Current research in amphibians: studies integrating endocrinology, behavior, and neurobiology

Modulation of sensory-motor integration as a general mechanism for context dependence of behavior

Social communication is context-dependent, with both the production of signals and the responses of receivers tailored to each animal's internal needs and external environmental conditions. We propose that this context dependence arises because of neural modulation of the sensory-motor transformation that underlies the social behavior. Neural systems that are restricted to individual behaviors may be modulated at early stages of the sensory or motor pathways for optimal energy expenditure. However, when neural systems contribute to multiple important behaviors, we argue that the sensory-motor relay is the likely site of modulation. Plasticity in the sensory-motor relay enables subtle context dependence of the social behavior while preserving other functions of the sensory and motor systems. We review evidence that the robust responses of anurans to conspecific signals are dependent on reproductive state, sex, prior experience, and current context. A well-characterized midbrain sensory-motor relay establishes signal selectivity and gates locomotive responses to sound. The social decision-making network may modulate this auditory-motor transformation to confer context dependence of anuran reproductive responses to sound. We argue that similar modulation may be a general mechanism by which vertebrates prioritize their behaviors.

Socially modulated cell proliferation is independent of gonadal steroid hormones in the brain of the adult green treefrog (Hyla cinerea)

Gonadal steroid hormones have been shown to influence adult neurogenesis in addition to their well-defined role in regulating social behavior. Adult neurogenesis consists of several processes including cell proliferation, which can be studied via 5-bromo-2'-deoxyuridine (BrdU) labeling. In a previous study we found that social stimulation altered both cell proliferation and levels of circulating gonadal steroids, leaving the issue of cause/effect unclear. In this study, we sought to determine whether socially modulated BrdU-labeling depends on gonadal hormone changes. We investigated this using a gonadectomy-implant paradigm and by exposing male and female green treefrogs (Hyla cinerea) to their conspecific chorus or control stimuli (i.e. random tones). Our results indicate that socially modulated cell proliferation occurred independently of gonadal hormone levels; furthermore, neither androgens in males nor estrogen in females increased cell proliferation in the preoptic area (POA) and infundibular hypothalamus, brain regions involved in endocrine regulation and acoustic communication. In fact, elevated estrogen levels decreased cell proliferation in those brain regions in the implanted female. In male frogs, evoked calling behavior was positively correlated with BrdU-labeling in the POA; however, statistical analysis showed that this behavior did not mediate socially induced cell proliferation. These results show that the social modulation of cell proliferation can occur without gonadal hormone involvement in either male or female adult anuran amphibians, and confirms that it is independent of a behavioral response in males.

Androgens modulate song effort and aggression in Neotropical singing mice

Androgens are an important class of steroid hormones involved in modulating the expression and evolution of male secondary sex characters. Vocalizations used in the context of aggression and mate attraction are among the most elaborate and diverse androgen-dependent animal displays as reflected in a rich tradition of studies on bird song and anuran calls. Male Alston's singing mice (Scotinomys teguina) commonly emit trilled songs that appear to function in male-male aggression. In this study, we experimentally manipulated androgens in singing mice to assess their role in modulating aggression and song effort. Testosterone- and DHT-treated animals retained aggressive and song attributes similarly. However, castrated mice administered empty implants showed more subordinate behavior and sang fewer songs that were shorter, lower in power, higher in frequency, and less stereotyped. The extensive effects of androgens on a suite of phenotypes highlight their role in linking gonadal status with decisions about investment in reproductive behaviors.

The neuroendocrine action potential. Winner of the 2008 Frank Beach Award in Behavioral Neuroendocrinology

Animals are remarkably well equipped to respond to changes in their environment across different time scales and levels of biological organization. Here, I introduce a novel perspective that incorporates the three main processes the nervous system uses to integrate and process information: electrophysiological, genomic, and neuroendocrine action potentials. After discussing several examples of neuroendocrine action potentials, I lay out the commonalities of these temporally organized responses and how they might be interrelated with electrophysiological activity and genomic responses. This framework provides a novel outlook on longstanding questions in behavioral neuroendocrinology and suggests exciting new avenues for further research that will integrate across disciplines and levels of biological organization.

