Phenotypic correlates of pelvic spine coloration in the threespine stickleback ( Gasterosteus aculeatus ): implications for function and evolution

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. Significance statement The interplay between color patches that are either readily concealable or always visible has been little studied, particularly in organisms with patches on a single individual that differ in capacity for concealment, such as the threespine stickleback ( Gasterosteus aculeatus ). While much work has been done on the evolution of coloration in stickleback, little has addressed patterns of expression and evolutionary functions of pelvic spine color, which can be facultatively concealed. We evaluated the expression characteristics of male spine coloration within the spawning period and how spine color relates to other color patches. Our work also examines the relationship between pelvic spine erections and presentation of spine color patches and raises the possibility of pelvic spine color being naturally selected and functioning across different behavioral contexts.