Dynamic endocrine responses to stress: evidence for energetic constraints and status dependence in breeding male green turtles

During reproduction, male vertebrates may exhibit a continuum of interactions between sex and adrenal steroids during stressful events, the outcome of which may be important in either reducing or promoting male reproductive success. We studied adult male green turtles (Chelonia mydas) to examine if they altered plasma corticosterone (CORT) and androgen levels in response to a standardized capture/restraint stressor as potential mechanisms to maintain reproductive activity during stressful events. At the population level, we found that migrant breeding males had a significantly smaller CORT response to the capture/restraint stressor compared to nonbreeding males and that this decreased response coincided with the generally poorer body condition of migrant breeders. In contrast, plasma androgen levels decreased significantly in response to the capture/restraint stressor in migrant breeding males, but not in nonbreeding and pre-migrant breeding males. For individual migrant breeding males, the magnitude of their CORT and androgen responses to the capture/restraint stressor was highly correlated with their body condition and body length, respectively. Our results demonstrate that male green turtles exhibit complex interactions in their endocrine responses to a capture/restraint stressor and that variation in these interactions is associated with differences in males' reproductive, energetic, and physical state. We hypothesize that interplay between physical status and plasma hormone responses to stressors could have important consequences for male green turtle reproduction.     

Ecological factors underlying the adrenocortical response to capture stress in arctic-breeding shorebirds.

Birds breeding in the Arctic must carefully balance survival and reproduction because of the often extreme environmental conditions and the very brief breeding season. Acute elevation of plasma corticosterone is one mechanism that birds apparently use to alter the balance in favor of survival at the expense of reproduction when faced with stressors such as storms, predators, or low food availability. To examine this relationship, we applied a standardized stressor, capture and handling, to four species of shorebirds (Scolopacidae) during their breeding season in the Arctic and measured the increase in corticosterone between 3 min and 30 min after capture (hereafter called the stress response). We tested two hypotheses in an effort to explain the individual variation observed in the stress response. The first hypothesis states that individuals most responsible for parental care have a lower stress response than individuals less responsible for parental care. In species with uniparental care (Pectoral Sandpiper Calidris melanotos, Red Phalarope Phalaropus fulicaria), the caregivers had a lower stress response than the opposite sex, although in the latter species the difference was not significant (P = 0.016 and P = 0.102, respectively). In a species with equal biparental care (Semipalmated Sandpiper C. pusilla), the stress response did not differ between the sexes (P = 0.99, Barrow population). In a species with unequal biparental care (Western Sandpiper C. mauri), the more parental sex (males) had a much lower stress response than the less parental sex (P = 0.002). The second hypothesis states that individuals breeding in the high Arctic have a lower stress response than individuals breeding in the low Arctic. The stress response in a low-Arctic population of Semipalmated Sandpipers (Nome) was higher than that in a high-Arctic population (Barrow, P < 0.05). Individuals at an additional high-Arctic location (Prudhoe Bay) exhibited stress responses most similar to those of the Barrow population (P > 0.05). Taken together, these results provide evidence of a mechanism that birds may use to breed in environments with a brief breeding season and under conditions that might be perceived as stressful, if not for their reduced sensitivity to potential stressors.     

Plasma Corticosterone Concentrations Associated with Acute Captivity Stress in Wild Loggerhead Sea Turtles (Caretta caretta)

Plasma corticosterone concentrations were measured in wild loggerhead sea turtles (Caretta caretta) in response to acute captivity (capture, serial bleeding, and restraint up to 6 hr). In general, concentrations of corticosterone dramatically increased 1 hr after capture, peaked at 3 hr, and decreased by 6 hr. Initial corticosterone concentrations were significantly lower in animals captured by tangle net than in those captured by trawl and were thought to more closely represent baseline levels. Significant effects of season and size class on corticosterone concentrations were found for turtles captured by trawl. Corticosterone concentrations of small turtles captured in summer were higher than those of large turtles captured in the same season and of all turtles captured during winter. In winter, corticosterone concentrations for small turtles were higher than those for large turtles at 3 hr after capture. Large turtles captured during winter experienced the slowest rate of increase in plasma corticosterone and a decline at 3 hr after capture. Although cloacal temperatures were significantly higher in summer samples, corticosterone concentrations of large turtles did not differ between seasons until 1 hr after capture. In addition, several large turtles during summer did not experience an increase in corticosterone concentrations 1 hr after capture. It is possible that the lower corticosterone response of large turtles captured during summer may be associated with reproductive condition.     

