Variation in somatic growth rates has interested biologists for decades because of the relationship between growth and other fitness-determining traits (i.e. fecundity, survival, and body size), and the corresponding effect of somatic growth on production of organisms humans use for food. The interaction between genetic variation in growth rates and environmentally induced variation in growth rates shows the pattern of growth across multiple environments (i.e. the reaction norm) that clarifies the history and potential future of evolutionary change in growth rates among populations. Theoretical predictions suggest variation in predator-induced mortality rates can influence mean growth rates and the shape of the reaction norm for growth. The adaptive growth hypothesis predicts that mean growth rates would evolve in response to environmental pressures, such as mortality rates, at different body sizes. Few studies, however, have focused on variation in reaction norms for growth in response to resource availability between high-predation and low-predation environments. We used juvenile Brachyrhaphis rhabdophora from high-predation and low-predation environments to test for variation in mean growth rates and for variation in reaction norms for growth at two levels of food availability in a common-environment experiment, and we compared field somatic growth rates in juveniles from the same two environments (high-predation and low-predation). In the common-environment experiment, mean growth rates did not differ between predation environments, but the interaction between predation environment and food level took the form of a crossing reaction norm for both growth in length and growth in mass. Fish from low-predation environments exhibited no significant variation in growth rate between high and low food amount treatments. In contrast, fish from high-predation environments exhibited wide variation in growth rates between low and high food treatments, with higher food availability resulting in higher growth rates. In the field, individuals in the high-predation environment grow at a faster rate than those in a low-predation environment at the smallest sizes (comparable to sizes in the common-environment experiment). These data provide no evidence for evolved differences in mean growth rates between predation environments. However, fish from high-predation environments exhibited greater plasticity in growth rates in response to resource availability suggesting that increased risk of predation could drive variation in food availability for prey and consequent selection for plasticity.

Individual organisms face trade-offs when dealing with predation—more time spent avoiding predators often results in less time allocated to energy acquisition and reproductive-related activities. Individuals that optimize this trade-off and respond appropriately to current risk levels in their environment should be at an advantage. What is less clear is whether this tradeoff changes when individuals are repeatedly exposed to a predation threat. There may be advantages to responding consistently to every signal of threat, but it might also be advantageous to modulate individual behavior. Moreover, it is unclear how evolutionary history of a population might affect such individual responses. Our study was designed to address two questions: (1) how do B. rhabdophora respond to repeated exposures of a predation cue; and (2) do repeated responses differ based on evolutionary history? To answer these questions, we used a predation cue stimulus to repeatedly expose B. rhabdophora individuals from both high- and low-predation populations. We measured the change in total distance traveled in a 15-minute trial before and after each cue exposure, and then compared the proportional change in response to the first cue to that of each successive cue (repeated four times) to see if a decrease in response occurred. We found that fish responded consistently to each cue exposure. Both populations showed similar decreases in activity in response to each exposure and did not return to normal baseline activity until the cue was removed from the test tank. That both high- and low-predation populations respond consistently to repeated cues of predation with no reinforcement prompts questions as to the potential importance of the relative length of risk and safe periods in affecting response variation. It also provides a starting point in understanding how recent risk exposure and the evolutionary history of risk in a population both interact to influence individual response to threats over time.