Trait Covariances in Eastern Box Turtles Do Not Support Pleiotropic Effects of the Melanocortin System on Color, Behavior, and Stress Physiology
Endocrine systems and individual behavioral differences (temperament) are often linked in animals. In particular, glucocorticoids (corticosterone [CORT]) have been implicated in animal coping styles, or syndromes of integrated temperamental and neuroendocrine variation. Typically, organisms with lower stress-induced elevations of CORT tend to exhibit more proactive behavior. Melanin-based coloration has been further linked to CORT physiology and temperament, with more melanistic individuals typically exhibiting more proactive coping styles. The melanocortin hypothesis proposes that variation in the melanocortin system could drive the repeated covariation in coloration, coping style, and CORT levels. We evaluated the relationships among the CORT stress response, boldness (i.e., responsiveness to risk), and melanization of the shell in Eastern Box Turtles (Terrapene carolina carolina), predicting that turtles with bolder temperaments would exhibit lower stress-induced CORT levels and possess darker shells. We also expected stress-induced CORT levels to be lower at cooler body temperatures. Our results generally failed to support the melanocortin hypothesis. We found no significant correlations among behavior, CORT, and melanization, and correlations that approached significance were weak. Moreover, the near significant relationship between CORT levels and boldness is in the opposite direction predicted. We also found that temperature had a strong positive effect on CORT levels, and there were population differences in plastron melanization and boldness.ABSTRACT
Example carapace (upper row) and plastron (lower row) images of turtles demonstrating a range of melanization from low (left) to high (right). Upper row and lower row images are not paired and represent different individual turtles.
Plasma corticosterone (CORT) levels from before and after confinement stress. Values from the same individual are linked by the lines. Values below 0.57 ng/mL (indicated by the horizontal dotted line) were below the detection limited and treated as censored in the analysis. The y-axis is presented on a logarithmic scale.
Relationships among behavioral, corticosterone (CORT), and shell melanization traits. Stress-induced plasma CORT and boldness (head emergence latency) are shown on a logarithmic scale. Censored values of CORT levels (<0.57 ng/mL) and boldness (head emergence latencies >600 sec) are indicated by triangular symbols.
Differences in boldness (A) and plastron melanization (B) between the two populations, Allee Memorial Woods (AMW) and Weiler–Leopold/Black Rock Barrens (WLBRB) nature preserves. Boldness (head emergence latency) is depicted on a logarithmic scale, and each circle represents a value for an individual turtle. Boxes represent the 25th to 75th percentiles, thick lines within the boxes represent the median, and whiskers (vertical lines) extend to all observed values within a range of 1.5 times the interquartile range from the boxes; points beyond 1.5 times the interquartile range are shown unconnected by whiskers in (B).
Effects of plastron temperature (A) and total stress exposure time (B) on stress-induced plasma corticosterone (CORT) levels. Total stress exposure time is the elapsed time from initial contact to completion of the final (stress-induced) blood sampling, including confinement, handling, and bleed times. CORT levels are shown on a logarithmic scale. Censored values of CORT levels (<0.57 ng/mL) appear below the horizontal line representing the detection limit.
Contributor Notes
Present address: Indiana Department of Environmental Management, Indianapolis, Indiana 46204, USA