Stress response of brown pelican nestlings to ectoparasite infestation

Measurement of corticosterone has become a useful tool for assessing the response of individuals to ecological stressors of interest. Enhanced corticosterone levels can promote survival of stressful events; however, in situations where a stressor persists and corticosterone levels remain elevated, the adrenocortical response can be detrimental. A potential ecological stressor for wild birds is parasitism by ectoparasites. We studied the stress response of 11-23-day-old brown pelican (Pelecanus occidentalis) nestlings by measuring plasma corticosterone levels in relation to the presence of the soft tick Carios capensis at two colonies in South Carolina in 2005. We expected to see higher baseline and stress-induced levels of corticosterone for parasitized chicks compared to those nestlings with no ticks. Although nestlings mounted a response to capture stress, tick category was not associated with corticosterone levels at either colony. Our results appear to contrast those of previous studies and indicate that the adrenocortical response of the host is likely dependent on the type of ectoparasite and the degree of infestation.     

Protecting embryos from stress: corticosterone effects and the corticosterone response to capture and confinement during pregnancy in a live-bearing lizard (Hoplodactylus maculatus)

Hormones in the embryonic environment, including those of the hypothalamo-pituitary-adrenal (HPA) axis, have profound effects on development in eutherian mammals. However, little is known about their effects in reptiles that have independently evolved viviparity. We investigated whether exogenous corticosterone affected embryonic development in the viviparous gecko Hoplodactylus maculatus, and whether pregnant geckos have a corticosterone response to capture and confinement that is suppressed relative to that in non-pregnant (vitellogenic) females and males. Corticosterone implants (5 mg, slow-release) administered to females in mid-pregnancy caused a large elevation of corticosterone in maternal plasma (P<0.001), probable reductions in embryonic growth and development (P=0.069-0.073), developmental abnormalities and eventual abortions. Cool temperature produced similar reductions in embryonic growth and development (P< or =0.036 cf. warm controls), but pregnancies were eventually successful. Despite the potentially harmful effects of elevated plasma corticosterone, pregnant females did not suppress their corticosterone response to capture and confinement relative to vitellogenic females, and both groups of females had higher responses than males. Future research should address whether lower maternal doses of corticosterone produce non-lethal effects on development that could contribute to phenotypic plasticity. Corticosterone implants also led to increased basking in pregnant females (P<0.001), and basal corticosterone in wild geckos (independent of reproductive condition) was positively correlated with body temperature (P<0.001). Interactions between temperature and corticosterone may have broad significance to other terrestrial ectotherms, and body temperature should be considered as a variable influencing plasma corticosterone concentrations in all future studies on reptiles.     

Effects of breeding season, testosterone and ACTH on the corticosterone response of free-ranging male fence lizards (Sceloporus undulatus)

An attenuated stress response during the breeding season has been reported for several vertebrate species, but the underlying physiological mechanism has received little attention, particularly in reptiles. Modulation could involve changes in the capacity of the adrenal gland to secrete glucocorticoids in addition to upstream changes in the pituitary or hypothalamus. In this study the magnitude of the corticosterone response to capture and confinement was compared between the breeding and postbreeding season in adult male eastern fence lizards, Sceloporus undulatus. Males were captured in both seasons and subjected to the identical stressor of 4 h of confinement. Plasma corticosterone levels in response to confinement were significantly lower in the breeding than the postbreeding season. The effect of testosterone on the stress response was tested by experimentally elevating plasma testosterone levels via silastic implants in free-living males during the postbreeding season. Males with experimentally elevated testosterone exhibited significantly weaker corticosterone responses to 1 h of confinement than sham-implanted males. Finally the capacity of the adrenal glands to secrete corticosterone during the breeding season was tested by challenging males with adrenocorticotropin (ACTH) injections. In spite of naturally suppressed corticosterone responses during the breeding season, males nonetheless responded robustly to ACTH. Altogether these results suggest that modulation resides upstream of the adrenal gland, as has been shown in some arctic-breeding avian species, and likely involves seasonal changes in testosterone levels.     

