Life history trade-offs and phenotypic plasticity in the reproduction of Galapagos lava lizards (Microlophus delanonis)

Trade-offs between traits are a fundamental component of life history theory. However, because individuals may differ in the amount of energy available for allocation between traits, expected negative relationships are often difficult to detect or may become positive. The latter can occur when both traits are phenotypically plastic in response to variation in environmental productivity. We tested this hypothesis by analyzing expected trade-offs between egg mass and clutch size, and total clutch mass and post-nesting condition, in a population of Galapagos lava lizards (Microlophus delanonis) that experienced marked annual and seasonal variation in rainfall. Due to its close relationship with overall mass (reproductive plus post-nesting mass), we considered rainfall to be a suitable indicator of environmental productivity. Females that reproduced during periods of low rainfall produced smaller and fewer eggs, and had lower post-nesting body mass independent of body size (snout-vent length). This pattern was found in comparisons both between and within individuals that nested in dry and wet periods, respectively. The relationships between egg mass and clutch size, and total clutch mass and post-nesting mass, were positive when data were combined across years and seasons. Restriction of the analysis to comparisons within time periods (in effect, holding productivity constant), resulted in negative correlations between egg mass and clutch size, and weakened relationships between total clutch mass and post-nesting mass. We conclude that phenotypic plasticity in reproduction obscures life history trade-offs and that plasticity may be more important to the fitness of females than a particular value for an individual reproductive trait.