I explored contributions of phenotypic plasticity and local genetic differentiation to resource allocation patterns by juvenile salamanders from populations that are known to differ in age and size at maturity. Salamanders were raised in a realistic field-experimental system with a reciprocal transplant design crossed with a prey addition treatment. This design allowed simultaneous assessment of (1) environmental effects (prey and garden), (2) genetic effects (populations), and (3) interactions between environmental and genetic effects on overall allocation patterns and on particular response variables. Genetic differences in maturity but not growth were detected, suggesting local adaptation in age at maturity. Prey supplementation accelerated growth and increased gonad and fat body mass similarly in both populations. Gonad maturation was also accelerated by supplemental food, but the populations differed in the timing of this effect. Gonad development of low-elevation salamanders, known to mature earlier in nature, responded to high prey levels at smaller sizes (younger ages) than did high-elevation animals. The plasticity of overall allocation patterns in response to supplemental prey also differed between the populations. The result that these populations differ in maturation rates but not in growth potential suggests that growth capacity and maturation timing and size are free to evolve independently. I argue that correlations between growth rates and age and size at maturity observed in many taxa do not necessarily indicate causation but probably reflect correlated responses to local environments.
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Joseph Bernardo (1994) studied this question.