Glucocorticoid response to food availability in breeding barn swallows (Hirundo rustica)

Parents feeding altricial nestlings have to trade-off the competing demands of self-maintenance and reproductive investment over their lifetime. Corticosterone, a glucocorticoid hormone released by birds in response to stressors, might play a key role in regulating parental investment when conditions unexpectedly deteriorate. However, birds breeding in unpredictable environmental conditions have been shown not to increase circulating levels of corticosterone as a response to bad weather to avoid nest abandonment when investment in offspring is high or when the probability of re-nesting is low. We investigated whether parent barn swallows Hirundo rustica, a passerine bird whose aerial insect food varies greatly in abundance depending on weather, also belongs to those species or whether it responds with an activation of the hypothalamo-pituitary-adrenal axis to natural variation in insect availability. We correlated plasma corticosterone levels of parents with weather conditions, the availability of aerial insects and parental body condition. Plasma corticosterone concentrations increased when mean daytime temperature declined, and consequently insect availability decreased and body condition of the parents deteriorated. Low temperatures also had a negative effect on body mass of the nestlings and there was a negative relationship between circulating corticosterone of parents and body mass of nestlings. We conclude that corticosterone is probably involved in the regulation of parental investment.     

The effect of exposure to 40 degrees on the heat production and the serum concentrations of triiodothyronine, thyroxine, and corticosterone in immature domestic fowl

Four-week-old fowl were exposed to 40 degrees for 35 days and the effects on the serum concentrations of triiodothyronine (T3), thyroxine (T4), and corticosterone were investigated. Changes in resting heat production, food intake, body temperature, body weight gain, and growth (both in terms of skeletal growth and weight of selected organs) were also measured. Body temperature was increased at 40 degrees, and body weight gain and skeletal growth were both reduced. The serum concentrations of T3 and T4 were reduced at 40 degrees. The decrease in serum T3 was more strongly correlated with the reduction in food intake than was the reduction in T4. Resting heat production was decreased by exposure to 40 degrees; the reduction in heat production was correlated with serum T3 and serum T4 concentrations. Exposure to 40 degrees had no effect on adrenal weight or on serum corticosterone concentration but weights of the bursa, spleen, and thymus glands were decreased. These results suggest that (a) T3 is the principle metabolically active thyroid hormone in the chicken and that the reduction in heat production at 40 degrees is related to the decline in T3; (b) the reduction in T3 is a consequence of the reduction in food intake; and (c) the response to a stressor need not involve increased adrenocortical activity.     

Effect of neonatal handling and sex on basal and chronic stress-induced corticosterone and leptin secretion

Neonatal handling is an experimental paradigm for early experiences. It affects the programming of hypothalamo-pituitary-adrenal (HPA) axis function, known to be sexually dimorphic. Recently leptin, a hormone related to energy balance and secreted mainly by adipocytes, has been implicated in the stress response. We thus determined the effect of neonatal handling on plasma concentrations of corticosterone and leptin of male and female rats under basal conditions and after two consecutive chronic stressors: chronic forced swimming stress and chronic restraint. Handling resulted in lower basal corticosterone levels in both males and females and in a more efficient HPA response, with a large corticosterone surge following the first chronic stressor and a return to basal levels following the second. Handling also resulted in decreased plasma leptin concentrations in males, thus abolishing the sex difference in leptin levels. Furthermore, handling increased body weight while it decreased food intake under basal conditions. Food intake and body weight gain during chronic forced swimming was lower in handled than in non-handled males, while in females these parameters were not influenced by handling. In both males and females, handling resulted in decreased food intake and increased body weight loss during chronic restraint stress. Body weight loss during chronic restraint stress, which is considered an index of maladaptation and 'depression', was particularly pronounced in the handled females. Our results also showed that non-handled females had higher corticosterone and lower leptin levels than males under basal conditions and following each of the two chronic stressors. The present work suggests that early experiences, such as the mother-infant relationship, interact with endogenous factors, such as gonadal hormones, to determine the organism's response to stressful stimuli during adulthood.     

