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.



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Life Sciences; Biology



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crossing reaction norm, growth rate, food availability, predation, Brachyrhaphis rhabdophora, life history, Costa Rica

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Biology Commons