Sexually dimorphic androgen and estrogen receptor mRNA expression in the brain of túngara frogs

Sex steroid hormones are potent regulators of behavior and they exert their effects through influences on sensory, motor, and motivational systems. To elucidate where androgens and estrogens can act to regulate sex-typical behaviors in the túngara frog (Physalaemus pustulosus), we quantified expression of the androgen receptor (AR), estrogen receptor alpha (ERalpha), and estrogen receptor beta (ERbeta) genes in the brains of male and females. To do so, we cloned túngara-specific sequences for AR, ERalpha, and ERbeta, determined their distribution in the brain, and then quantified their expression in areas that are important in sexual communication. We found that AR, ERalpha, and ERbeta were expressed in the pallium, limbic forebrain (preoptic area, hypothalamus, nucleus accumbens, amygdala, septum, striatum), parts of the thalamus, and the auditory midbrain (torus semicircularis). Males and females had a similar distribution of AR and ER expression, but expression levels differed in some brain regions. In the auditory midbrain, females had higher ERalpha and ERbeta expression than males, whereas males had higher AR expression than females. In the forebrain, females had higher AR expression than males in the ventral hypothalamus and medial pallium (homolog to hippocampus), whereas males had higher ERalpha expression in the medial pallium. In the preoptic area, striatum, and septum, males and females had similar levels of AR and ER expression. Our results suggest that sex steroid hormones have sexually dimorphic effects on auditory processing, sexual motivation, and possibly memory and, therefore, have important implications for sexual communication in this system.

Treatment with arginine vasotocin alters mating calls and decreases call attractiveness in male túngara frogs

The peptide hormone arginine vasotocin (AVT) and its mammalian homolog arginine vasopressin modulate a variety of social behaviors in vertebrates. In anurans, AVT influences the production of advertisement calls, the acoustic signals that males use to attract females and repel rival males. In this study, we investigate the effects of AVT on call characteristics in the túngara frog (Physalaemus pustulosus). Túngara frogs produce a "whine" that is important for species recognition; they may also produce a second, attractive call component, the "chuck". We used a field playback experiment to determine changes in male calling behavior following treatment with AVT. A previous study showed that AVT alters call rate and the production of chucks; in the current analysis, we focus on changes in the whine. Males produce shorter whines with higher initial frequencies following treatment with AVT. Call changes do not vary with a social stimulus. We also used female phonotaxis experiments to investigate the effects of call changes on female mate choice. Females disfavor the calls produced by males treated with exogenous AVT. We suggest that AVT influences motivation to call and the motor control of call production, but that over-stimulation of the vocal system limited the production of attractive calls in this experimental context.

Sex-specific modulation of cell proliferation by socially relevant stimuli in the adult green treefrog brain (Hyla cinerea)

Social experience plays an important role in regulating the neural, physiological and hormonal changes that accompany the expression of reproductive behavior in vertebrates. This suite of functions is sexually dimorphic, with different neural control areas preeminent in males and females. In anuran amphibians, social experience comes in the form of acoustic communication, which is central to their reproductive behavior. We sought to determine whether acoustic cues regulate cell proliferation in the brain of adult green treefrogs (Hyla cinerea). Our results show that both male and female treefrogs that heard a conspecific chorus during the breeding season exhibited increased brain cell proliferation compared to animals that heard random tones. Increased cell proliferation, as assessed by the number of 5-bromo-2'-deoxyuridine-immunoreactive (BrdU+) cells, were found near the ventricles of acoustically sensitive brain regions such as the preoptic area (POA) and the infundibular hypothalamus (IF). Sex differences emerged in the location of this socially modulated cell proliferation: increases occurred primarily in the male POA and the female IF. In addition, gonadal steroid hormones might have played a role in the social modulation of cell proliferation: by statistically control- ling for hormone level, we revealed that androgens might influence socially induced increases in BrdU+ cells in the male POA, but estrogen did not contribute to socially induced increases in the female IF. These results indicate that the reception of social cues increases cell proliferation in brain regions mediating sexual behavior and endocrine regulation, and moreover that social modulation of cell proliferation occurs in a sexually differentiated fashion.

