Search PubMed⌕ Search

Biomedical subjects

D Crews

Publications and source records attributed to D Crews.

At least 163 records · Page 9Linked to original sources
PubMed1986

Sex steroid hormones in natural populations of a sexual whiptail lizard Cnemidophorus inornatus, a direct evolutionary ancestor of a unisexual parthenogen.

The lizard genus Cnemidophorus consists of both sexual species and unisexual, all-female species. We characterized changes in circulating levels of gonadal sex steroid hormones in males and females in one of the sexual species, C. inornatus, to compare them to previously measured levels in a unisexual, parthenogenetic species, C. uniparens. Reproductively active male C. inornatus have high levels of dihydrotestosterone and somewhat lower levels of testosterone. These levels are highest immediately after females become sexually receptive and decrease later at the onset of testicular regression. Female C. inornatus have high levels of estradiol and low levels of progesterone during the previtellogenic and vitellogenic phases of the ovarian cycle. During the postovulatory phase, they have low levels of estradiol and high levels of progesterone. We could not detect circulating levels of androgen at any phase of the ovarian cycle. The patterns of hormone secretion in the female C. inornatus are virtually identical to those of its direct evolutionary descendant, C. uniparens. This confirms our previous conclusion that the evolution of the parthenogenetic mode of reproduction and expression of male-like pseudosexual behavior that are characteristic of the unisexual C. uniparens has not been accomplished by evolutionary modifications in the pattern of sex steroid hormone secretion. Rather it is the response to this pattern of secretion that has been modified.

Animals↗
PubMed1986

Ovarian development in red-sided garter snakes, Thamnophis sirtalis parietalis: relationship to mating.

Female red-sided garter snakes (Thamnophis sirtalis parietalis) exhibit a complex array of behavioral patterns. Females may mate once, more than once, or not at all in the spring of the year, and a high percentage of females may mate in late summer before returning to winter hibernacula. The role of mating in the induction of ovarian growth in this species is complicated by this pattern of spring and late summer mating. Females hibernated in the laboratory that mate on emergence in the spring exhibit higher frequencies of ovarian recrudescence than females that do not mate when exposed to males. Yet neither male courtship nor mating in the spring is required for ovarian development. However, mated and unmated females collected in the field in the spring of the year do not differ in frequencies of ovarian recrudescence. Ovarian growth in unmated females collected in the spring parallels the incidence of mating in the previous late summer. Thus, mating may influence the initiation of ovarian growth, but the effect can occur immediately (from spring mating) or be stored over winter (from late summer mating). Large year-to-year variation in reproductive rates of female red-sided garter snakes suggests that other factors such as nutrition and stored energy reserves may modify the effects of mating on ovarian growth.

Animals↗
PubMed1986

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.

Animals↗
PubMed1986

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.

Animals↗
PubMed1986

Effects of prostaglandin F2 alpha on sexual behavior and ovarian function in female garter snakes (Thamnophis sirtalis parietalis).

Sexual behavior, plasma steroid hormone levels, and ovarian growth of female red-sided garter snakes (Thamnophis sirtalis parietalis) treated with prostaglandin F2 alpha (PGF2 alpha; 0.5-5.0 micrograms/g BW) were observed. The largest dose of PGF2 alpha significantly inhibited receptivity (mating) within 0.5 h of treatment and resulted in a significant decrease in sexual attractively 24 h later. These changes precisely parallel those seen in females that have mated. Lower doses of PGF2 alpha significantly increased the latency of females to mate by 3-fold in both the laboratory and the field. In contrast to the inhibitory effects of PGF2 alpha on sexual behavior, females injected with PG had levels of steroid hormones (progesterone, testosterone, 17 beta-estradiol, and corticosterone) similar to those in unmated controls 24 h after treatment, while females that had mated 24 h previously had significantly elevated plasma levels of 17 beta-estradiol. There was no significant difference in the incidence of vitellogenesis 6-8 weeks later among unmated females, unmated females treated with PG, and mated females. These results suggest that complex neuroendocrine responses occur after mating. One response may involve mating-induced release of PGs and suppression of sexual behavior, while a second independent response may involve a PG-independent mechanism that influences ovarian function. These findings are interpreted within the context of the garter snake's reproductive physiology and ecology.

