EVIDENCE FOR A TRADE-OFF BETWEEN DEFENSIVE MORPHOLOGY AND STARTLE-RESPONSE PERFORMANCE IN THE BROOK STICKLEBACK (CULAEA INCONSTANS)

It is generally believed that predation pressure may drive the evolution of long spines, a robust pelvic girdle, and a deep body in sticklebacks (Pisces: Gasterosteidae). However, the lack of such traits in environments under intense predation pressure suggests that there may be a limit to which these apparently defensive structures benefit sticklebacks. In some environments, well-developed defensive structures may not increase stickleback survival, but may actually reduce fitness if there is a cost associated with them. This paper focuses on a trade-off between defensive morphology and escape-response performance in the brook stickleback (Culaea inconstans). Our study of four populations of brook stickleback reveals that the population with the largest pelvic girdles and deepest bodies has a poorly developed escape response (i.e., small displacement, low maximum velocity, and low acceleration), while the population with the smallest pelvic girdles and shallowest bodies has a highly developed escape response. The two populations with intermediate defensive structures are intermediate in escape-response performance. Consideration of predation regimes in different environments may help us understand selection pressures that favor heavily versus poorly armored stickleback morphs.