Phenylethanolamine-N-methyl transferase activity in the avian adrenal following immobilization or adrenocorticotropin

The acute response of the avian adrenal to stress was evaluated by quantitatively measuring the adrenal and plasma levels of the major adrenal hormones (corticosterone (B), epinephrine (E), and norepinephrine (NE) along with assaying the adrenal activity of phenylethanolamine-N-methyltransferase (PNMT) in 8- to 9-week-old White Leghorn cockerels. The nonspecific stressor used in this study was a single 3-hour period of immobilization or a single dose of adrenocorticotropin (ACTH).It was found that the administration of ACTH or immobilization resulted in a significant increase in plasma E and NE concentrations with an accompanying decrease in their adrenal levels. ACTH-trcatment or immobilization was also effective in significantly increasing plasma B concentration and adrenal PNMT activity, but had no effect on the adrenal level of B.It is suggested that the rapid increase in PNMT activity, occurring with the acute application of stressors in chickens, may provide a means of sustaining an increased outflow of E in times of “stress.” The role that B may play in the elevation of PNMT, occurring in the avian stress response, is also discussed.     

A single exposure to an acute stressor has lasting consequences for the hypothalamo-pituitary-adrenal response to stress in free-living birds

In vertebrates, activation of the hypothalamo-pituitary-adrenal (HPA) axis in response to unpredictable events results in elevated glucocorticoid secretion. Repeated exposure to stressors alters subsequent glucocorticoid secretion, either by inducing chronic stress or as a result of habituation. However, most studies of repeated stress focus on the impacts of multiple and frequent exposures to acute stressors, and few have been carried out in free-living animals. We investigated whether a single exposure to a novel stressor was sufficient to produce long-lasting alterations in HPA function in free-living eastern bluebirds (Sialia sialis). We subjected adult females to a capture/restraint protocol in which we collected serial blood samples over an hour of restraint to be analyzed for corticosterone. We administered this protocol to three groups of females during the nestling phase of their first and/or second brood of the season: Repeaters (sampled during brood 1 and brood 2), Naïve-Brood 1 (sampled only during brood 1), and Naïve-Brood 2 (sampled only during brood 2). Repeaters had attenuated corticosterone responses to the second restraint bout compared to the first, and in brood 2, Repeaters had lower responses than Naïve-Brood 2 females. However, Naïve-Brood 1 and Naïve-Brood 2 birds did not differ in their responses to restraint. Thus, as little as one prior experience with an acute stressor was sufficient to alter subsequent HPA responsiveness, and this effect was not due to a natural change in HPA responsiveness as the breeding season progressed. These data may have important implications for understanding how acute stressors can alter a free-living animal’s ability to cope in the face of subsequent stressors, and for longitudinal field studies in which individuals are repeatedly sampled for glucocorticoid responsiveness.     

A serotonin receptor antagonist, but not melatonin, modulates hormonal responses to capture stress in two populations of garter snakes (Thamnophis sirtalis parietalis and Thamnophis sirtalis concinnus)

