What drives variation in the corticosterone stress response between subspecies? A common garden experiment of swamp sparrows (Melospiza georgiana).

Although differences in the corticosterone stress response have frequently been reported between populations or closely related subspecies, their origin remains unclear. These differences may appear because individuals adjust their corticosterone stress response to the environmental conditions they are experiencing. However, they may also result from selection that has favoured individuals with specific corticosterone stress response or from environmental factors that have affected the development of the corticosterone stress response during early life. We investigated these hypotheses by studying the corticosterone stress response of two closely related subspecies of swamp sparrows (Melospiza sp.). We showed for the first time that two closely related subspecies can differ in their corticosterone stress response when raised at the laboratory and held in similar conditions for a year. Thus, we demonstrated that selection, developmental processes or a conjunction of both of these processes can account for variation in the stress response between closely related subspecies.

The role of corticosterone in supporting migratory lipogenesis in the dark-eyed Junco, Junco hyemalis: a model for central and peripheral regulation.

The functional role of corticosterone (CORT) in regulating migratory hyperphagia and lipogenesis was investigated in an annual migrant, the dark-eyed junco (Junco hyemalis). Intraperitoneal injections of either dexamethasone (9 microg DXM/500 microL of 5% EtOH in saline, n=10) to inhibit an increase in baseline CORT or saline (5% EtOH, n=9) were given every 48 h for 15 d after transfer from short (10.5L:13.5D) to long (15.5L:8.5D) days. Food intake, body mass, furcular fat deposition scores, and nocturnal migratory activity were recorded for 29 d after photostimulation. Both groups showed the same increase in daily food intake over the study period (DXM=52%, control=41%). Controls began to increase baseline CORT and mass about 2 wk after photostimulation. DXM-treated birds maintained low CORT and did not increase mass or CORT until injections ceased, at which time they gained mass at the same rate shown earlier by controls. DXM-treated birds did not show greater levels of migratory activity despite experiencing an increase in energy intake during the CORT-inhibited period. Collectively, the results support the migration modulation hypothesis, illustrating how an increase in baseline CORT is needed to support the development of migratory condition. We address the apparent conflict with earlier studies on CORT and migratory food intake and propose a model in which migratory hyperphagia is supported by changes in centrally regulated responses to CORT that can occur even if CORT remains low and lipogenesis is regulated predominantly by peripheral mechanisms that require an increase in baseline CORT.

An alternative method for delivering adrenocorticotropin hormone in birds.

In birds, intrajugular injection (i.j.) of exogenous adrenocorticotropin hormone (ACTH) has been used to challenge adrenocortical tissue when investigating various dynamics of the hypothalamic-pituitary-adrenal (HPA) axis. Although this method has been shown to deliver ACTH efficiently, i.j. injection can be difficult and potentially damaging to the bird, especially to young birds and small species. Intramuscular (i.m.) injection has been shown to be an alternative method for delivering ACTH to large birds; however, small songbirds have relatively less muscle mass in which to absorb i.m. injections and, in very small birds, even small needles may injure the delicate muscle tissue. Therefore, intraperitoneal (i.p.) administration of ACTH was investigated as a means of avoiding the potential problems associated with i.j. and i.m. injections when conducting adrenocortical pathway studies on small birds. Dark-eyed Juncos, Junco hyemalis, were first treated with dexamethasone to suppress the endogenous signal cascade for corticosterone release. Twelve hours later, birds were sampled for baseline corticosterone concentration and then immediately given an i.p. injection of either ACTH or physiological saline. A second blood sample was taken from all birds 30 min later to assess the effectiveness of i.p. ACTH administration on adrenocortical tissue. While saline-treated birds showed no significant increase in endogenous plasma corticosterone in response to the capture and handling protocol, ACTH-treated birds showed a significant increase in plasma corticosterone concentration, illustrating the effectiveness of the i.p. administration of exogenous ACTH. The adrenocortical response of ACTH-treated birds was similar in magnitude and rate of increase to that found in free-living, noninjected Juncos. Intraperitoneal injection of ACTH is thus a valid alternative to i.j. and i.m. injections when challenging adrenocortical tissue in small birds.

