexual behaviors may exist in the anole.

Androgen metabolism in the brain of the green anole lizard (Anolis carolinensis)

Activities of the steroid metabolizing enzymes aromatase and 5 alpha-reductase have been documented in species ranging from fish to humans, yet relatively little work has been done in this area in reptiles. In the green anole lizard, Anolis carolinensis, steroid hormones are critical to the display of both masculine and feminine sexual behaviors. For example, dihydrotestosterone and testosterone can stimulate both masculine courtship and copulatory behaviors, and estrogen and testosterone facilitate feminine receptivity. These results suggest roles for both aromatase (which catalyzes the conversion of testosterone to estradiol) and 5 alpha-reductase (which converts testosterone to dihydrotestosterone). This study documents the presence of both enzymes in the brain of the green anole and indicates that the activity of 5 alpha-reductase is much higher than that of aromatase in whole brain homogenates. However, differences exist among brain regions, such that aromatase activity is higher in preoptic area/hypothalamic dissections, whereas 5 alpha-reductase is much more active in the brain stem. These variations in regional enzyme activity may influence steroid hormone regulation of specific sexual behaviors in male and female anoles.

Intrahypothalamic implantation of progesterone in castrated male whiptail lizards (Cnemidophorus inornatus) elicits courtship and copulatory behavior and affects androgen receptor- and progesterone receptor-mRNA expression in the brain

A primary tenet of behavioral neuroendocrinology is that gonadal steroid hormones act on limbic nuclei to activate mating behavior in vertebrates. Traditionally, research has focused on the regulation of male-typical sexual behavior by testicular androgens and female-typical sexual behavior by ovarian estrogen and progesterone. Indeed, progesterone generally is regarded as an antiandrogen, acting centrally to inhibit sexual behavior in males. However, experiments with lizards, and more recently with rats, have challenged this paradigm. For example, exogenous progesterone induces mating behavior in some, but not all, castrated male whiptail lizards. The present study determined that implantation of progesterone into the anterior hypothalamus preoptic area of castrated, progesterone-sensitive males completely restored sexual behavior but failed to elicit sexual activity in castrated, progesterone-insensitive males. Further, androgen receptor -and progesterone receptor-mRNA expression in specific brain regions was significantly different in progesterone-sensitive versus progesterone-insensitive animals. Progesterone-sensitive males showed significantly higher relative abundance of androgen receptor-mRNA in the preoptic area, amygdala, and lateral septum, as compared with progesterone-insensitive animals receiving the same treatment. In contrast, progesterone receptor-mRNA abundance was lower in preoptic area of progesterone-sensitive males than in progesterone-insensitive males. No differences were found in the baseline abundance of androgen receptor-or progesterone receptor-mRNA in these nuclei between control groups of progesterone-sensitive and progesterone-insensitive males who were castrated but not implanted. This suggests that progesterone differentially regulates its own receptor as well as androgen receptor in areas of the brain involved in the control of sexual behavior of males and that the nature of this regulation shows individual variability.

Progesterone modulation of androgen-dependent sexual behavior in male rats

The present study examines the effects of physiological levels of progesterone (P) on copulatory behavior in sexually naive male rats. Two weeks after gonadectomy males were implanted with either empty Silastic capsules (BL) or Silastic capsules containing testosterone (T), P, or both (P+T). When tested with an estrous female, all of the gonadally intact males (intact) and none of the BL controls exhibited mounting/intromission behaviors. Mounting was observed in 75% of the T-alone males. More than half (64%) of the P-alone males and 100% P+T males exhibited mounting. In most cases, mounting was followed by intromission responses. Subsequently, intact and gonadectomized males received daily injections of the P antagonist RU486 along with hormone treatment. After receiving RU486, only 63% of the intact males and 71% of the T-alone males mounted successfully. The facilitatory effects of P on copulatory behavior were completely abolished by RU486 treatment. The present studies provide the first evidence in mammals suggesting that P-dependent mechanisms influence neurochemical pathways involved in copulation.

Progesterone and sexual behavior in males

Previous investigations into the effects of progestins on copulatory behavior have suggested that progesterone inhibits the expression of androgen-dependent sexual behaviors in males. However, virtually all of those studies utilized pharmacological dosages of progesterone. Such experiments, although essential for understanding the behavioral effects of progesterone, yield little insight into the function of endogenous progesterone in masculine sexual responses. In this brief review, attention is focused on the role of physiological levels progesterone in copulatory behavior in male reptiles and mammals. Efforts are made to promote a reevaluation of the behavioral effects of progestins in males, similar to ongoing studies which are reexamining neural mechanisms involved in progestin-mediated reproductive behavior in the female.