Sexual hearing: the influence of sex hormones on acoustic communication in frogs

The majority of anuran amphibians (frogs and toads) use acoustic communication to mediate sexual behavior and reproduction. Generally, females find and select their mates using acoustic cues provided by males in the form of conspicuous advertisement calls. In these species, vocal signal production and reception are intimately tied to successful reproduction. Research with anurans has demonstrated that acoustic communication is modulated by reproductive hormones, including gonadal steroids and peptide neuromodulators. Most of these studies have focused on the ways in which hormonal systems influence vocal signal production; however, here we will concentrate on a growing body of literature that examines hormonal modulation of call reception. This literature suggests that reproductive hormones contribute to the coordination of reproductive behaviors between signaler and receiver by modulating sensitivity and spectral filtering of the anuran auditory system. It has become evident that the hormonal systems that influence reproductive behaviors are highly conserved among vertebrate taxa. Thus, studying the endocrine and neuromodulatory bases of acoustic communication in frogs and toads can lead to insights of broader applicability to hormonal modulation of vertebrate sensory physiology and behavior.

Hormones and acoustic communication in anuran amphibians

Circulating hormone levels can mediate changes in the quality of courtship signals by males and/or mate choice by females and may thus play an important role in the evolution of courtship signals. Costs associated with shifts in hormone levels of males, for example, could effectively stabilize directional selection by females on male signals. Alternatively, if hormone levels affect the selection of mates by females, then variation in hormone levels among females could contribute to the maintenance of variability in the quality of males' signals. Here, I review what is known regarding the effects of hormone levels on the quality of acoustic signals produced by males and on the choice of mates by females in anuran amphibians. Surprisingly, despite the long history of anuran amphibians as model organisms for studying acoustic communication and physiology, we know very little about how variation in circulating hormone levels contributes to variation in the vocal quality of males. Proposed relationships between androgen levels and vocal quality depicted in recent models, for example, are subject to the same criticisms raised for similar models proposed in relation to birds, namely that the evidence for graded effects of androgens on vocal performance is often weak or not rigorously tested and responses seen in one species are often not observed in other species. Although several studies offer intriguing support for graded effects of hormones on calling behavior, additional comparative studies will be required to understand these relationships. Recent studies indicate that hormones may also mediate changes in anuran females' choice of mates, suggesting that the hormone levels of females can influence the evolution of males' mating signals. No studies to date have concurrently addressed the potential complexity of hormone-behavior relationships from the perspective of sender as well as receiver, nor have any studies addressed the costs that are potentially associated with changes in circulating hormone levels in anurans (i.e., life-history tradeoffs associated with elevations in circulating androgens in males). The mechanisms involved in hormonally induced changes in signal production and selectivity also require further investigation. Anuran amphibians are, in many ways, conducive to investigating such questions.

Sex differences and androgen influences on midbrain auditory thresholds in the green treefrog, Hyla cinerea

Reproductive hormones can modulate communication-evoked behavior by acting on neural systems associated with motivation; however, recent evidence suggests that modulation occurs at the sensory processing level as well. The anuran auditory midbrain processes communication stimuli, and is sensitive to steroid hormones. Using multiunit electrophysiology, we tested whether sex and circulating testosterone influence auditory sensitivity to pure tones and to the natural vocalization in the green treefrog, Hyla cinerea. Sex did not influence audiogram best frequencies although sexes did differ in the sensitivities at those frequencies with males more sensitive in the lower frequency range. Females were more sensitive than males in response to the natural vocalization, despite showing no difference in response to pure tones at frequencies found within the advertisement call. Thresholds to frequencies outside the range of the male advertisement call were higher in females. Additionally, circulating testosterone increased neural thresholds in females in a frequency-specific manner. These results demonstrate that sex differences are limited to frequency ranges that relate to the processing of natural vocalizations, and depend on the type of stimulus. The frequency-dependent and stimulus-dependent nature of sex and testosterone influences suggests that reproductive hormones influence the filtering properties of the auditory system.