Differential migration and an endocrine response to stress in wintering dark-eyed juncos (Junco hyemalis).

The dark-eyed junco (junco hyemalis) exhibits differential migration in autumn that, in general, results in females overwintering south of males, and young within each sex overwintering north of older birds. Individuals overwintering at higher latitudes face less predictable and more challenging environmental conditions. Rapid increases in circulating levels of the energy-regulating glucocorticosteroid, corticosterone, occur in response to environmental stressors. To establish whether the strength of acute corticosterone secretion was correlated with the probability of encountering poor environmental conditions, we compared the corticosterone stress response (e.g. initial plasma concentrations at the time of capture and 30 min later) in dark-eyed juncos overwintering in Mississippi (MS), USA, near the southern limit of their wintering range, with juncos overwintering in New York (NY), USA, near the northern limit of their wintering range. During two winters, 22 males and one female were sampled in NY; 13 males, 12 females and one bird of undetermined sex were sampled in MS. Not unexpectedly, NY birds carried greater fat reserves that resulted in a significantly higher value of energetic condition (mass corrected for wing cord cubed). There was no difference between the two winters sampled at either site, nor was there an effect of sex on patterns of corticosterone secretion in MS birds. With sexes pooled, MS and NY birds had similar baseline corticosterone levels. However, as predicted, NY birds exhibited significantly higher corticosterone concentrations 30 min after capture. These results support the hypothesis that birds wintering in less predictable, more extreme environments show a higher amplitude corticosterone response, which may enable them to adjust their behaviour and physiology more rapidly in response to environmental stressors such as storms. Adrenocortical sensitivity may be a part of the physiological milieu associated with differential migration in juncos; whether it results from endogenous differences in the migratory programmes of individuals or from acclimatization to local environmental conditions remains to be determined.

Development of the corticosterone stress response in young northern mockingbirds (Mimus polyglottos).

In birds, additional adrenocortical secretion in response to stressors often redirects an individual's ongoing activities toward immediate life-saving activities, usually by facilitating an increase in food searching and food intake needed to meet periods of increased energy demand. We asked whether young birds, who are entirely dependent on parents for food acquisition and therefore unable to manipulate their own food intake, fail to show an adult-like adrenocortical response to the acute stress of capture and handling. In 1998, plasma profiles of acute corticosterone secretion (e.g., samples taken at the time of capture and 30 min later) were compared across seven age classes of Northern Mockingbirds (Mimus polyglottos) representing various age-related stages of foraging ability and opportunity. As predicted, young birds less able, or entirely unable, to readjust their own foraging effort exhibited significantly lower stress responses compared to adults. The magnitude of the stress response (at 30 min postcapture) increased and approached that of adults as young birds approached independence. Energetic condition was not correlated with the magnitude of the stress response at any age, suggesting that variation in its expression was most likely due to age alone. We also investigated at what level within the hypothalamic-pituitary-adrenal (HPA) axis the corticosterone response may be controlled in young birds. In 1999, baseline corticosterone samples were taken in 8-day-old nestlings and were immediately followed by intrajugular injections of adrenocorticotropic hormone (ACTH) or saline. While plasma corticosterone concentrations did not change in saline-injected nestlings, ACTH-injected nestlings showed a significant increase in plasma corticosterone concentrations similar to 30-min samples taken from adults. These results indicate that, while young birds do not normally show the corticosterone response, the adrenocortical tissue has the capacity to do so, and the control appears to be within the hypothalamic-pituitary component of the HPA axis. Collectively, our results indicate that the expression of the corticosterone stress response develops in concert with a young, altricial bird's ability to utilize it as it approaches independence; the reduced corticosterone secretion may also allow young, rapidly growing birds to avoid potential deleterious exposure to elevated glucocorticosteroid concentrations.

Effects of exogenous corticosterone on locomotor activity in the red-eared slider turtle, Trachemys scripta elegans.