Animals↗
PubMed1985

Behavioral facilitation of reproduction in sexual and parthenogenetic Drosophila.

In a normal bisexual laboratory strain of Drosophila mercatorum, females housed with either fertile or sterile males lay more eggs than do females housed in pairs or as isolates. Females of a derived parthenogenetic strain have suffered genetic loss of this behavioral facilitation of egg production, a loss comparable to the loss of sexual receptivity. Despite these losses there has been a large increase in fecundity in the parthenogenetic strain. These findings are compared with those in a parthenogenetic lizard.

Animals↗
PubMed1985

Role of progesterone in the control of female sexual receptivity in Anolis carolinensis.

The synergism of progesterone (P) with estrogen (estradiol benzoate, EB) in the facilitation of receptivity in female green anole lizards (Anolis carolinensis) is independent of time since ovariectomy, but is dependent on the time interval between EB and P administration and on the time of year. There is a brief window of time during which P facilitates receptivity in females primed with a subthreshold dose of EB. Sexual receptivity is induced if the time interval between EB and P is 24 hr but not if it is 48 hr. Furthermore, female A. carolinensis appear to be more sensitive to hormone treatment during different times of the year. Ovariectomized green anole females injected with EB and P during the summer are sexually receptive, but if given the same hormone regimen during the winter, they remain unreceptive.

Animals↗
PubMed1985

Sex steroid hormones during the ovarian cycle of an all-female, parthenogenetic lizard and their correlation with pseudosexual behavior.

Cnemidophorus uniparens is a unisexual lizard that reproduces by parthenogenesis. Individuals of this species display male-like and female-like copulatory behaviors during different phases of the ovarian cycle suggesting that these pseudocopulatory behaviors are hormonally activated. To learn more about both the endocrinology of parthenogenesis and the possible hormonal activation of male-like copulatory behavior in female individuals, we (1) characterized changes in plasma levels of the sex steroid hormones progesterone, 5 alpha-dihydrotestosterone, testosterone, and 17 beta-estradiol during the ovarian cycle in both free-living and captive individuals, and (2) measured sex steroid hormones in plasma collected from captive individuals immediately after they expressed male-like or female-like copulatory behavior. In general, the pattern of secretion of ovarian hormones in C. uniparens appears to be similar to that of other oviparous vertebrates with similar reproductive cycles. Estradiol is elevated only during the preovulatory phase, whereas progesterone increases slightly during vitellogenesis and then increases dramatically following ovulation. Circulating levels of androgen are very low and are generally below the sensitivity of our radioimmunoassay at all stages of the ovarian cycle. The hormonal correlates of female-like copulatory behavior suggest that, as in other vertebrates, female receptivity is activated by a synergism of estradiol and progesterone. There is no evidence that the hormonal cycle has been altered to produce elevated levels of androgens during the phase of the cycle when male-like behavior is expressed. Rather it seems more likely that the central nervous system has evolved a novel response to a typical pattern of ovarian steroid hormone secretion. At present, the best hormonal correlate of male-like behavior is that changes in plasma levels of progesterone closely parallel changes in probability of expressing male-like behavior.

Animals↗
PubMed1985

Role of the anterior hypothalamus-preoptic area in the regulation of courtship behavior in the male Canadian red-sided garter snake (Thamnophis sirtalis parietalis): intracranial implantation experiments.