Hormonal control of sex differences in the brain, behavior and accessory sex structures of whiptail lizards (Cnemidophorus species)

The effects of steroid hormones on sexual dimorphisms in the brain, behavior and accessory sex structures were investigated in two species of whiptail lizards. The studies were conducted both in adults and hatchlings of a sexually reproducing species (Cnemidophorus inornatus) and an all-female species (C. uniparens) which displays 'sexual' behaviors typical of males and females. Adults were gonadectomized and approximately 3 months later given either a Silastic capsule filled with sex steroid or an empty capsule. Young animals of both species were left intact and given a capsule on the day of hatching. An additional group of C. uniparens was ovariectomized on the day of hatching. Following treatment, measures of oviduct (estrogen-dependent), renal sex segment (androgen-dependent) and wolffian duct (androgen-dependent) hypertrophy were taken in some experiments. Animals were also tested for sexual behavior in some of the studies. The volumes of the anterior hypothalamus-preoptic area and ventromedial hypothalamus were measured in each individual. Estrogen, testosterone and dihydrotestosterone stimulated peripheral structures at both time periods in both sexes and species. The hormones also stimulated courtship and copulatory behaviors in many of the adult animals. However, testosterone in the anterior hypothalamus-preoptic area of male C. inornatus was the only treatment which produced parallel effects on the volume of a brain area and the behaviors which it controls. These data add whiptail lizards to the list of species in which steroid hormones affect the volume of brain regions in adulthood, but suggest that such changes in morphology are not necessarily predictive of functional differences.

Sex differences in adrenal function in the lizard Cnemidophorus sexlineatus: I. Seasonal variation in the field

In order to document sex differences in adrenal function and how this relates to gonadal function during the period of seasonal activity, blood samples from male and female six-lined racerunners, Cnemidophorus sexlineatus, were taken immediately after capture in the field for determination of plasma corticosterone and gonadal steroid concentrations. Plasma testosterone and dihydrotestosterone levels for males, and 17 beta-estradiol and progesterone levels for females, were measured. Trends in the concentration of plasma corticosterone differed significantly between males and females. In males the highest concentrations of corticosterone were measured in late spring and the lowest concentrations were measured in late summer. Whereas half of the variation in corticosterone levels among males could be explained as seasonal change, less than 1% of the variation among females could be explained as seasonal change. In males plasma corticosterone and androgens exhibited similar seasonal decreases. Corticosterone levels for females were not correlated with progesterone or 17 beta-estradiol levels. Sex differences in seasonal variation in plasma corticosterone concentrations suggest that corticosterone may be involved in the different reproductive strategies and energy requirements of males and females during the seasonal period of activity.

Interactions between progesterone and androgens in the stimulation of sex behaviors in male little striped whiptail lizards, Cnemidophorus inornatus

Progesterone is believed to have a suppressive or inhibitory role in regulating androgen-dependent sex behaviors in male mammals and birds. Previous studies in this laboratory have revealed that in the little striped whiptail lizard (Cnemidophorus inornatus), progesterone (P) can stimulate sex behavior in a proportion of the males. The present study sought to determine (i) interactions between androgens and P in activating sex behaviors, and (ii) the overlap in behavioral sensitivities to androgens and P in male C. inornatus. With an increasing length of castration the behavioral sensitivity of males to exogenous P was reduced. However, priming of castrated males with subthreshold doses of exogenous dihydrotestosterone (DHT) greatly facilitated subsequent behavioral responses to exogenous P. Progesterone treatments of castrated males were more effective at reinstating sex behaviors in males that exhibited high-intensity sex behaviors prior to castration compared to males that exhibited low-intensity sex behaviors. Finally, exogenous DHT is more effective at reinstating sex behaviors in P-sensitive males than in P-insensitive males. These data are discussed in light of possible mechanisms underlying the unusual behavioral effects of P in the Cnemidophorus model system.

Individual variation in intensity of sexual behaviors in captive male Cnemidophorus inornatus

The present studies investigated the source of individual variation in intensity of sexual behaviors in captive male whiptail lizards, Cnemidophorus inornatus. No correlation was found between an individual's circulating concentration of dihydrotestosterone, testosterone, or corticosterone at the time of capture or in the laboratory and their level of sexual behaviors observed in the laboratory. A large percentage of males that initially exhibited low intensity courtship remained low intensity courters, although some became more reliable courters following 6 months of acclimation to the laboratory. Similarly, following castration and androgen replacement, most low intensity courters continued to exhibit weak and infrequent sexual behaviors. The data suggest that individual variation in sexual behaviors exhibited by captive male C. inornatus is not due to (i) low circulating concentrations of androgens, (ii) elevated circulating concentrations of corticosterone, or (iii) different profiles of testicular steroidogenesis. Rather, the source of differences may lie in (i) an inability to respond to androgens, (ii) an inability to exhibit sexual behavior, or (iii) non-hormonal stress related to captivity.

