Tales of testosterone: Advancing our understanding of environmental endocrinology through studies of neotropical birds.

Studies of birds have greatly advanced our understanding of how testosterone modulates complex phenotypes, specifically its role in mediating male reproductive and associated behaviors. Yet most of the foundational studies have been limited to northern latitude breeding species despite the fact that they represent only a small fraction of worldwide avian diversity. In contrast, phylogenetic, life-history, and mating system diversity all reach their apex in neotropical avifauna and yet these birds, along with more southern latitude species, remain very poorly understood from an endocrine perspective. Despite the relatively limited previous work on taxa breeding in Central and South America, empirical findings have had a disproportionately large impact on our understanding of testosterone's role in everything from geographic variation to behavioral roles and neuroplasticity. Here, we synthesize how studies of neotropical breeding avifauna have advanced our understanding of how testosterone's actions can and are associated with the broad patterns of phenotypic diversity that we see in birds. In addition, we outline how these studies can be used individually or in a comparative context to address fundamental questions about the environmental endocrinology of testosterone and to understand the diversity of roles that testosterone plays in mediating behavioral variation, reproductive strategies, and associated life-history trade-offs.

Genetic population structure in an equatorial sparrow: roles for culture and geography.

Female preference for local cultural traits has been proposed as a barrier to breeding among animal populations. As such, several studies have found correlations between male bird song dialects and population genetics over relatively large distances. To investigate whether female choice for local dialects could act as a barrier to breeding between nearby and contiguous populations, we tested whether variation in male song dialects explains genetic structure among eight populations of rufous-collared sparrows (Zonotrichia capensis) in Ecuador. Our study sites lay along a transect, and adjacent study sites were separated by approximately 25 km, an order of magnitude less than previously examined for this and most other species. This transect crossed an Andean ridge and through the Quijos River Valley, both of which may be barriers to gene flow. Using a variance partitioning approach, we show that song dialect is important in explaining population genetics, independent of the geographic variables: distance, the river valley and the Andean Ridge. This result is consistent with the hypothesis that song acts as a barrier to breeding among populations in close proximity. In addition, songs of contiguous populations differed by the same degree or more than between two populations previously shown to exhibit female preference for local dialect, suggesting that birds from these populations would also breed preferentially with locals. As expected, all geographic variables (distance, the river valley and the Andean Ridge) also predicted population genetic structure. Our results have important implications for the understanding whether, and at what spatial scale, culture can affect population divergence.

Weathering the storm: parental effort and experimental manipulation of stress hormones predict brood survival.

BACKGROUND: Unpredictable and inclement weather is increasing in strength and frequency, challenging organisms to respond adaptively. One way in which animals respond to environmental challenges is through the secretion of glucocorticoid stress hormones. These hormones mobilize energy stores and suppress non-essential physiological and behavioral processes until the challenge passes. To investigate the effects of glucocorticoids on reproductive decisions, we experimentally increased corticosterone levels (the primary glucocorticoid in birds) in free-living female tree swallows, Tachycineta bicolor, during the chick-rearing stage. Due to an unprecedented cold and wet breeding season, 90 % of the nests in our study population failed, which created a unique opportunity to test how challenging environmental conditions interact with the physiological mechanisms underlying life-history trade-offs. RESULTS: We found that exogenous corticosterone influenced the regulation of parental decisions in a context-dependent manner. Control and corticosterone-treated females had similar brood failure rates under unfavorable conditions (cold and rainy weather), but corticosterone treatment hastened brood mortality under more favorable conditions. Higher female nest provisioning rates prior to implantation were associated with increased probability of brood survival for treatment and control groups. However, higher pre-treatment male provisioning rates were associated with increased survival probability in the control group, but not the corticosterone-treated group. CONCLUSIONS: These findings reveal complex interactions between weather, female physiological state, and partner parental investment. Our results also demonstrate a causal relationship between corticosterone concentrations and individual reproductive behaviors, and point to a mechanism for why naturally disturbed populations, which experience multiple stressors, could be more susceptible and unable to respond adaptively to changing environmental conditions.