Hormonal and behavioral responses to a stressor depend on many factors, including the influence of other hormones. We examined the role of melatonin in modulating hormonal responses to capture stress in two populations of male garter snakes, Thamnophis sirtalis. Studies of red-sided (T. sirtalis parietalis) and red-spotted (T. sirtalis concinnus) garter snakes were conducted in the field with free-living snakes. Populations of red-sided garter snakes in south-central Manitoba, Canada undergo a period of winter dormancy for approximately 8 months each year followed by an attenuated mating season (4-5 weeks) in early spring. In contrast, the mid-latitude red-spotted garter snake in western Oregon, USA has an extended breeding season and can be active during 10-12 months of the year given appropriate environmental conditions. We chose to study these two populations of garter snakes to investigate possible variation in melatonin function among snakes with different suites of environmental adaptations. To better address these questions, we also examined the effects of 5-hydroxytryptophan (a precursor of melatonin synthesis) and ketanserin (a serotonergic type 2A receptor antagonist) on hormonal responses to capture stress. We observed a trend of increased corticosterone and decreased androgen concentrations in northern-latitude red-sided garter snakes (T. sirtalis parietalis) subjected to 4 h of capture stress during the spring. However, these differences were not statistically significant. During the fall, red-sided garter snakes showed no change in corticosterone or androgen concentrations in response to the capture stress treatments. We speculate that northern-latitude red-sided garter snakes suppress hormonal responses to capture stress during preparation for winter dormancy. Treatment with melatonin, 5-hydroxytryptophan, or ketanserin did not significantly influence corticosterone or androgen concentrations of northern-latitude red-sided garter snakes during the spring or fall. Mid-latitude red-spotted garter snakes (T. sirtalis concinnus) from Oregon showed a statistically significant increase in corticosterone concentrations in response to 4 h of capture stress; treatment with melatonin, 5-hydroxytryptophan, or ketanserin prior to capture stress had no significant influence on plasma corticosterone concentrations. Androgen concentrations of mid-latitude red-spotted garter snakes in response to capture stress were significantly lower than those of non-stressed control snakes. Neither melatonin nor 5-hydroxytryptophan influenced the change in androgen concentrations during capture stress. However, androgen concentrations of snakes treated with ketanserin prior to 4 h of capture stress did not differ significantly from those of non-stressed control snakes. These studies suggest that melatonin does not modulate hormonal responses to capture stress in this ectothermic model. Our results also suggest that a serotonin-regulated system may play a role in modulating the activity of the hypothalamic-pituitary-gonadal axis during physiological stress responses.     

Transitory corticosterone responses of ducks (Anas platyrhynchos) to exercise

Horizontal treadmill exercise induced a marked (P less than 0.001) but transitory increase in the level of circulating corticosterone in the plasma of adult male ducks. The decline in corticosterone concentration during exercise is unlikely to be due to a depletion of adrenocortical stores since a marked (P less than 0.001) corticosterone response to adrenocorticotrophin (ACTH) administration was observed immediately after exercise, and was of similar magnitude to that induced in nonexercised controls. The corticosterone response to repeated exercise is also transitory, habituating completely within 28 days of the start of daily training. The corticosterone response (P less than 0.001) to ACTH challenge is not, however, diminished by training. These results indicate that the habituation of the corticosterone response to either acute or repeated exercise is due to a reduction in endogenous ACTH secretion. A similar mechanism appears to be responsible for the habituation of the corticosterone response to handling and confinement in nonexercised control birds. A marked (P less than 0.001) increase in the level of circulating corticosterone was, however, elicited when these birds were exercised, indicating that despite adaptation to handling and confinement the birds remained responsive to the novel stressor of exercise. The corticosterone response of these birds was, however, less than that in untrained birds indicating that the corticosterone response in untrained birds is due to both workload and the stress of handling and confinement.     

Corticosterone in relation to body mass in Adelie penguins (Pygoscelis adeliae) affected by unusual sea ice conditions at Ross Island, Antarctica

Penguins naturally fast each year during breeding and again whilst moulting, and may lose more than 40% of body mass during a fast. Fasting in emperor (Aptenodytes forsteri) and king (Aptenodytes patagonicus) penguins has been divided into three phases, with phase III characterised by an increased rate of body mass loss, increased plasma corticosterone concentrations, and a change in behaviour leading to abandonment of the breeding attempt and return to sea to feed. Initial corticosterone concentrations and corticosterone responses to a handling stressor were measured in the current study to determine if they increase during phase III of fasting in Adelie penguins (Pygoscelis adeliae). The study was conducted in 2001 at the northern Cape Bird colony on Ross Island, Antarctica. Penguin breeding on Ross Island was disrupted in the 2001-2002 summer by a large iceberg (B15A) which stopped the normal movement of sea ice in the Ross Sea. Penguins departing from the Cape Bird colony were lighter than returning or incubating birds (3.39+/-0.10cf. 4.16+/-0.06 and 4.07+/-0.08kg). It is likely that the departing birds were males that had been lighter than normal when they arrived at the colony. Initial plasma corticosterone concentrations were higher in departing than returning or incubating penguins (6.89+/-1.69cf. 2.36+/-0.42 and 1.08+/-0.19ng/ml). Corticosterone responses to handling were also greater in departing penguins. Initial plasma corticosterone, concentrations at 30min and total and corrected integrated corticosterone responses were inversely related to body mass in departing penguins, whereas there were no relationships in arriving penguins. beta-hydroxybutyrate and uric acid concentrations were consistent with departing birds having entered phase III of fasting. The results indicate that corticosterone and corticosterone responses are elevated in phase III of fasting in the Adelie penguin.     