The effects of progesterone on sexual behavior in male green anole lizards (Anolis carolinensis)

It is well known that androgen-dependent sexual behaviors in male mammals and birds are inhibited by exogenous progesterone (P). However, recent research on male whiptail lizards (Cnemidophorus inornatus) indicates that P can stimulate sexual and copulatory behavior. We report here both antiandrogenic and synandrogenic actions of P on sexual behavior in males of another reptile, the green anole lizard (Anolis carolinensis). Earlier reports on birds and mammals are reviewed and discussed in relation to a possible physiological role of P in influencing sexual behavior in male vertebrates.

Is the snark still a boojum? The comparative approach to reproductive behavior

One of Frank Beach's many achievements was his stimulating influence on the comparative study of behavior. This review honors that legacy by categorizing and describing the many kinds of comparative approaches in use today for the study of reproductive behavior. The categorization is based on the motives and goals of the researcher, the kinds of questions that can be answered, the number and phylogenetic relatedness of the species being compared, and the method used for analyzing the results. Each approach is illustrated with specific examples from recent research, using studies from the field of hormones and behavior whenever possible.

Effects of intracranial implantation of dihydrotestosterone on sexual behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a parthenogenetic lizard

Dihydrotestosterone was implanted directly into the brain of castrated male Cnemidophorus inornatus, a direct sexual ancestor of the parthenogenetic species C. uniparens. Only implants located in the anterior hypothalamus--preoptic area (AH-POA) induced male-typical sexual behavior. Implants in other brain regions, including the ventromedial hypothalamus, failed to elicit courtship or copulatory behavior. Radioimmunoassay revealed no significant difference in the concentrations of circulating androgens between the responding and nonresponding animals. Previous data from this laboratory demonstrated that the AH-POA controls male-like pseudosexual behavior in C. uniparens. The current results support the hypotheses that (i) the AH-POA is the major area of hormone action in the brain controlling male-typical sexual behavior in C. inornatus as in other vertebrates and (ii) the neural circuits controlling male-typical behavior have been conserved in the evolution of the parthenogen C. uniparens.

Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards

All-female, parthenogenetic species afford a unique test of hypotheses regarding the nature and evolution of sexuality. Mating behavior accomplishes the transfer of gametes and stimulates the coordination of reproductive activity of the male and female. Cnemidophorus uniparens, a parthenogenetic species, is believed to have resulted from the hybridization of two extant gonochoristic species, Cnemidophorus inornatus and Cnemidophorus gularis. C. uniparens regularly and reliably perform behaviors identical in form to those performed during mating by male C. inornatus. We have determined experimentally that individuals of the parthenogenetic species demonstrating male-like pseudosexual behavior also share a similarity in function with males of the sexually reproducing species. The number of female C. inornatus ovulating increases, and the latency to ovulation decreases, if a sexually active conspecific male is present. A similar facilitatory effect on ovarian recrudescence occurs in the all-female C. uniparens in the presence of a male-like individual. These results show that behavioral facilitation of ovarian recrudescence is important in sexual and unisexual species. This may represent a potent selection pressure favoring the maintenance of male-typical behaviors, thus accounting for the display of behavioral traits usually associated with males in unisexual species of hybrid origin.

Progesterone induction of pseudocopulatory behavior and stimulus-response complementarity in an all-female lizard species

Individuals of the all-female whiptail lizard species (Cnemidophorus) exhibit male-like and female-like pseudocopulatory behaviors that are correlated with stages of the ovarian cycle. Here we report on the hormonal bases of these behaviors. Parthenogenetic C. uniparens were ovariectomized and given Silastic implants containing either progesterone (P) or estradiol (E2); untreated controls received blank implants. Ten pairs of the following combinations were observed: P females paired with E2 females, P females paired with blank females, and E2 females paired with blank females. Each pair was observed at regular intervals 4 hr a day for 6 days. Pseudocopulations were observed between P and E2 animals; P animals consistently assumed the male-like role while E2 females assumed the female-like role. No pseudosexual behavior was observed between individuals of either P and blank or E2 and blank pairs. These data indicate that the postovulatory surge in P mediates male-like pseudosexual behaviors and the preovulatory surge in E2 mediates female-like pseudosexual behaviors in C. uniparens. Further, a complementarity in the behavior and physiology of both participants (male-typical mounting and female-typical receptivity) are important factors in pseudocopulatory behavior.