Slight differences in incubation temperature affect early growth and stress endocrinology of wood duck (Aix sponsa) ducklings.

Early developmental experiences, such as incubation conditions, can have important consequences for post-hatching fitness in birds. Although the effects of incubation temperature on phenotype of avian hatchlings are poorly understood, recent research suggests that subtle changes in incubation conditions can influence hatchling characteristics, including body size and condition. We designed an experiment to explore the effects of incubation temperature on hatching success, survival to 9 days post hatch, growth and the hypothalamo-pituitary-adrenal (HPA) axis in wood ducks (Aix sponsa). Wood duck eggs were collected from nest boxes and experimentally incubated at three temperatures (35.0, 35.9 and 37.0 degrees C), each falling within the range of temperatures of naturally incubated wood duck nests. Survival and growth were monitored in ducklings fed ad libitum for 9 days post hatch. In addition, baseline and stress-induced plasma corticosterone concentrations were measured in 2 and 9 day old ducklings. Hatching success and survival to 9 days was greatest in ducks incubated at the intermediate temperature. Ducklings incubated at 35.9 degrees C and 37.0 degrees C had 43% higher growth rates than ducklings incubated at 35.0 degrees C. In addition, ducklings incubated at 35.0 degrees C had higher baseline (17-50%) and stress-induced (32-84%) corticosterone concentrations than ducklings incubated at 35.9 degrees C and 37.0 degrees C at 2 and 9 days post hatch. We also found a significant negative correlation between body size and plasma corticosterone concentrations (baseline and stress-induced) in 9 day old ducklings. To our knowledge, this is the first study to demonstrate that thermal conditions experienced during embryonic development can influence the HPA axis of young birds. Our results illustrate that subtle changes (<1.0 degrees C) in the incubation environment can have important consequences for physiological traits important to fitness.

The Cloacal Microbiome Changes with the Breeding Season in a Wild Bird.

The symbiotic microbial communities, or "microbiomes," that reside on animals are dynamic, and can be affected by the behavior and physiology of the host. These communities provide many critical beneficial functions for their hosts, but they can also include potential pathogens. In birds, bacteria residing in the cloaca form a complex community, including both gut and sexually-transmitted bacteria. Transmission of cloacal bacteria among individuals is likely during the breeding season, when there is direct cloacal contact between individuals. In addition, the major energetic investment in reproduction can draw resources away from immune responses that might otherwise prevent the successful establishment of microbes. We assessed dynamic variation in the cloacal microbiome of free-living rufous-collared sparrows (Zonotrichia capensis) through sequential breeding and non-breeding seasons. We found that the cloacal bacterial communities differed between the sexes when they were in breeding condition. Further, in males, but not in females, the bacterial community became more diverse with the onset of reproduction, and then decreased in diversity as males transitioned to non-breeding condition. Individuals sampled across sequential breeding seasons did not accumulate more bacterial taxa over seasons, but bacterial community composition did change. Our results suggest that the cloacal microbiome in birds is dynamic and, especially in males, responsive to breeding condition.

Glucocorticoids and "Stress" Are Not Synonymous.

Reference to glucocorticoids as "stress hormones" has been growing in prevalence in the literature, including in comparative and environmental endocrinology. Although glucocorticoids are elevated in response to a variety of stressors in vertebrate animals, the primary functions of glucocorticoids are not responding to stressors and they are only one component of complex suite of physiological and behavioral responses to stressors. Thus, the use of the short-hand phrase "stress hormone" can be misleading. Further, simply measuring glucocorticoids is not equivalent to measuring a stress response, nor is manipulating glucocorticoids equivalent to exposing an animal to a stressor. In this commentary we highlight the problems with using functional names for hormones, and of treating cortisol or corticosterone as synonymous with stress. We provide recommendations to add clarity to the presentation of research on this topic, and to avoid conflation of glucocorticoids with stressors and the stress response in the design of experiments.

