The threat-sensitive predator avoidance hypothesis suggests that prey animals should minimize
the costs of antipredator behaviors by only responding to predators that pose a potentially lethal threat.
Thus, prey must use risk assessment strategies to determine which predators present a great enough threat
to merit a response. A rich literature demonstrates that chemical signals can communicate information about
predator identity, diet, and size. However, the relationship between predator size and prey size has been largely overlooked. What remains unknown is how prey respond to odors from predators along a continuum
of relative size. Here, we tested the responses of crayfish (prey) to odor cues from two species of predatory
fish along a gradient of relative size relationships. Crayfish and fish were paired according to crayfish carapace width and the fish’s gape width to represent different relative size pairings. Foraging and shelter use
video assays were used to evaluate changes in crayfish behavior under the different levels of threat presented. The analysis revealed significant increases in macrophyte consumption, foraging effort, and overall
activity among crayfish that were small relative to their predators, independent of the absolute size of fish or
crayfish size. Crayfish that were small relative to predator size showed significantly less shelter use than
crayfish that were large relative to predator gape size. The results demonstrate that the crayfish are assessing
threat based on their size relative to the predator gape size solely using chemical signals. As a result of this
assessment, crayfish are changing their resource use to potentially grow larger to limit future predatory
threats. Overall, this research supports the threat-sensitive predator avoidance hypothesis and provides an
opportunity to refine the landscape of fear concept to accommodate more detailed risk assessments by prey.