Plasticity in butterfly egg size: Why larger offspring at lower temperatures?

Dividing sister pairs of the butterfly Bicyclus anynana (reared in a common environment) between high and low temperature shows that oviposition temperature induces a plastic response in egg size. Females at a lower temperature laid significantly larger (but fewer) eggs than their sisters kept at a higher temperature, whereas total reproductive investment increased with temperature. Cross-transfer experiments demonstrated that this plastic response in egg size is reversible. Interestingly, this pattern parallels an almost universal temperature-induced developmental response in ectotherm body size. In both cases, however, we do not yet understand the underlying mechanisms or the potential adaptive significance. By cross-transferring the experimentally manipulated eggs between temperatures, we showed that the larger eggs produced at a lower temperature had a higher hatching success, and yielded larger hatchlings with a slightly higher probability of reaching maturity and shorter larval development time (at the lower temperature) compared to the smaller eggs produced at a higher temperature. Overall, the lower temperature was slightly more detrimental for smaller than for larger eggs. The slight differences in survival across temperatures, together with more substantial differences in fecundity, indicate that it could pay off to produce fewer but larger offspring (with increased fitness) at a lower temperature, but more and smaller offspring at a higher temperature where offspring survival was generally high. Our data present for the first time evidence that selection may favor larger eggs at a lower temperature, and thus that a. plastic response. of egg size to temperature in ectotherms could be adaptive.