Los Glucocorticoides y el "Estrés" no Son Sinónimos (Glucocorticoids and "Stress" Are Not Synonymous) La referencia a los glucocorticoides como "hormonas del estrés" ha aumentado en prevalencia en la literatura, incluso en endocrinología comparativa y ecológica. Aunque los glucocorticoides están elevados en respuesta a una variedad de factores de estrés en animales vertebrados, las funciones primarias de los glucocorticoides no responden a los factores de estrés y son solo un componente de un conjunto complejo de respuestas fisiológicas y de comportamiento a los factores de estrés. Por lo tanto, el uso de la frase abreviada "hormona del estrés" puede ser engañoso. Además, simplemente medir glucocorticoides no es equivalente a medir una respuesta al estrés, ni manipular glucocorticoides equivalente a exponer a un animal a un factor estresante. En este comentario destacamos los problemas con el uso de nombres funcionales para las hormonas y con el tratamiento del cortisol o la corticosterona como sinónimo de estrés. Brindamos recomendaciones para agregar claridad a la presentación de investigaciones sobre este tema y para evitar la combinación de glucocorticoides con factores estresantes y la respuesta al estrés en el diseño de los experimentos. Translated to Spanish by J. Heras (herasj01@gmail.com).

Glucocorticóides e "Stress" Não São Sinônimos (Glucocorticoids and "Stress" Are Not Synonymous) A referência aos glicocorticóides como "hormônios do estresse" vem se tornando prevalente em literatura, inclusive na endocrinologia comparada e ecológica. Embora os glicocorticóides sejam elevados em resposta a uma variedade de estressores em vertebrados, as funções primárias de glicocorticóides são a de não responder aos causadores do stress e eles são apenas um componente dentro de um complexo conjunto de respostas fisiológicas e comportamentais aos estressores. Assim, o uso da frase curta "hormônio do estresse" pode ser errôneo. Além disso, simplesmente medir glicocorticóides não é equivalente a medir uma resposta ao estresse, nem a manipulação de glicocorticoides é equivalente a expor um animal a um estressor. Neste comentário, destacamos os problemas com o uso de nomes funcionais para hormônios e o tratamento de cortisol ou corticosterona como sinônimos de estresse. Nós provemos recomendações para adicionar clareza à apresentação de pesquisas deste tópico e para evitar a o tratamento direto de glicocorticoides como estressores e a resposta ao estresse no planejamento de experimentos. Translated to Portuguese by Diego Vaz (dbistonvaz@vims.edu).

A Mathematical Modeling Approach to the Cort-Fitness Hypothesis.

The Cort-Fitness Hypothesis has generated much interest from investigators integrating field endocrinology with evolutionary biology, ecology, and conservation. The hypothesis was developed to test the assumption that if glucocorticoid levels increase with environmental challenges and fitness decreases with environmental challenges, then there should be a negative relationship between baseline glucocorticoid levels and fitness. Indeed, studies across diverse taxa have found that the relationship between baseline glucocorticoid levels and fitness are not consistent: some studies show a positive relationship, others negative, and some show no correlation. Hence, a deeper understanding of the mechanisms underlying the relationship between baseline glucocorticoid levels, environmental challenges, and fitness is needed. We propose a mathematical model representing the links between baseline glucocorticoid levels, environmental challenges, and fitness. Our model describes how variation in the predictability and intensity of environmental challenges, reproductive strategies, and fitness metrics can all contribute to the variability observed in empirical tests of the Cort-Fitness Hypothesis. We provide qualitative results showing that much of the inconsistency in previous studies can be explained and we discuss how the model can be used to inform future Cort-Fitness studies.