Testosterone stimulates mounting behavior and arginine vasotocin expression in the brain of both sexual and unisexual whiptail lizards

In nonmammalian vertebrates the abundance of arginine vasotocin (AVT) neurons in the brain is sexually dimorphic, a pattern that is modulated by testicular androgen. This peptide is thought to be involved in the control of male-typical mounting behaviors. The all-female desert-grasslands whiptail (Cnemidophorus uniparens) reproduces by obligate parthenogenesis and in nature no males exist, but eggs treated with aromatase inhibitor hatch into individuals (called virago C. uniparens) having testes, accessory sex structures, high circulating concentrations of androgens, and exhibiting only male-like copulatory behavior. To examine the 'sexual' dimorphism of AVT-containing neurons in these animals, we compared AVT immunoreactivity in gonadectomized control and virago C. uniparens, with that of gonadectomized male and female Cnemidophorus inornatus, a sexual species that is the maternal ancestor to the parthenogenetic species. Mounting behavior is elicited in both species and both sexes by testosterone, and it was predicted that the distribution and abundance of AVT cell bodies and fibers would reflect the propensity of males and females of the two species to display male-typical copulatory behavior. Since both this propensity and AVT abundance are controlled by androgens, we compared testosterone-implanted and control animals within each group. Testosterone treatment generally increased AVT abundance, except in lab-reared parthenoforms, in which testosterone treatment was the least effective in inducing male-like copulatory behavior.

Sex differences in the response to environmental cues regulating seasonal reproduction in birds

Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo-pituitary-gonadal axis that might be important.

Amphibians used in research and teaching

Amphibians have long been utilized in scientific research and in education. Historically, investigators have accumulated a wealth of information on the natural history and biology of amphibians, and this body of information is continually expanding as researchers describe new species and study the behaviors of these animals. Amphibians evolved as models for a variety of developmental and physiological processes, largely due to their unique ability to undergo metamorphosis. Scientists have used amphibian embryos to evaluate the effects of toxins, mutagens, and teratogens. Likewise, the animals are invaluable in research due to the ability of some species to regenerate limbs. Certain species of amphibians have short generation times and genetic constructs that make them desirable for transgenic and knockout technology, and there is a current national focus on developing these species for genetic and genomic research. This group of vertebrates is also critically important in the investigation of the inter-relationship of humans and the environment based on their sensitivity to climatic and habitat changes and environmental contamination.

Reproductive hormones modify reception of species-typical communication signals in a female anuran

In many vertebrates, the production and reception of species-typical courtship signals occurs when gonadotropin and gonadal hormone levels are elevated. These hormones may modify sensory processing in the signal receiver in a way that enhances behavioral responses to the signal. We examined this possibility in female túngara frogs (Physalaemus pustulosus) by treating them with either gonadotropin (which elevated estradiol) or saline and exposing them to either mate choruses or silence. Expression of an activity-dependent gene, egr-1, was quantified within two sub-nuclei of the auditory midbrain to investigate whether gonadotropin plus chorus exposure induced greater egr-1 induction than either of these stimuli alone. The laminar nucleus (LN), a sub-nucleus of the torus semicircularis that contains steroid receptors, exhibited elevated egr-1 induction in response to chorus exposure and gonadotropin treatment. Further analysis revealed that neither chorus exposure nor gonadotropin treatment alone elevated egr-1 expression in comparison to baseline levels whereas gonadotropin + chorus exposure did. This suggests that mate signals and hormones together produce an additive effect so that together they induce more egr-1 expression than either alone. Our previously published studies of female túngara frogs reveal that (1) gonadotropin-induced estradiol elevations also increase behavioral responses to male signals, and (2) reception of male signals elevates estradiol levels in the female. Here, we report data that reveal a novel mechanism by which males exploit female sensory processing to increase behavioral responses to their courtship signals.

