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Responses of prey animals to odor cues from two predator species across interspecific size gradients

Aquatic
Fauna
Project Abstract: 
Prey animals must perform a balancing act between foraging for food, finding mates, and seeking shelter, while avoiding predation (Sih, 1980). If prey animals are overly cautious and respond to every predator cue, then they will expend unnecessary energy on predator avoidance behaviors (Brown, 1999). Such an aversion to risk will also severely limit the prey’s access to resources (Kats & Dill, 1998). If prey do not respond to predator cues, the probability that they will encounter a predator is increased. However, prey that risk ignoring predator cues have more time to acquire resources and can utilize more of their habitat (Lima & Dill, 1990). Neither extreme case is commonly observed in natural systems because prey use factors like predator proximity, species, relative size, and diet to only respond to the most threatening predators. Information relevant to all these factors can be extracted from the odor cues released by predators. Prey animals can differentiate the odors of predators from those of non-threatening species, recognize distinct predator species, and respond accordingly to predators with different hunting strategies (Gherardi et al., 2011). Odor cues also contain information about the predator’s diet. Predators that have recently consumed conspecific prey are perceived as greater threats than those that have not (Chivers and Mirza, 2001). Odor cues can provide information about the size of the predator as well. An individual prey animal may respond differently to a large or small predator of the same species (Apfelbach et al., 2005). Once prey have a detailed assessment of the threat posed by a predator, they can weigh the value of the resource against the risk required to obtain it. Currently, it is unknown how prey combine information about predator species and predator size relative to individual prey to assess risk and make resource use decisions. Parsons et al. (2018) specifically calls for studies which address this gap in knowledge by quantifying the food consumption and behavioral responses of prey animals exposed to different levels of threat via video analysis of prey responses. An ideal predator-prey system to examine this relationship exists in northern Michigan which involves predatory Largemouth and Smallmouth Bass, and the crayfish prey that they feed upon. Crayfish have well stereotyped predator avoidance behaviors and are known to feed on aquatic plants as adults.
Years Active: 
2019
Methods: 
A two by five fully factorial experiment will be conducted using predator species as a two-level factor and the ratio of prey size to predator gape size as a five-level factor. Largemouth Bass Exposure Smallmouth Bass Exposure Size Ratio N N < 80% 10 10 80 – 89.9% 10 10 90 – 99.9% 10 10 100 – 109.9% 10 10 ≥ 110% 10 10 Total N = 100 trials This research will be performed at the University of Michigan Biological Station Stream Research Facility, beginning in May 2019. During trials, predators and prey will be housed in two-part flow through stream mesocosms. A single predator (either largemouth or smallmouth bass) will be placed into the upstream section of each mesocosm to produce odor cues. Then, a single crayfish will be placed into the downstream section. A screened opening will separate the two sections but will allow predator odors to flow into the prey section. The crayfish will forage on samples of macrophyte tissue and will have access to shelter while being exposed to predator odor. Crayfish responses will be quantified through measurements of macrophyte consumption, foraging effort, shelter use, and movement frequency. Macrophyte samples will be weighed before and after each trial to assess consumption under threat. Behavioral variables will be assessed from infrared video recordings that occur each night from 0000 to 0400. Each trial will end by offering the live crayfish from the trial to the bass involved in the trial. Crayfish chelae will be fixed closed with super-glue to prevent them from potentially injuring the fish. Predators that consume the crayfish within an hour will be scored as a genuine threat while those that do not will be scored as a non-threat. Analyses will examine differences in crayfish behavior between predator species and across the predator gape size to prey size ratios. The effects of predator species and relative size ratio may interact as well. Based on the predator consumption of crayfish, we will also estimate the prey size preference of each predator species.