Animal color patches may be static or plastic in expression and concealable or continuously visible, yet the evolution and function of these aspects of coloration have seldom been studied together. We investigated such color pattern elements using the threespine stickleback (Gasterosteus aculeatus). Despite a rich history of study of stickleback nuptial color pattern evolution, disagreement persists regarding selection pressures and function and only limited research has addressed the role of pelvic spine coloration, a potentially important, and substantially concealable, color pattern element. We investigated (i) whether male pelvic spine (along with throat and body) coloration is relatively static or plastic across the reproductive cycle, (ii) when pelvic spines are raised versus concealed across behavioral contexts, and (iii) associations between color patches and behavior in males. We found no significant variation in spine color across reproductive stages whereas body color was more plastic and intensely red during courtship and egg/fry care. Conspicuousness of pelvic spine coloration instead varied behaviorally, through increased erection frequency during social interactions and in response to a model predator. Spine erection frequency was positively associated with behaviors that enhance spine color visibility, i.e. flees and leads to nest. These findings suggest that stickleback use pelvic spines to display an intensely red color patch facultatively, either as a complement to similar body coloration or possibly as a substitute.

Keywords: Gasterosteus aculeatus; animal behavior; animal coloration; evolution; pelvic spine; threespine stickleback.