Androgen receptor expression and morphology of forebrain and neuromuscular systems in male green anoles displaying individual differences in sexual behavior

Investigating individual differences in sexual performance in unmanipulated males is important for understanding natural relationships between behavior and morphology, and the mechanisms regulating them. Among male green anole lizards, some court and copulate frequently (studs) and others do not (duds). To evaluate potential factors underlying differences in the level of these behaviors, morphology and androgen receptor expression in neuromuscular courtship and copulatory structures, as well as in the preoptic area and amygdala, were compared in males displaying varying degrees of sexual function. This study revealed that individual differences in behavior among unmanipulated males, in particular the extension of a throat fan (dewlap) used during courtship, were positively correlated with the size of fibers in the associated muscle and with soma size in the amygdala. The physiological response to testosterone, as indicated by the height of cells in an androgen-sensitive portion of the kidney, was also correlated with male sexual behavior, and predicted it better than plasma androgen levels. Androgen receptor expression was not related to the display of courtship or copulation in any of the tissues examined. The present data indicate that higher levels of male courtship behavior result in (or are the result of) enhanced courtship muscle and amygdala morphology, and that androgen-sensitive tissue in studs may be more responsive to testosterone than duds. However, some mechanism(s) other than androgen receptor expression likely confer this difference in responsiveness.

Effects of season, testosterone and female exposure on c-fos expression in the preoptic area and amygdala of male green anoles

Expression of the immediate early gene, c-fos, was used to investigate changes in neuronal activity in forebrain regions involved in male sexual behavior following social, hormonal and/or seasonal manipulations in the male green anole. These factors all influence behavior, yet it is unclear how they interact to modify neuronal activity in forebrain regions, including the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY). These regions are involved in the display of sexual behaviors in male green anoles as in many other vertebrates. To determine the effects of seasonal, hormonal and social cues on these brain areas, we investigated c-fos under environmental conditions typical of the breeding or non-breeding season in adult male green anoles that were castrated and implanted with either testosterone (T) or blank (Bl) capsules. We also manipulated social cues by exposing only half of the animals in each group to females. T enhanced courtship and copulatory behaviors, but decreased c-fos expression in the AMY. A similar, although not statistically significant, pattern was observed in the POA, and the density of c-fos+ cells was negatively correlated in that region with the number of extensions of a throat fan (dewlap) used during courtship. Therefore, it appears that in the male green anole, T may diminish c-fos expression (likely in inhibitory neurons) in the POA and AMY to create a permissive environment in which the appropriate behavioral response can be displayed.

Nitric Oxide Synthase and NADPH Diaphorase Distribution in the Bullfrog (Rana catesbeiana) CNS: Pathways and Functional Implications

The gas nitric oxide (NO) is emerging as an important regulator of normal physiology and pathophysiology in the central nervous system (CNS). The distribution of cells releasing NO is poorly understood in non-mammalian vertebrates. Nitric oxide synthase immunocytochemistry (NOS ICC) was thus used to identify neuronal cells that contain the enzyme required for NO production in the amphibian brain and spinal cord. NADPH-diaphorase (NADPHd) histochemistry was also used because the presence of NADPHd serves as a reliable indicator of nitrergic cells. Both techniques revealed stained cells in all major structures and pathways in the bullfrog brain. Staining was identified in the olfactory glomeruli, pallium and subpallium of the telencephalon; epithalamus, thalamus, preoptic area, and hypothalamus of the diencephalon; pretectal area, optic tectum, torus semicircularis, and tegmentum of the mesencephalon; all layers of the cerebellum; reticular formation; nucleus of the solitary tract, octaval nuclei, and dorsal column nuclei of the medulla; and dorsal and motor fields of the spinal cord. In general, NADPHd histochemistry provided better staining quality, especially in subpallial regions, although NOS ICC tended to detect more cells in the olfactory bulb, pallium, ventromedial thalamus, and cerebellar Purkinje cell layer. NOS ICC was also more sensitive for motor neurons and consistently labeled them in the vagus nucleus and along the length of the rostral spinal cord. Thus, nitrergic cells were ubiquitously distributed throughout the bullfrog brain and likely serve an essential regulatory function.