Un enfoque de modelado matemático para la hipótesis de Cort-Fitness (A Mathematical Modeling Approach to The Cort-Fitness Hypothesis) La hipótesis de Cort-Fitness ha generado mucho interés por parte de los investigadores que integran la endocrinología de campo con la biología evolutiva, la ecología y la conservación. La hipótesis se desarrolló para probar el supuesto de que si los niveles de glucocorticoides aumentan con los desafíos ambientales y la aptitud física disminuye con los ambientales, entonces debería existir una relación negativa entre los niveles de glucocorticoides de referencia y la aptitud física. De hecho, los estudios en diversos taxones han encontrado que la relación entre los niveles de glucocorticoides de referencia y la aptitud no son consistentes: algunos estudios muestran una relación positiva, otros negativos y otros no muestran correlación. Por lo tanto, se necesita una comprensión más profunda de los mecanismos subyacentes en la relación entre los niveles de glucocorticoides de referencia, los desafíos ambientales y la aptitud. Proponemos un modelo matemático que representa los vínculos entre los niveles basales de glucocorticoides, los desafíos ambientales y la aptitud. Nuestro modelo describe cómo la variación en la previsibilidad e intensidad de los desafíos ambientales, las estrategias reproductivas y las métricas de aptitud pueden contribuir a la variabilidad observada en las pruebas empíricas de la hipótesis de Cort-Fitness. Proporcionamos resultados cualitativos que muestran que gran parte de la variación en estudios anteriores se puede explicar y discutimos cómo se puede usar el modelo para informar futuros estudios de Cort-Fitness. Translated to Spanish by J Heras (herasj01@gmail.com).

Uma Abordagem Matématica de Modelagem para a Hipótese Cort-Fitness (A Mathematical Modeling Approach to The Cort-Fitness Hypothesis) A hipótese de cort-fitness gerou muito interesse de pesquisadores ao integrar endocrinologia de campo com biologia evolutiva, ecologia e conservação. A hipótese foi desenvolvida para testar a suposição de que, se os níveis de glicocorticóides aumentam com os desafios ambientais e o fitness diminui, então deve haver uma relação negativa entre os níveis basais de glicocorticóides e o fitness. De fato, estudos com diversos táxons descobriram que a relação entre os níveis basais de glicocorticoides e o fitness não são consistentes: alguns mostram uma relação positiva, outros uma negativa e há ainda os que não mostram correlação alguma. Assim, é necessária uma compreensão mais profunda dos mecanismos subjacentes à relação entre os níveis basais de glicocorticóides, os desafios ambientais e o fitness. Propomos um modelo matemático que representa as ligações entre os níveis basais de glicocorticoides, os desafios ambientais e o fitness. Nosso modelo descreve como a variação na previsibilidade e intensidade dos desafios ambientais, estratégias reprodutivas e métricas de fitness podem contribuir para a variabilidade observada nos testes empíricos da hipótese cort-fitness. Nós fornecemos resultados qualitativos mostrando que grande parte da variação em estudos anteriores pode ser explicada e discutimos como o modelo pode ser usado para informar futuros estudos sobre o cort-fitness. Translated to Portuguese by G Sobral (gabisobral@).

Do Hormones, Telomere Lengths, and Oxidative Stress form an Integrated Phenotype? A Case Study in Free-Living Tree Swallows.

Synopsis All organisms must anticipate and balance energetic demands and available resources in order to maximize fitness. As hormones coordinate many interactions between an organism's internal condition and the external environment, they may be key in mediating the allocation of resources to meet these demands. However, given that individuals differ considerably in how they react to changes in energetic demand, we asked whether variations in endocrine traits also correspond with life history variation. We tested whether natural variation in glucocorticoid hormone levels, oxidative stress measurements, and condition related to reproductive effort in a free-living songbird, the tree swallow, Tachycineta bicolor We then tested whether any of these traits predicted the probability of a particular individual's return to the local population in the following two years, an indicator of survival in this philopatric species. We found that males and females with longer telomeres had lighter nestlings. Moreover, individuals with lower plasma antioxidant capacity and higher reactive oxygen metabolites (i.e., greater oxidative stress) were less likely to return to the population. However, none of these traits were related to glucocorticoid levels. Our findings suggest a trade-off between reproduction and survival, with individuals with shorter telomeres having heavier nestlings but potentially paying a cost in terms of higher oxidative stress and lower survival. Interestingly, the evidence of this trade-off was unrelated to natural variation in glucocorticoids.

Bumpus in the snake den: effects of sex, size, and body condition on mortality of red-sided garter snakes.