Testosterone (T) was administered intracranially to intact adult male Canadian red-sided garter snakes (Thamnophis sirtalis parietalis) in the fall and in the summer. Implants in the anterior hypothalamus-preoptic area (AH-POA) as well as other regions of the brain, including the medial and basal hypothalamus, thalamus, medial cortex, and third ventricle, failed to elicit courtship behavior. The spermatogenic stage was more advanced, and the circulating level of androgens was significantly higher, in animals that received implants of T in the AH-POA. These findings suggest the sex steroid-concentrating sites in the AH-POA of the garter snake are involved in feedback regulation of pituitary gonadotropin secretion and not in the control of courtship behavior. Hematocrit was found to be higher in animals that received implants of T, regardless of location, a response that may be related to changes in blood chemistry prior to hibernation. These findings support previous research indicating that in the adult male red-sided garter snake, a species exhibiting a dissociated reproductive tactic, courtship behavior is independent of testicular androgens.

Androgens↗
PubMed1985

Behavioral and physiological control of yolk synthesis and deposition in the female red-sided garter snake (Thamnophis sirtalis parietalis).

Ovarian recrudescence in female garter snakes, Thamnophis sirtalis parietalis, follows spring emergence from hibernation and mating. In the laboratory, courtship and mating stimuli significantly increased the proportion of female garter snakes becoming pregnant, although some noncourted nonmated controls also became pregnant. Females given artificial mating stimuli under anesthetic, without courtship stimuli, were no more likely than either noncourted, nonmated or anesthetized controls to become pregnant. Hormonal changes and yolk synthesis rapidly followed mating in both laboratory and field females; serum estradiol increased more than 10-fold in 2 days and serum calcium, a measure of yolk precursor lipoprotein (vitellogenin) concentration, increased more than two times in 10 days. Administration of exogenous estradiol stimulated yolk synthesis, but did not result in yolk deposition into ovarian follicles. However, administration of ovine follicle-stimulating hormone induced both hepatic yolk synthesis and yolk deposition. Our results are consistent with the hypotheses that courtship and copulation are facilitatory to ovarian recrudescence but neither alone nor in combination is necessary nor sufficient, and in this species yolk synthesis and yolk deposition are separately regulated.

Animals↗
PubMed1985

Effects of early sex steroid hormone treatment on courtship behavior and sexual attractivity in the red-sided garter snake, Thamnophis sirtalis parietalis.

Previous research has shown that exogenous androgen fails to elicit courtship (chin-rubbing) behavior in adult male or female red-sided garter snakes (Thamnophis sirtalis parietalis). However, gonadectomized and intact newborn male and female red-sided garter snakes given silastic capsules containing testosterone exhibit chin-rubbing behavior; gonadectomized and untreated animals do not show this behavior. Both males and females also exhibit chin-rubbing behavior when treated with testosterone as yearlings. Hibernation stimulates chin-rubbing behavior only in males that have received androgen treatment as a neonate or as a yearling. Previous research has also shown that adult females, but not adult males, are courted if given estrogen treatment. Both newborn males and females will elicit chin-rubbing behavior from adult males if given estrogen treatment, indicating production and release of an attractiveness pheromone characteristic of adult females. Male red-sided garter snake breed for the first time on emergence from their second hibernation, whereas females probably do not breed until their third year of life. These data suggest that in the red-sided garter snake, a species that exhibits a dissociated reproductive tactic, sex steroid hormones act to organize central nervous system mechanisms subserving courtship behavior such that temperature, and not hormonal, fluctuations activate sexual behavior in the adult organism.

Age Factors↗
PubMed1985

Role of the anterior hypothalamus-preoptic area in the regulation of courtship behavior in the male Canadian red-sided garter snake (Thamnophis sirtalis parietalis): lesion experiments.

Lesions of the medial preoptic area and/or the ventromedial hypothalamus resulted in an abrupt and immediate decline in courtship behavior in adult male Canadian red-sided garter snakes (Thamnophis sirtalis parietalis). Lesions of the more anterior portions of the preoptic area resulted in a more gradual, delayed decline in courtship behavior. Male snakes sustaining lesions dorsal, ventral, or caudal to the anterior hypothalamus-preoptic area exhibited no change in courtship behavior relative to controls. Measurements of testis size, spermatogenic stage, and circulation levels of androgens revealed no differences between any of the groups. There were marked differences in the change in hematocrit over time between the groups. These results indicate that the anterior hypothalamus-preoptic area is involved in the control of courtship behavior in the adult male red-sided garter snake. Moreover, it is suggested that the stimulus affecting the anterior hypothalamus-preoptic area to activate courtship is temperature related.

