Fine‐tuned responses to chemical landscapes: crayfish use predator odors to assess threats based on relative size ratios

Abstract

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.

Are you scared yet? Variations to cue indices elicit differential prey behavioral responses even when gape-limited predators are relatively small

“…However, as the results show, crayfish are responding to more than one source of chemosensory information. In line with previous studies, crayfish respond to increasing bass gape ratio by decreasing foraging time and choosing to shelter more (Wood and Moore 2020). The present study highlights that crayfish shelter more often when bass gape ratio and bass abundance interact, but significance is only seen for the highest bass abundance treatment (Fig.…”

“…Water flowed into the upstream predator section of each mesocosm before overflowing through a screened opening (28 cm × 12 cm opening with 1 mm × 1 mm screening) in a partial wall into the downstream prey section. The water overflowing through the screened opening did not exceed 5 mm in depth, which is an inadequate depth for crayfish to see bass in the predator section of the arena (Wood and Moore 2020). The water exited from the downstream end of the mesocosm through another screened opening.…”

“…The three different components were the number of predators present, the summed total length of the predators present, and the average gape ratio of the predators present. The number of predators serves as a proxy manipulation of the overall surface area of predator skin available to release predatory odors (Wisenden 2015;Wood and Moore 2020). Number of predators ranged from zero (controls) to a maximum of three.…”

“…Bass were chosen in a pseudorandom way, as we wanted to have a range of gape ratios present in this study. Gape ratios ranged from 0.311 to 0.753 and were calculated by dividing the gape of the bass by the carapace width of the crayfish and taking the average of this measure if multiple bass predators were present (Wood and Moore 2020). Historically, gape ratios of less than 0.9 for these predators indicated that the bass cannot consume or swallow the crayfish (Hill et al 2004).…”

“…Upon completion of trials, the screened openings between the predator and crayfish section and the outflow from the crayfish section were brushed to remove any debris that might inhibit water/odor flow. Because only four trials were run each day, mesocosms were allowed to flush for at least 24 h between trials, which is sufficient time for cue dissipation to occur (Wood and Moore 2020).…”