Biomedical subjects

D Crews

Publications and source records attributed to D Crews.

At least 145 records · Page 8Linked to original sources

PubMed1988

Embryonic temperature determines adult sexuality in a reptile.

Gonadal differentiation in amniote vertebrates is controlled by one of two mechanisms: genotypic sex determination (GSD) or environmental sex determination (ESD). After differentiation the fetal gonad produces sex steroid hormones which govern the development of other components of sexuality. Thus, the primary sex determiner is thought to operate solely as a trigger that initiates a cascade of events culminating in adult sex differences. In the leopard gecko (Eublepharis macularius), gonadal and morphological sex is determined by incubation temperature, with relatively 'hot' temperatures (32 degrees C) resulting in mostly male offspring and relatively 'cold' temperatures (26 degrees C) resulting in only female offspring. We report here that the reproductive behaviour and endocrine physiology of an adult is influenced by the temperature experienced as an embryo. Also, the perception of a female to courtship by a male is influenced by incubation temperature. These data indicate that incubation temperature, the primary determiner of sex in this species, has differential effects on adult sexuality.

Animals↗

PubMed1988

Sex reversal by estradiol in three reptilian orders.

Administration of exogenous estradiol caused embryos to develop ovaries rather than testes in alligators, softshell turtles, and leopard geckos. Alligators and leopard geckos are known to have environmental sex determination; softshell turtles lack both environmental sex determination and heteromorphic sex chromosomes. Sensitivity to gonadal feminization by estradiol thus appears to be widespread in amniotes.

Alligators and Crocodiles↗

PubMed1988

Psychobiology of sexual behavior in a whiptail lizard, Cnemidophorus inornatus.

This study investigated the effects of social environment on gonadal recrudescence and sexual behavior in male and female Little Striped Whiptail lizards (Cnemidophorus inornatus). The presence of sexually active males facilitates ovarian recrudescence in conspecific females. Similarly, the presence of reproductively active females facilitates testicular recrudescence in conspecific males. Males housed with females, however, had lower average circulating concentrations of testosterone and dihydrotestosterone, and higher average concentrations of corticosterone compared to intact males housed in isolation. In other studies, the presence of reproductively active females partially restored courtship behavior in castrated males compared to castrated males housed in isolation. Despite the stimulatory effects of females on castrates, exogenous androgens are required for complete restoration of all components of sexual behavior in male C. inornatus. Females are receptive to male courtship and copulatory behavior only during the vitellogenic stages; females in previtellogenic or postovulatory ovarian stages aggressively reject male courtship advances. These findings demonstrate reciprocal effects of sexual behaviors of males and females upon each other's reproductive behavior and physiology.

Animals↗

PubMed1988

Plasma steroid concentrations change in response to sexual behavior in Bufo marinus.

Steroid hormone concentrations change in response to social or environmental stimuli in many vertebrates. To test this phenomenon in an amphibian, we examined plasma androgen (A) and corticosterone (B) concentrations in male marine toads (Bufo marinus), a tropical species exhibiting intermale competition, amplectic clasping of females, and bouts of breeding behavior following rains. When males clasped females for 0, 1, 2, or 3 hr, plasma A concentrations were significantly and positively correlated with hours spent in amplexus. In field-sampled males, plasma A concentrations were higher in amplexing males than in single males. Among single males those found closer to breeding ponds had higher A concentrations than those more distant. These data support the hypothesis that sexual stimuli enhance plasma A concentrations in this amphibian. In 3-hr experimental tests and field-sampled males, B concentrations were higher in amplexing than in single males. Unlike some amphibians, short-term elevations of B apparently are not associated with decreased reproductive function. However, as in other amphibians in which high B concentrations are associated with stress-induced inhibition of reproduction, after 48-72 hr in captivity male toads showed high B concentrations and low plasma androgen concentrations. The bursts of sexual activities exhibited by B. marinus following heavy rains were associated with no changes in A concentration and with increased B concentration.

Animals↗

PubMed1988

Sexual coloration, plasma concentrations of sex steroid hormones, and responses to courtship in the female keeled earless lizard (Holbrookia propinqua).

The reproductive condition, steroid hormone concentrations in the plasma, and behavior of bright and plain female Holbrookia propinqua were examined. Bright females performed significantly more aggressive rejection of courtship than plain females. Bright females were significantly more likely than plain females to have follicles larger than 5.0 mm or oviductal eggs; females with large follicles or oviductal eggs had significantly higher concentrations of progesterone and androgen than those with small follicles and lacking oviductal eggs. Plasma progesterone, androgen, and estradiol levels in the females studied behaviorally were significantly higher for the bright females than for the plain ones. Females undergoing rapid brightening were significantly more likely to be sexually receptive and copulate than were either plain or fully brightened females. The role of sex steroid hormones in coloration and behavior and the adaptive value of chromatic signalling by females are discussed.