Androgens↗
PubMed1985

Small male body size in garter snake depends on testes.

In the red-sided garter snake (Thamnophis sirtalis parietalis) adult females are larger than adult males; this difference is apparent within 3 wk of birth, a time coinciding with high circulating levels of androgens. To study the ontogeny and regulation of this sexual dimorphism, male neonates were either castrated, castrated and given Silastic capsules containing testosterone or estradiol, or given a sham operation at 8, 9, or 10 wk of age. Female neonates were either given a Silastic capsule containing testosterone or dihydrotestosterone or given a sham operation at 8, 9, 10, or 14 wk of age. The sex difference in body size and growth rate in neonates was abolished by castration; the pattern of growth of castrated males was similar to sham-operated females. Androgens in the amounts administered failed to reverse the effects of castration, because castrated male and female neonates receiving exogenous androgens grew at the same rate as did sham-operated females. Males castrated as adults grow larger than adult males given a sham operation, indicating the inhibitory role of the testes on body size exists after sexual maturity. Treatment of adult males with testosterone, however, prevented the increase in body size after castration, suggesting that the mechanism regulating weight gain in the garter snake depends on gonadal androgen.

Androgens↗
PubMed1985

Social dynamics of group courtship behavior in male red-sided garter snakes (Thamnophis sirtalis parietalis).

Mating in red-sided garter snakes is characterized by the formation of mating balls. Up to 100 males simultaneously court single females. The social dynamics of the formation of these mating balls was examined to determine whether the mating balls are formed simply because of a common attraction to the female or whether males are stimulated by the mating balls themselves. A sexually attractive female garter snake appears to be even more attractive to a male when she is being courted by other males than when she is alone. Male garter snakes courted females more actively when other males were also courting the female than when they were alone with her. There is a positive correlation between the number of additional males present and the amount of courtship activity shown by the test male toward the stimulus female. The extent to which the courtship activity of the test males was stimulated by the presence of additional courting males was not influenced by how actively the additional males courted.

Animals↗
PubMed1984

Social status, gonadal state, and the adrenal stress response in the lizard, Anolis carolinensis.

Adult males of the small arboreal iguanid lizard, Anolis carolinensis, form social dominance hierarchies when placed in habitats with limited resources. Skin color changes occur during hierarchy formation, most conspicuously in subordinates, who appear darker (more brown) than dominants (more green). Because skin color in this species is under the control of hormones frequently associated with physiological stress, radioimmunoassay of plasma levels of the principal reptilian adrenal steroid, corticosterone, was performed. To examine the influence of gonadal androgen, known to influence the aggression that attends hierarchy formation, lizard pairs were constituted in which one or both members were castrated. Corticosterone levels of intact subordinates were significantly elevated, whereas those of castrated subordinates or dominants showed levels comparable to those of isolates. No significant differences in spermatogenic stage could be detected between intact dominants or subordinates.

Animals↗
PubMed1984

Gamete production, sex hormone secretion, and mating behavior uncoupled.

A common observation for seasonally breeding vertebrates is that the reproductive processes of gamete production, sex steroid hormone secretion, and mating behavior coincide, and further, that sex steroid hormones activate mating behavior. The postulate of hormone-dependence of mating behavior is based primarily on detailed studies of laboratory and domesticated species. However, comparison of a wide array of vertebrates reveals numerous exceptions to this supposed rule. Consideration of these species indicates that there is no fixed or intrinsic causal association among gamete production, sex hormone secretion, and mating behavior within any of the classes of vertebrates.

Animals↗