Repeated, but not acute, stress suppresses inflammatory plasma extravasation

Clinical findings suggest that inflammatory disease symptoms are aggravated by ongoing, repeated stress, but not by acute stress. We hypothesized that, compared with single acute stressors, chronic repeated stress may engage different physiological mechanisms that exert qualitatively different effects on the inflammatory response. Because inhibition of plasma extravasation, a critical component of the inflammatory response, has been associated with increased disease severity in experimental arthritis, we tested for a potential repeated stress-induced inhibition of plasma extravasation. Repeated, but not single, exposures to restraint stress produced a profound inhibition of bradykinin-induced synovial plasma extravasation in the rat. Experiments examining the mechanism of inhibition showed that the effect of repeated stress was blocked by adrenalectomy, but not by adrenal medullae denervation, suggesting that the adrenal cortex mediates this effect. Consistent with known effects of stress and with mediation by the adrenal cortex, restraint stress evoked repeated transient elevations of plasma corticosterone levels. This elevated corticosterone was necessary and sufficient to produce inhibition of plasma extravasation because the stress-induced inhibition was blocked by preventing corticosterone synthesis and, conversely, induction of repeated transient elevations in plasma corticosterone levels mimicked the effects of repeated stress. These data suggest that repetition of a mild stressor can induce changes in the physiological state of the animal that enable a previously innocuous stressor to inhibit the inflammatory response. These findings provide a potential explanation for the clinical association between repeated stress and aggravation of inflammatory disease symptoms and provide a model for study of the biological mechanisms underlying the stress-induced aggravation of chronic inflammatory diseases.     

Effects of temperature on urinary corticosterone metabolite responses to short-term capture and handling stress in the cane toad (Rhinella marina)

Extreme temperature can cause metabolic, immune and behavioural changes in amphibians. Short-term stress hormonal response via increased secretion of corticosterone enables amphibians to make necessary physiological and behavioural adjustments for coping with stressors. The effect of temperature on short-term corticosterone responses has not been studied in amphibians. In this study, this relationship was evaluated in adult male cane toads (Rhinella marina). We acclimated male toads (n=24 toads per group) at low, medium and high temperature (15, 25 or 35°C) under controlled laboratory conditions for a 14day period. After thermal acclimation, short-term corticosterone responses were evaluated in the toads subjected to a standard capture and handling stress protocol over a 24h period. Corticosterone metabolites in toad urine were measured via enzyme-immunoassay. During acclimation, mean baseline urinary corticosterone level increased after transfer of the toads from wild into captivity and returned to baseline on day 14 of acclimation for each of the three temperatures. At the end of the 14days of thermal acclimation period, baseline corticosterone level were highest for toad group at 35°C and lowest at 15°C. All toads generated urinary corticosterone responses to the standard capture and handling stressor for each temperature. Both individual and mean short-term corticosterone responses of the toads were highest at 35°C and lowest at 15°C. Furthermore, Q(10) values (the factor by which the reaction rate increases when the temperature is raised by 10°) were calculated for mean corrected integrated corticosterone responses as follows; (15-35°C) Q(10)=1.51, (15-25°C) Q(10)=1.60; (25-35°C) Q(10)=1.43. Both total and corrected integrated corticosterone responses were highest for toads at 35°C followed by 25°C and lowest for the 15°C toad group. Overall, the results have demonstrated the thermodynamic response of corticosterone secretion to short-term capture and handling stress in an amphibian species.     