Huge breeding aggregations of red-sided garter snakes (Thamnophis sirtalis parietalis) at overwintering dens in Manitoba provide a unique opportunity to identify sources of mortality and to clarify factors that influence a snake's vulnerability to these factors. Comparisons of sexes, body sizes, and body condition of more than 1000 dead snakes versus live animals sampled at the same time reveal significant biases. Three primary sources of mortality were identified. Predation by crows, Corvus brachyrhynchos (590 snakes killed), was focussed mostly on small snakes of both sexes. Crows generally removed the snake's liver and left the carcass, but very small snakes were sometimes brought back to the nest. Suffocation beneath massive piles of other snakes within the den (301 dead animals) involved mostly small males and (to a lesser extent) large females; snakes in poor body condition were particularly vulnerable. Many emaciated snakes (n = 142, mostly females) also died without overt injuries, probably due to depleted energy reserves. These biases in vulnerability are readily interpretable from information on behavioral ecology of the snakes. For example, sex biases in mortality reflect differences in postemergence behavior and locomotor capacity, the greater attractiveness of larger females to males, and the high energy costs of reproduction for females.

Gonadotropin-inhibitory hormone in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii): cDNA identification, transcript localization and functional effects in laboratory and field experiments.

The neuropeptide control of gonadotropin secretion is primarily through the stimulatory action of the hypothalamic decapeptide, GnRH. We recently identified a novel hypothalamic dodecapeptide with a C-terminal LeuPro-Leu-Arg-Phe-NH2 sequence in the domestic bird, Japanese quail (Coturnix japonica). This novel peptide inhibited gonadotropin release in vitro from the quail anterior pituitary; thus it was named gonadotropin-inhibitory hormone (GnIH). GnIH may be an important factor regulating reproductive activity not only in domesticated birds but also in wild, seasonally breeding birds. Thus, we tested synthetic quail GnIH in seasonally breeding wild bird species. In an in vivo experiment, chicken gonadotropin-releasing hormone-I (cGnRH-I) alone or a cGnRH-I/quail GnIH cocktail was injected i.v. into non-breeding song sparrows (Melospiza melodia). Quail GnIH rapidly (within 2 min) attenuated the GnRH-induced rise in plasma LH. Furthermore, we tested the effects of quail GnIH in castrated, photostimulated Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), using quail GnIH or saline for injection. Again, quail GnIH rapidly reduced plasma LH (within 3 min) compared with controls. To characterize fully the action of GnIH in wild birds, the identification of their endogenous GnIH is essential. Therefore, in the present study a cDNA encoding GnIH in the brain of Gambel's white-crowned sparrow was cloned by a combination of 3' and 5' rapid amplification of cDNA ends and compared with the quail GnIH cDNA previously identified. The deduced sparrow GnIH precursor consisted of 173 amino acid residues, encoding one sparrow GnIH and two sparrow GnIH-related peptides (sparrow GnIH-RP-1 and GnIH-RP-2) that included Leu-Pro-Xaa-Arg-Phe-NH2 (Xaa=Leu or Gln) at their C-termini. All these peptide sequences were flanked by a glycine C-terminal amidation signal and a single basic amino acid on each end as an endoproteolytic site. Although the homology of sparrow and quail GnIH precursors was approximately 66%, the C-terminal structures of GnIH, GnIH-RP-1 and GnIH-RP-2 were all identical in two species. In situ hybridization revealed the cellular localization of sparrow GnIH mRNA in the paraventricular nucleus (PVN) of the hypothalamus. Immunohistochemical analysis also showed that sparrow GnIH-like immunoreactive cell bodies and terminals were localized in the PVN and median eminence respectively. Thus, only the sparrow PVN expresses GnIH, which appears to be a hypothalamic inhibitory factor for LH release, as evident from our field injections of GnIH into free-living breeding white-crowned sparrows. Sparrow GnIH rapidly (within 2 min) reduced plasma LH when injected into free-living Gambel's white-crowned sparrows on their breeding grounds in northern Alaska. Taken together, our results indicate that, despite amino acid sequence differences, quail GnIH and sparrow GnIH have similar inhibitory effects on the reproductive axis in wild sparrow species. Thus, GnIH appears to be a modulator of gonadotropin release.

Behavioral and hormonal responses to corticosterone in the male red-sided garter snake, Thamnophis sirtalis parietalis.