Regulation of male traits by testosterone: implications for the evolution of vertebrate life histories

The negative co-variation of life-history traits such as fecundity and lifespan across species suggests the existence of ubiquitous trade-offs. Mechanistically, trade-offs result from the need to differentially allocate limited resources to traits like reproduction versus self-maintenance, with selection favoring the evolution of optimal allocation mechanism. Here I discuss the physiological (endocrine) mechanisms that underlie optimal allocation rules and how such rules evolve. The hormone testosterone may mediate life-history trade-offs due to its pleiotropic actions in male vertebrates. Conservation in the actions of testosterone in vertebrates has prompted the 'evolutionary constraint hypothesis,' which assumes that testosterone signaling mechanisms and male traits evolve as a unit. This hypothesis implies that the actions of testosterone are similar across sexes and species, and only the levels of circulating testosterone concentrations change during evolution. In contrast, the 'evolutionary potential hypothesis' proposes that testosterone signaling mechanisms and male traits evolve independently. In the latter scenario, the linkage between hormone and traits itself can be shaped by selection, leading to variation in trade-off functions. I will review recent case studies supporting the evolutionary potential hypothesis and suggest micro-evolutionary experiments to unravel the mechanistic basis of life-history evolution.

Arginine Vasotocin Promotes Calling Behavior and Call Changes in Male Túngara Frogs

In the túngara frog, Physalaemus pustulosus, males alter calling behavior with changes in their social environment, adding 'chucks' to their advertisement calls in response to the calls of conspecific males. Other studies demonstrate that adding chucks increases the attractiveness of calls to females but also increases the risk of bat predation. In the current study, subcutaneous injections of the neuropeptide hormone arginine vasotocin (AVT) significantly increased chuck production in male túngara frogs. The effects of AVT on chuck production did not depend on the presence of playback stimuli, suggesting that AVT increased either the males' general motivation to produce chucks or their responsiveness to the calls of distant males. Injections of AVT also increased the probability that males would call and decreased the latency to call after injection, supporting the hypothesis that AVT influences motivation to call. Finally, AVT inhibited a drop in call rate after the termination of a playback stimulus and increased call rate at a lower dose of AVT. The effects of AVT on chucks and call rate appear to be independent of each other, as there was no correlation between change in chuck production and change in call rate in individual males. We conclude that AVT may play an important role in socially-mediated call changes that result from competition for mates. The behavioral changes induced by AVT might increase a male's attractiveness to females, and also may be consistent with an aggressive response to another túngara frog male.

Direct action of gonadotropin in brain integrates behavioral and reproductive functions

Essential roles for gonadotropins in gonadal development and reproduction are well established. Over the past decade, however, the expression of luteinizing hormone receptor (LHR) has also been reported in the brain of various mammals and birds. Although suggestive, it has not yet been determined whether this expression pattern supports a novel function for gonadotropins. Here, we demonstrate a CNS-mediated role of gonadotropins in a reproductive behavior: the courtship songs of the South African clawed frog, Xenopus laevis. Male advertisement calling in this species depends on a nongonadal action of gonadotropin. To determine whether this effect is due to action on the CNS, we administered gonadotropin intracerebroventricularly (ICV) or systemically to intact or castrated males with or without concomitant androgen replacement. In intact and androgen-replaced gonadectomized males, gonadotropin significantly increased calling within 1 h after ICV injection. The effective dosage via ICV injections was less than one hundredth of the effective systemic dose. In situ hybridization with a cloned fragment of Xenopus LHR revealed strong expression in ventral forebrain areas important for vocal control. Further, gonadotropin treatment of brain in vitro up-regulates immunoreactivity for the LHR downstream target, egr-1, specifically in these vocal forebrain areas. Up-regulation occurs even when synaptic transmission is suppressed by incubation in Ca2+ free/high magnesium saline. These results demonstrate a neural role for gonadotropin in the control of calling behavior, potentially mediated via LHRs in forebrain vocal nuclei. Gonadotropin may play a novel integrative role in modulating both reproductive physiology and behavior.

