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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.

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® 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. © 1999 Wiley‐Liss, Inc.

The effect of the simulated drying of a pond on the behavior and corticosterone secretion of Trachemys scripta was measured in a field situation. Slider turtles were held in experimental and control ponds (12 x 15 m) enclosed with a drift fence integrated with spring-triggered livetraps. The experimental pond water level was dropped 10 cm per day for 8 d, until water was completely drained. Slider turtles responded to the draining of the pond by the emigration of the majority (75%) of the experimental population. Emigrating turtles had significantly elevated corticosterone at Time 0 (blood sample within 10 min of handling=4.48 ng/mL+/-0.503SE) when compared with turtles captured in a control pond (Time 0=0.954 ng/mL+/-0.121SE), where conditions were held constant. Turtles emigrated during the final 72 hr of pond draining when ponds reached 30 cm depth and lower and water temperature was at least 30.8 degrees C or higher. Additionally, the effect of trapping using spring-activated livetraps was tested. Turtles held in livetraps (n=6) for 45-110 min showed a characteristic corticosterone response (Time 0=0.957 ng/mL+/-0.091SE; Time 30=2.85 ng/mL+/-0.131SE), indicating that this trapping technique alone does not stimulate corticosterone secretion. The findings of the study met our predictions that turtles would respond to the draining of the pond behaviorally by emigrating from the habitat concurrent with an elevated corticosterone concentration. This supports the view that corticosterone is involved in stress avoidance mechanisms that allow organisms to respond to environmental perturbations.

Fish can have strong predatory impacts on aquatic food webs. Indeed, fish are known to have strong effects on amphibians, with some species being excluded from communities where fish are present. Most research with amphibians and fish has focused on lower latitudes and very little is known of amphibian–fish interactions at higher latitudes. Therefore, we conducted an enclosure experiment in a subarctic natural wetland to examine the predatory effects of two species of fish, brook sticklebacks (Culaea inconstans (Cuvier, 1829)) and ninespine sticklebacks (Pungitius pungitius (L., 1758)), on the survival and growth of Wood Frogs (Lithobates sylvaticus (LeConte, 1825)). We found no significant difference in survival and size at metamorphosis among the two fish species treatments and fish-free treatments. We found that individuals from fish-free treatments metamorphosed earlier than those from either fish species present treatment. Our work suggests that stickleback fish predation may not have a major impact on Wood Frog tadpole survival and growth in a subarctic wetland. Sticklebacks may still have an impact on earlier developmental stages of Wood Frogs. This work begins to fill an important gap in potential factors that may impact larval amphibian survival and growth at higher latitudes.

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