Stress and glucocorticoids are generally thought to suppress reproductive function at multiple levels. We tested the hypotheses that exogenous corticosterone would suppress sexual behavior in a dose-dependent manner, as well as drive a decrease in plasma testosterone levels in the male red-sided garter snake, Thamnophis sirtalis parietalis. We examined this by challenging individual males with intraperitoneal injections of exogenous corticosterone, and subsequently exposing them to sexually attractive females or taking a blood sample. Previous work has demonstrated a hormonal but no behavioral response to stress in this species. In this study, increasing concentrations of exogenous corticosterone rapidly suppressed mating behavior in a threshold manner. However, exogenous corticosterone had no effect on plasma levels of testosterone. Thus, these data suggest that the mechanism is in place for corticosterone to suppress mating behavior in this species and that these effects do not occur because of an indirect effect on plasma levels of testosterone but rather are the direct effect of the hormone itself. In addition, the negative relationship observed previously between plasma levels of corticosterone and testosterone in this species was probably not the direct result of corticosterone acting on the hypothalamic-pituitary-gonadal (HPG) axis. Rather, our results seem to indicate that the negative associations between the hypothalamic-pituitary-adrenal axis (HPA) and the HPG axis occur at other levels of these neuroendocrine pathways.

Sympathetic mediation of stress and aggressive competition: plasma catecholamines in free-living male tree lizards.

The sympathetic nervous system and adrenal catecholaminergic tissue act to prepare an animal for "fight or flight" by release of catecholamines into synapses and plasma. However, few studies have measured plasma catecholamines in nonmammalian vertebrates and none have measured them in free-living animals. We report plasma levels of norepinephrine (NE), epinephrine (EPI) and dopamine (DA) in free-living tree lizards (Urosaurus ornatus) bled in the field: 1) immediately after capture, 2) after 10 minutes of restraint, 3) immediately after a staged territorial encounter and 4) four minutes after the end of a staged territorial encounter. Time to capture and time to bleed after capture were also recorded for each sample. Time to capture had little effect on plasma catecholamines suggesting that plasma catecholamines do not rise during the brief pursuit by the investigator necessary to capture the lizards in the field. In contrast, plasma NE and EPI increased during blood collection. However, this response was very consistent allowing comparisons to be made using analysis of covariance to control for time to bleed. Results indicate that restraint stress caused a large increase in plasma levels of NE, E and DA. Plasma levels of NE and E, but not plasma DA, were also significantly elevated both immediately and 4 minutes after aggressive encounters, although less so than following restraint stress. These studies show that stress and aggression activate the sympatho-adrenal system. This activation could be involved in behavioral changes during aggression, meeting metabolic demands of the encounter, or both.

Relationships between annual cycles of testosterone, corticosterone, and body condition in male red-spotted garter snakes, Thamnophis sirtalis concinnus.

Over a 2-yr period, we investigated the annual cycles of plasma testosterone and corticosterone and the relationships between these hormones and body condition in a wild population of male red-spotted garter snakes, Thamnophis sirtalis concinnus. In the 10 mo that were sampled, a peak in testosterone was observed in late summer during gametogenesis and declining through the spring breeding period. Corticosterone and testosterone cycles were positively correlated, in contrast to many vertebrates, suggesting the lack of a direct negative interaction between the two hormones. Body condition, defined as the residual of the regression of mass on snout-vent length, also cycled annually, with individuals being more robust during the summer than during the spring or fall. Individuals with a positive body condition had significantly lower plasma levels of corticosterone than did individuals with a negative body condition, supporting the energetic role of glucocorticoids. There was no relationship between body condition and testosterone. This study suggests that annual cycles of testosterone, corticosterone, and body condition can be associated with one another, and considering all three simultaneously is necessary to understand their control and function.

Endocrine differentiation of fetal ovaries and testes of the spotted hyena (Crocuta crocuta): timing of androgen-independent versus androgen-driven genital development.