We investigated the effects of exogenous corticosterone on the locomotor activity of captive red-eared slider turtles, Trachemys scripta elegans. An increase in plasma corticosterone often increases locomotor activity in mammals and birds, but there are no reported findings for turtles. In this study turtles implanted with corticosterone-filled Silastic((R)) implants showed a significant increase in caged locomotor activity when compared to control animals with empty implants. Corticosterone-treated turtles also showed a significant increase in plasma corticosterone concentration when pre-trial plasma samples were compared to post-trial plasma samples, while control turtles exhibited no such increase, validating the effectiveness of our implants to deliver corticosterone. Although corticosterone remained high at the end of the activity trials, the increase in activity was ephemeral in nature, peaking within 48 hr after the implant was in place. This suggests that the effects of corticosterone on behavior may be context-dependent (i.e., whether the turtles can find food) and concentration-dependent, and that there are underlying physiological mechanisms, perhaps mediated at the receptor level in the brain, involved in locomotor activity behavior in slider turtles. Environmental perturbations that cause a reduction in available food resources may cause the organism to increase its level of locomotor activity to increase food encounter rate but later reduce activity to conserve energy reserves. These data are important when considering behavioral and physiological mechanisms involved in a turtle's response to changing conditions in habitat quality. J. Exp. Zool. 284:637-644, 1999.

Changes in patterns of corticosterone secretion concurrent with migratory fattening in a neotropical migratory bird.

Several studies on free-living birds have shown a change in corticosterone secretion (elevated baseline levels and a reduced corticosterone response to stress) during migration. It was not known, however, if this change was concurrent with the development of migratory condition or if it was an independent response to unknown environmental stressors experienced by the birds prior to capture. In this study, a Neotropical annual migrant, the yellow-rumped warbler (Dendroica coronata), held under controlled laboratory conditions, was used to test the Migration Modulation Hypothesis (MMH): during the migratory period migrants exhibit (1) elevated baseline corticosterone to facilitate migratory fattening and (2) a reduced corticosterone stress response, a means by which skeletal muscle needed for migration can be protected against catabolism by high levels of corticosterone. Fifteen hatching-year warblers were maintained on insect larvae and water ad libitum for 43 weeks, experiencing two transitions from a short- to long-day photoperiod to bring them into spring migratory condition. Corticosterone profiles comprising three blood samples from each individual (baseline at the time of initial disturbance and 30 and 60 min later), body mass, fat reserves, molt, and state of cloacal protuberance (males only) were measured at key intervals throughout the study. Over the entire study, mean baseline corticosterone levels were positively correlated with mean body mass, which increased predictably in response to long days. Individual baseline corticosterone was not correlated with individual body mass at any time. During periods when the birds were lean and held on short days, the corticosterone stress profiles were characterized by low initial hormone concentration followed by a significant increase in corticosterone with handling time. In response to long days, the warblers showed a significant increase in body mass and fat reserves concurrent with corticosterone stress profiles characterized by significantly elevated baseline levels and no further increase in corticosterone with handling time. These results support both components of the MMH illustrating changes in corticosterone secretion concurrent with migratory fattening but the exact nature of this change is unknown.

Corticosterone secretion in response to capture and handling in free-living red-eared slider turtles.

The corticosterone response to capture and handling was measured in free-living red-eared slider turtles, Trachemys scripta elegans. To determine the ability of this species to exhibit this endocrine response, slider turtles were bled at the time of removal from hoop nets and again at 30 and 60 min following capture to create plasma profiles of acute corticosterone secretion from individuals. Plasma corticosterone concentration increased significantly with handling time. The greatest rise in corticosterone was within the first 30 min following capture and handling, with this rate of increase declining over the next 30 min of restraint. There was no correlation between corticosterone levels at the time of capture and the length of time it took to get the sample if the sample was taken within the first 10 min after capture. However, when these samples were included with those taken from other turtles sampled 11 to 25 min after capture, hormone levels were significantly correlated with handling time. This suggests that the critical time to obtain an initial sample that best represents the predisturbance level in slider turtles is within 10 min. There was no correlation between the turtles' energetic condition and initial corticosterone concentrations. Plasma corticosterone values at all sampling times were comparable to those observed in other reptile species. The results from this study may be used to investigate the effects of unpredictable resources on reproductive success and survival in freshwater turtles.