Behavioural and physiological responses to simulated predator-induced stress in the eastern box turtle, Terrapene carolina carolina
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In spite of the fully-hinged shell being the defining characteristic of box turtles, few studies have quantified the behavioural use of the shell in response to predator encounters. While it is often assumed that box turtles simply withdraw into their shell in response to threats, we expected variability among individuals in shell usage and in the degree of head and limb retraction because withdrawal is likely to be costly. We tested the behavioural and physiological responses of free-ranging eastern box turtles to stimuli of varying intensity: observation only, a visual stimulus, a poke on the leg, or a pinch on the leg before being flipped. The more intense the simulated predatory stimulus, the more likely turtles were to fully close their shell. However, stimulus intensity did not affect plasma corticosterone or lactate levels. This may be due to too brief or weak of a stimulus, increased parasympathetic activity, or seasonal effects. Upon capture and during handling, almost all turtles withdrew into their shells and closed their anterior plastron, although fewer closed their posterior plastron, and a few engaged in other behaviours such as biting and urinating. Older, larger turtles were able to close their shells with greater strength than younger and smaller turtles. Our results indicate that in spite of their fully closable shell, box turtles exhibit a graded response to perceived threats. This suggests that full closure of the shell is likely to be costly and occurs only in response to the most intense stimuli.