Female spotted hyenas (Crocuta crocuta) have an erectile peniform clitoris and a pseudoscrotum but no external vagina, all established by day 35 of a 110-day gestation. Recent studies indicate that these events are androgen-independent, although androgen secretion by fetal ovaries and testis was hypothesized previously to induce phallic development in both sexes. We present the first data relating to the capacity of the ovaries and testes of the spotted hyena to synthesize androgens at different stages of fetal life. Specifically, spotted hyena fetal gonads were examined by immunohistochemistry at GD 30, 45, 48, 65, and 95 for androgen-synthesizing enzymes, as related to the morphological development. Enzymes included 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17), cytochrome b5, 3beta-hydroxysteroid dehydrogenase (3betaHSD), and cholesterol side-chain cleavage cytochrome P450 (P450scc). Anti-Müllerian-hormone (AMH) expression was also examined. AMH was strongly expressed in fetal Sertoli cells from GD 30 and after. P450c17 expression was detected in Leydig cells of developing testes and surprisingly in Müllerian duct epithelium. Fetal ovaries began to organize and differentiate by GD 45, and medullary cells expressed P450c17, cytochrome b5, 3betaHSD, and P450scc. The findings support the hypothesis that external genital morphology is probably androgen-independent initially, but that fetal testicular androgens modify the secondary, male-specific phallic form and accessory organs. Fetal ovaries appear to develop substantial androgen-synthesizing capacity but not until phallic differentiation is complete, i.e. after GD 45 based on circulating androstenedione concentrations. During late gestation, fetal ovaries and testes synthesize androgens, possibly organizing the neural substrates of aggressive behaviors observed at birth in spotted hyenas. These data provide an endocrine rationale for sexual dimorphisms in phallic structure and reveal a potential source of androgenic support for neonatal aggression in female and male C. crocuta.

Environmental and seasonal adaptations of the adrenocortical and gonadal responses to capture stress in two populations of the male garter snake, Thamnophis sirtalis.

Stress and reproduction are generally thought to work in opposition to one another. This is often manifested as reciprocal relationships between glucocorticoid stress hormones and sex steroid hormones. However, seasonal differences in how animals respond to stressors have been described in extreme environments. We tested the hypothesis that garter snakes, Thamnophis sirtalis, with limited reproductive opportunities will suppress their hormonal stress response during the breeding season relative to conspecifics with an extended breeding season. The red-sided garter snake, T.s. parietalis, of Manitoba, Canada, has a brief breeding season during which males displayed no change in either plasma levels of testosterone or corticosterone, which were both elevated above basal levels, in response to capture stress. During the summer, capture stress resulted in increased plasma corticosterone and decreased testosterone. During the fall, when mating can also occur, males exhibited a significant decrease in testosterone but no increase in corticosterone in response to capture stress. The red-spotted garter snake, T.s. concinnus, of western Oregon, has an extended breeding season during which males displayed a stress response of increased plasma corticosterone and decreased testosterone levels. The corticosterone response to capture stress was similar during the spring, summer, and fall. In contrast, the testosterone response was suppressed during the summer and fall when gametogenesis was occurring. These data suggest that male garter snakes, in both populations, seasonally adapt their stress response but for different reasons and by potentially different mechanisms. J. Exp. Zool. 289:99-108, 2001.

Differential mechanisms for regulation of the stress response across latitudinal gradients.

We examined plasticity of the stress response among three populations of the white-crowned sparrow (Zonotrichia leucophrys). These populations breed at different elevations and latitudes and thus have breeding seasons that differ markedly in length. We hypothesize that in populations where birds raise only one or rarely two broods in a season, the fitness costs of abandoning a nest are substantially larger than in closely related populations that raise up to three broods per season. Thus individuals with short breeding seasons should be less responsive to stressors and therefore less likely to abandon their young. In our study, baseline and handling-induced corticosterone levels were similar among populations, but corticosteroid-binding globulins differed, leading to a direct relationship between stress-induced free corticosteroid levels and length of breeding season. There were also population-specific differences in intracellular low-affinity (glucocorticoid-like) receptors in both liver and brain tissue. Although investigations of population-based differences in glucocorticoid secretion are common, this is the first study to demonstrate population-level differences in binding globulins. These differences could lead to dramatically different physiological and behavioral responses to stress.