Does Exposure to Predator Cues Influence Movement Behavior of Postmetamorphic Juvenile Rana aurora?
Exposure to predators during early life stages can alter behavioral and physical traits during ontogeny. This is important for biphasic amphibian species because juvenile movement behavior can be shaped by carry-over effects of the larval environment and direct effects of abiotic conditions in terrestrial habitats. We explored the interaction of aquatic predator cue exposure, juvenile morphology, and abiotic terrestrial conditions on the movement behavior of postmetamorphic Northern Red-legged Frogs (Rana aurora). We quantified carry-over effects of aquatic predator cues and direct effects of substrate conditions on individual movement behavior using a combination of seminatural enclosed runways and powder tracking assays. We found a latent effect of aquatic condition, such that crayfish-exposed individuals had greater relative body condition at metamorphosis, and all predator-exposed individuals had larger body lengths relative to controls. Terrestrial conditions were the dominant factor shaping movement behavior. During runway movement assays, individual conditional movement displacement was likely greater in dry soil and low humidity conditions than in moist conditions with high humidity. For nocturnal powder tracking assays, individuals were more likely to have greater path distances during warmer temperatures, whereas paths were less meandering in cooler, dry conditions. Morphologically larger individuals moved greater distances only during the powder tracking assays. We observed latent effects of aquatic predator exposure on juvenile morphology; however, the relationship between these changes and the relative importance of direct environmental conditions in shaping movement behavior is unclear. Our results demonstrate the interconnected nature of aquatic and terrestrial environments in shaping the movement of juvenile amphibians.ABSTRACT
The four morphological measurements taken on juvenile Northern Red-Legged Frog (Rana aurora) including head width, arm length, hind leg length, and snout–vent body length (also known as snout–vent length).
Morphological traits of each larval treatment group (Ctl = control, Cray = crayfish cue exposure, Trout = Rainbow Trout cue exposure). Circles indicate mean values; error bars illustrate ± 1 standard error. For morphological measures other than body length, we used a residual index independent of body length created by residuals from a log–log linear regression of each trait with individual body length multiplied by 100. In total we had 7 morphological responses of A: body length, B: relative body condition (residual index of mass), C: relative leg length (residual index of hind leg length), D: relative arm length (residual index of arm length), E: relative head width (residual index of head width), F: relative emergence size (residual index of body size at emergence), and G: relative emergence condition (residual index of mass at emergence). Morphological traits with significant post hoc univariate ANOVAs are marked with an asterisk by the plot label. Individuals from control conditions had the smallest body length, whereas individuals from crayfish and trout treatments were 4.84% and 3.43% larger, respectively. After controlling for the effect of postmetamorphic body length, individuals exposed to crayfish cue were also more likely to have greater relative body condition at emergence compared with control and trout treatments.
Partial residual plots of the impact of ambient relative humidity and runway condition on the conditional movement displacement of individuals that initiated movement in the runway assay. Shaded areas represent 95% confidence intervals. Response is modelled based on an individual from the control treatment of average body length and relative size at emergence. Individuals that initiated movement in the runway assay had the greatest movement displacement distances in the dry runway treatments and when the relative humidity was low.
Partial residual plots of the mean response in total path distance in the powder tracking assay from the impacts of ambient temperature (A) and body length in combination with runway movement initiation (B). The color in panel B represents the individuals runway initiation behavior, which interacted with their body length, with red representing individuals that did not initiate movement, and blue for those that did. Shaded areas represent 95% confidence intervals. Response is modelled based on an individual from the control treatment of average body length and relative size at emergence. In panel A, we can see that for each Celsius degree increase in temperature, total path distances increased by 12.4% (95% CI = 5.86 to 19.3%). In panel B, total path distances increased on average by 12.5% (95% CI = 5.9 to 19.6%) for each millimeter increase in an individual's body length. Additionally, individuals that initiated movement in the earlier runway assay reduced their total movement distance in this powder tracking assay by 16.7% (95% CI = 2.6 to 28.7%).
Partial residual plots of the mean path straightness index in the powder tracking assay based on the interaction of ambient temperature and relative humidity. The straightness index represents a ratio of net displacement with distance traveled. An index of 1 is a straight line, while a twisting circular path can approach 0. Color gradient represents the mean response with each combination of temperature and humidity, with red shades representing more-direct movement paths and blue shades representing more tortuous paths. Response is modelled based on an individual from the control treatment of average body length and relative size at emergence with average temperature and humidity. Individual path straightness index was highest (most efficient) in low temperatures and humidity, while warmer temperatures in combination with higher relative humidity increase path tortuosity.
Partial residual plots of the mean straightness index in the powder tracking assay based on the movement initiation behavior in the runway assay. The straightness index represents a ratio of net displacement with distance traveled. An index of 1 is a straight line, while a twisting circular path can approach 0. Shaded areas represent 95% confidence intervals. Response is modelled based on an individual from the control treatment of average body length and relative size at emergence with average temperature and humidity. Individual path straightness indices were reduced on average by 0.085 (95% CI = 0.013 to 0.156) for individuals that initiated movement during the earlier runway assay compared with those that did not initiate movement.
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