Plasma hormone levels in the green turtles Chelonia mydas during peak period of nesting at Ras Al-Hadd-Oman

Circulating estradiol (E(2)), progesterone (Pro), testosterone, and corticosterone (B) levels were monitored in the green turtles Chelonia mydas during different nesting phases. Successful nesting includes emergence from sea, chamber and nest excavation, oviposition, burying the nest, and returning to sea. Unsuccessful nesting includes chamber and nest excavations but without oviposition. Blood samples were taken from the cervical sinus and collected within 5-min of capture to minimize stress. The samples were collected between 2000 and 0100 h during the peak season (May-October). High-performance liquid chromatography using a u.v. detection system coupled with tandem quadrupole mass spectrometry was used to measure B. Plasma B levels were significantly higher in successful and unsuccessful phases over emergence and excavation phases. However, B levels in successful versus unsuccessful or emergence versus excavation phases were not significantly different. Plasma steroid levels were measured by the Coat-A-Count RIA technique. Pro levels were significantly higher (P<0.005) in successful over unsuccessful turtles and also successful turtles over turtles in the other phases (P<0.01). The Pro levels immediately after nesting were found to be higher than that reported previously. Plasma testosterone values were higher in successful turtles but not significantly different from the turtles in other phases. Estrogen levels were undetected in all phases. Overall, the hormone values during different phases of nesting may play a major role in formulating the nesting behavior and physiology of the nesting activities in the green turtle.     

The prolactin response to an acute stressor in relation to parental care and corticosterone in a short-lived bird, the Eurasian hoopoe

Prolactin plays an important role in mediating parental care in birds, but little is known about changes in prolactin levels when animals disrupt their reproductive behaviour during emergency life-history stages. We investigated the variation of prolactin levels with breeding stage, sex, body condition and as a response to a standardized acute stressor in a small short-lived bird, the Eurasian hoopoe Upupa epops under natural field conditions. We found higher baseline levels of prolactin in females during the brooding phase than in their mates which feed them and their chicks at this stage. Moreover, this is the first report of a differential prolactin stress-response between sexes with contrasting parental care within a breeding phase. Capture, handling and restraint induced a clear decrease of prolactin levels which was less pronounced in females at the very early stage of brooding compared to females in later stages. In contrast, the prolactin stress response in males remained nearly constant over the breeding stages and was stronger than in females. Baseline levels of prolactin, but not handling-induced levels, were positively correlated with body condition. We found a weak relationship between the decrease in prolactin due to acute handling stress and handling-induced levels of corticosterone. Taken together, both baseline and stress response levels of prolactin were related to the amount of parental care, although we found no relationship with reproductive success. It appears that the response to an acute stressor in prolactin levels is finely tuned to parental duties and investment. Hence, prolactin appears to be involved in mediating the trade-off between current reproduction versus self-maintenance and future reproduction.     

Plasma glucocorticoid concentrations and body mass in ground squirrels: seasonal variation and circannual organization

We examined variation in plasma glucocorticoid concentrations of free-living Belding's ground squirrels (Spermophilus beldingi) and captive golden-mantled ground squirrels (Spermophilus lateralis) housed in constant environmental conditions. Plasma corticosterone concentrations were not significantly correlated with time held captive in traps prior to collection of blood samples in males or non-breeding females, but these variables were significantly correlated in breeding females during the pre-mating, lactation, and post-lactation periods. Among male S. beldingi, plasma corticosterone concentrations increased over the course of the active season, and were significantly higher in non-breeding than breeding individuals. Corticosterone concentrations also increased in non-breeding females throughout the active period. In breeding females, baseline (non-stress) corticosterone concentrations, determined from blood samples collected within 3 min of capture, increased during gestation and declined during lactation, whereas stress values of corticosterone, determined from blood samples collected within 4-11 min of capture, increased during gestation and then again during the period after young emerged from the natal burrow. Changes in plasma corticosterone concentrations of S. beldingi paralleled changes in body mass. Among S. lateralis, plasma concentrations of cortisol were elevated when males and females were in reproductive condition and lower in reproductively quiescent squirrels. The annual peak in plasma cortisol concentrations occurred just prior to increases in body mass associated with pre-hibernation fattening. Collectively, these results suggest that concentrations of circulating glucocorticoids fluctuate with circannual rhythmicity in conjunction with annual cycles of change in body mass and activity; within these circannual cycles glucocorticoid concentrations are influenced by stress and breeding status.