A field study of brook stickleback morphology: multiple predators and multiple traits

Abstract

This field-based study examined a suite of morphological traits expressed by brook sticklebacks ( Culaea inconstans (Kirtland, 1840)) coexisting with two types of predator communities. “Small-predator” communities included gape-limited predators only; “mixed-predator” communities included both gape-limited and non-gape-limited predators. Across 26 study sites in three drainages, sticklebacks in small-predator communities had longer bodies, longer spines, and deeper midbodies than those in mixed-predator communities. The observed differences were consistent with the hypothesis that stickleback morphology in small-predator communities would be better adapted for avoiding gape-limited predators and that in mixed-predator communities should be better adapted for avoiding large, non-gape-limited predators. Morphological differences were apparent in 1-year-old sticklebacks and likely resulted from predator–prey interactions early in the sticklebacks’ life history. Correlation among morphological traits involved in antipredator defense (i.e., morphological integration) was hypothesized to occur in small-predator communities, characterized by a single gape-limited predation pressure, but not mixed-predator communities where sticklebacks coexisted with multiple predator types in each site. Contrary to expectations, length-at-age, spine length, and body depth were not correlated in either predator community. Numerous variables, not present in controlled laboratory studies, may constrain morphological expression in the field environment.