Aggression↗

PubMed1988

Hormonal correlates of sexual behavior in the female musk shrew: the role of estradiol.

Plasma levels of several steroid hormones in the female musk shrew, (Suncus murinus), are reported along with experiments on the causational role steroids play in female sexual behavior. Gonadally intact females sampled directly after a brief (about 7 minutes on average) sexual interaction with a male had equivalent plasma levels of estradiol (E2) but significantly higher plasma progesterone (P4) levels than resting females. In Experiment 2, females that engaged in nonsexual activities with conspecifics other than gonadally intact males had plasma levels of E2 and P4 equivalent to those of females that engaged in sexual behavior with males. Taken together, the results of these two experiments suggest that the relatively higher levels of plasma P4 noted in active musk shrews is not a response specific to sexual interactions with males. In Experiment 3, ovariectomized musk shrews were treated with E2 implants. These tonically administered physiological doses of E2 were not able to activate sexual behavior. However, two steroid-sensitive reproductive target tissues, the cervix and flank pad, were responsive to E2 treatment. Supplemental injections of P4 given to E2 treated ovariectomized females were also ineffective in restoring sexual behavior. These results suggest that physiological doses of E2 do not have direct effects on the expression of female sexual behavior in this mammalian species.

Androgens↗

PubMed1988

Effects of progestins on sexual behaviour in castrated lizards (Cnemidophorus inornatus).

In many male vertebrates, androgens stimulate, while progesterone inhibits, sexual behaviour. Testicular androgens also control sexual behaviour in males of the lizard Cnemidophorus inornatus. However, administration of progesterone will reinstate sexual behaviour in castrated animals. Increased doses of progesterone did not result in increased numbers of responders; at all doses tested approximately 36% (n = 45) of animals treated with progesterone responded as reliable courters. A high percentage (60-68%) of progesterone-sensitive individuals maintained their sensitivity over a period of months. A synthetic progestin 17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione with low rates of metabolism and conversion, also stimulated sexual behaviour in castrated C. inornatus. Finally, the synthetic antiprogestin 11 beta-4-dimethylaminophenyl-17 beta-hydroxy-17 alpha-1-propinyl- 4,9-oestradiene-3-one inhibited the stimulatory effects of progesterone on sexual behaviour but failed to inhibit the stimulatory effects of androgens. These findings indicate an unusual departure from the characteristic pattern of steroid sensitivities of neural centres controlling sexual behaviour.

Animals↗

PubMed1987

Annual reproductive cycle in the male red-sided garter snake, Thamnophis sirtalis parietalis: field and laboratory studies.

The reproductive cycle of male red-sided garter snakes (Thamnophis sirtalis parietalis) was studied in a laboratory-maintained population and in nature. Both populations exhibited a dissociated reproductive pattern in which spermatogenesis was initiated after the breeding season and sperm were stored in the vas deferens over winter. Although animals in both the laboratory and the field exhibited a dissociated reproductive pattern, temporal differences were evident in the two populations. Animals maintained in the laboratory initiated and completed spermiation much sooner than animals collected in the field. In the captive population, the renal sexual segment (RSS) could be identified but did not exhibit a seasonal fluctuation, indicating perhaps that androgen production was continuous, but at a reduced level. In nature, the RSS was hypertrophied at emergence, regressed following the breeding season, and again hypertrophied as circulating androgens increased in the fall, prior to hibernation. In both laboratory-maintained and field animals the circulating concentrations of testosterone and dihydrotestosterone at emergence were initially similar to the levels observed prior to hibernation but by 2 weeks postemergence had decreased to a relatively low level. Both the high androgen levels prior to hibernation and the rapid decrease during the spring courtship season may result from temperature influences on clearance rates. Plasma levels of corticosterone varied during the spring, first increasing upon emergence and then declining approximately 2 weeks following emergence.

Androgens↗

PubMed1987

Plasma steroid hormone levels of female red-sided garter snakes, Thamnophis sirtalis parietalis: relationship to mating and gestation.

