Trait Covariances in Eastern Box Turtles Do Not Support Pleiotropic Effects of the Melanocortin System on Color, Behavior, and Stress Physiology

Abstract

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.