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 these aspects of coloration, and
their consequences, have been little studied. We address them here 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. However,
little 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. In addition, elevated spine
raising in the presence of a model predator, together with the presence of
red spine coloration in females, raises the possibility that red spine
coloration may also have an anti-predator function.