Plasma levels of progesterone (P), testosterone (T), estradiol (E2), and corticosterone (B) of female red-sided garter snakes were measured during the period of ovarian development. Differences in hormone levels were analyzed with respect to three factors: whether the female mated in the spring, ovarian condition, and time after emergence from hibernation. The influence of these three factors on steroid hormone levels of two groups of females were then compared. In experiment I, females were obtained in the fall, subjected to an artificial dormancy period, and placed on warm, summer-like conditions in the laboratory. In experiment II, females were collected in the spring and sampled in the field. They were held in the field on fluctuating conditions for several weeks and then returned to the laboratory for sampling during early vitellogenesis. Females in experiment I had a shortened but otherwise normal ovarian and gestational cycle, whereas females in experiment II had an ovarian and gestational cycle typical of females in the field. In spite of these differences, the steroid hormone levels in relation to the ovarian cycle were remarkably similar for the two groups of females. We confirmed that mating in the spring induces a surge in E2; E2 also was elevated in a single sample obtained from animals collected in the fall. This elevation in plasma levels of E2 in the fall occurs at a time when the majority of females have recently deposited sperm in their oviducts. Plasma levels of T, P, and B were not significantly influenced by mating. Unlike previous reports of other viviparous snakes, plasma levels of P were low and mostly nondetectable, even during late gestation. Plasma T was significantly elevated around the time of late vitellogenesis and ovulation, and there was a tendency for E2 levels to be elevated at this time. In the field, plasma B levels were initially high immediately after capture and declined with time. Plasma B was significantly elevated in all females several weeks after emergence, suggesting that levels of B may vary with other annual cycles.

Animals↗

PubMed1987

Neural control of male-like pseudocopulatory behavior in the all-female lizard, Cnemidophorus uniparens: effects of intracranial implantation of dihydrotestosterone.

Dihydrotestosterone was implanted directly into the brain of ovariectomized all-female Cnemidophorus uniparens. Only implants located in the anterior hypothalamus-preoptic area (AH-POA) induced male-like pseudocopulatory behavior. Implants in other brain regions, including the ventromedial hypothalamus (VMH), failed to elicit mounting and intromission behavior. Implants in the VMH also failed to elicit female-like receptive behavior. Radioimmunoassay revealed no significant difference in circulating levels of androgen between the responding and the nonresponding animals. These findings support previous studies implicating the AH-POA as the major integrative area for male-typical mounting and intromission behavior.

Animals↗

PubMed1987

Pinealectomy blocks vernal courtship behavior in red-sided garter snakes.

Male red-sided garter snakes (Thamnophis sirtalis parietalis) court only on emergence from winter dormancy. The salient proximate factor responsible for the initiation of courtship behavior appears to be warm ambient temperatures subsequent to sustained exposure to cold temperatures. In the present study, autumn-captured garter snakes were pinealectomized or sham-pinealectomized 1-2 weeks prior to hibernation. After 17 weeks of maintenance in several ambient lighting and temperature conditions, all males were tested for the presence of courtship behavior. No pinealectomized animals exhibited any indications of reproductive behavior, whereas 53% of sham-pinealectomized males actively courted females. Photoperiod did not influence the prevalence of mating behavior. These results suggest that the pineal gland mediates non-photoperiodic seasonal information in the garter snake. This is the first demonstration, in any vertebrate species, that the pineal gland can directly influence reproductive behavior.

Animals↗

PubMed1987

Facilitation of courtship behavior in the male red-sided garter snake (Thamnophis sirtalis parietalis) following lesions of the septum or nucleus sphericus.

Courtship behavior in male red-sided garter snakes is limited to the period immediately following spring emergence. Lesions of the septum or nucleus sphericus prior to hibernation facilitate courtship behavior upon emergence the following spring. These lesions also had a significant effect on the reproductive physiology of the male snake. Snakes with lesions in the nucleus sphericus had a significantly higher concentration of circulating androgens compared to the control group. While animals with lesions in the septum exhibited elevated levels of circulating androgens, they were not statistically elevated when compared to the control group. The quantity of sexual granules in the epithelium of the renal sexual segment, a secondary sexual characteristic of male squamate reptiles, was significantly greater in lesioned animals. These results are the first to demonstrate a central inhibitory control of sexual behavior in a snake.

Animals↗

PubMed1987

Hibernation in garter snakes (Thamnophis sirtalis parietalis): seasonal cycles of cold tolerance.