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

Interactions among reproductive season, testosterone (T) and female presence were investigated on the structure and function of forebrain and neuromuscular systems controlling courtship and copulation in the green anole lizard. Under breeding (BS) or non-breeding (NBS) environmental conditions, male green anoles were implanted with either T or blank capsules and exposed to one of three female stimulus conditions: physical, visual or no female contact. T and at least visual exposure to females increased courtship displays (extension of a throat fan, or dewlap), and these effects were greater during the BS than NBS. T also facilitated copulation, and did so to a greater extent in the BS. The hormone increased soma size in the preoptic area (POA) and amygdala (AMY), and in the AMY the effects were greater in the BS than NBS. Cross-sectional areas of copulatory organs and associated muscle fibers were enhanced by T, and more so in the BS than NBS. However, no effects on morphology of dewlap motoneurons or muscles or copulatory motoneurons were detected. Thus, (1) changes in behavior and neural and/or muscular morphology are not always parallel and (2) differences in responsiveness to T exist across seasons and among tissues.

Use of ethanol for preserving steroid and indoleamine hormones in bird plasma

Endocrinological research on wild animals inhabiting remote areas has been hampered by the need to store plasma samples at subzero temperatures. In an attempt to remedy this logistical issue, we here investigate the use of ethanol as an alternative to freezing for the preservation of steroid and indoleamine hormones in avian plasma. Known quantities of the steroids 5alpha-dihydrotestosterone (DHT), testosterone, 17beta-estradiol, corticosterone, and the indoleamine melatonin were added to a stripped pool of chicken plasma. Samples were either immediately frozen at -40 degrees C, or treated with pure ethanol. Ethanol-treated samples were either immediately frozen, or-to simulate storage conditions at various field locations-left sitting at room temperature for one to two months, or incubated at 36 degrees C for one month before all treatment groups were frozen at -40 degrees C. All samples were then analyzed by radioimmunoassay. For DHT and estradiol there were no differences among treatment groups suggesting that ethanol-treatment is as effective as immediate freezing in preserving plasma steroid concentrations. For testosterone, corticosterone and melatonin ethanol-treated samples differed significantly from immediately frozen samples suggesting that caution is needed when comparing absolute concentrations of hormones between samples preserved in different ways. However, differences among ethanol-treated samples in general were small, demonstrating the feasibility of this preservation method in the field at remote locations.

Social regulation of plasma estradiol concentration in a female anuran

The behavior of an individual within a social aggregation profoundly influences behavior and physiology of other animals within the aggregation in such a way that these social interactions can enhance reproductive success, survival and fitness. This phenomenon is particularly important during the breeding season when males and female must synchronize their reproductive efforts. We examined whether exposure to conspecific social cues can elevate sex steroid levels, specifically estradiol and androgens, in female túngara frogs (Physalaemus pustulosus). We compared plasma estradiol and androgen concentrations in wild-caught females before and after exposure to either natural mate choruses or random tones. After exposure to mate choruses for 10 consecutive nights, estradiol concentrations were significantly elevated whereas there was no significant elevation in estradiol concentrations in the group of females exposed to random tones for 10 nights. Plasma androgen concentrations were not significantly changed after exposure to either natural mate choruses or random tones for 10 consecutive nights. Social modulation of estradiol concentrations may be important in maintaining a female's reproductive state while males are chorusing. To our knowledge, this is the first study to demonstrate social regulation of estradiol concentration in female anurans.