1. The red-sided garter snake hibernates for about six months each year in Manitoba, Canada, where winter temperatures are often as low as -40 degrees C. Mammalian hibernators typically undergo profound changes in preparation for hibernation, but little is known about corresponding changes in reptiles. 2. We tested the importance of seasonal changes in the ability of red-sided garter snakes to hibernate successfully by exposing them to winter conditions at different times of year. 3. Animals that began hibernation in the fall were more likely to survive hibernation than animals that began hibernation in either early or late summer, despite the fact that the animals were kept on seasonally constant conditions prior to hibernation. 4. We suggest that these changes are derived from endogenous components of the yearly hibernation cycle of red-sided garter snakes but that only part of the cycle proceeds endogenously, i.e. it is not a completely endogenous circannual cycle.

Acclimatization↗

PubMed1987

Sex and seasonal differences in the skin lipids of garter snakes.

1. This study investigates the skin lipids of male and female red-sided garter snakes both in the breeding season and in the non-breeding season. 2. Skin lipids were analyzed by means of thin-layer chromatography (TLC) and gas chromatography/mass spectrometry (GC/MS). 3. Distinct differences exist in the skin lipids of males and females. 4. Samples obtained during the breeding season were qualitatively different from those acquired during the non-breeding season.

Acclimatization↗

PubMed1987

Temperature activation of courtship behavior in the male red-sided garter snake (Thamnophis sirtalis parietalis): role of the anterior hypothalamus-preoptic area.

Adult male red-sided garter snakes (Thamnophis sirtalis parietalis) received radio-frequency lesions prior to 17 weeks of low-temperature hibernation. Animals found to have bilateral lesions of the anterior hypothalamus-preoptic area (AH-POA) failed to exhibit courtship behavior on emergence from hibernation. Those individuals in which the major portion of the destruction was centered in the anterior POA also exhibited deficits in thermoregulatory behavior. Animals that received unilateral lesions of the AH-POA initiated courtship behavior after controls and had an abbreviated period of courtship; these animals exhibited normal responses to thermal stimuli. Male snakes with lesions outside the AH-POA courted normally and demonstrated no differences in thermoregulatory behavior compared with the surgical controls. These results indicate that in male red-sided garter snakes, an intact AH-POA is critical for the integration of thermal stimuli that activate seasonal courtship behavior.

Animals↗

PubMed1986

Evolution of mechanisms controlling mating behavior.

The proximate mechanisms underlying mating behavior in naturally occurring species can be fundamentally different from those in more commonly studied laboratory and domesticated forms. In naturally occurring species, reproductive strategies are much more diverse, and mechanisms controlling behavior are correspondingly diverse. A variety of hormonal, environmental, and social cues can be used to activate mating behavior. Which cues are used by particular species depends on differences in environmental and physiological constraints imposed by particular reproductive strategies. Study of this diversity of mechanisms promises to identify specific selective forces that have shaped their evolution. This evolutionary perspective leads to widely applicable generalizations and provides a useful context within which to conceptualize differences between species, populations, and individuals.

Animals↗

PubMed1986

The effects of sex steroid treatments on sexual differentiation in a unisexual lizard, Cnemidophorus uniparens (Teiidae).

Cnemidophorus uniparens is a parthenogenetic unisexual species of lizard in which each individual develops as a female, making it a unique animal model for the study of sexual differentiation. In one study, administration of exogenous testosterone before and/or after hatching influenced the development of the gonads, the accessory reproductive ducts, the renal sex segment of the mesonephric kidney, and the femoral glands, a secondary sex character. Testosterone treatment also affected the cross-sectional area of the gonad and the proportions of cortical and medullary tissues present in the developing gonad. The oviducts and femoral glands of testosterone-treated individuals were hypertrophied; the collecting tubules of the kidney of these animals contained granules, an androgen-dependent, sexually dimorphic character in squamate reptiles. In another study, testosterone, dihydrotestosterone, or estradiol were administered to C. uniparens embryos. No treatment effects on gonadal development were detected on the day of hatching. However, estradiol, but not testosterone and dihydrotestosterone, stimulated development of the oviducts. Taken together these studies suggest that androgen aromatization may play a role in sexual differentiation in lizards.

Animals↗

PubMed1986

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

The hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, one of the gonochoristic ancestral species of the all-female, parthenogenetic C. uniparens, was determined. Significantly fewer castrated males courted females than intact and sham-castrated control males. Silastic capsules containing dihydrotestosterone or testosterone reinstated courtship and copulatory behavior in a significant number of castrates. While significantly greater numbers of castrates treated with androgens resumed courtship, some of the castrates receiving progesterone implants also resumed intense courtship and copulatory behavior. Exogenous progesterone also maintained courtship behaviors in a significant number of intact males at a time when control males ceased to court. The implications of these findings for our understanding of the evolution of hormone-brain-behavior relationships and sex steroid hormone mechanisms of action are discussed.

Animals↗