Fitness consequences of temperature-mediated egg size plasticity in a butterfly

1. By randomly dividing adult females of the butterfly Bicyclus anynana, reared in a common environment, among high and low temperatures, it is demonstrated that oviposition temperature induces a plastic response in egg size. Females kept at a lower temperature laid significantly larger eggs than those ovipositing at a higher temperature. 2. Cross-transferring the experimentally manipulated eggs between temperatures and investigating hatching success showed that a lower rearing temperature is more detrimental for the smaller eggs produced at a higher temperature than for the larger eggs produced at a lower temperature, supporting an adaptive explanation. 3. However, when examining two potential mechanisms for an increased fitness of larger offspring (higher desiccation resistance of larger eggs and higher starvation resistance of larger hatchlings), no direct link between egg size and offspring fitness was found. Throughout, i.e. even under benign conditions, larger offspring had a higher fitness. 4. Therefore, egg size should be viewed as a conveniently measurable proxy for the plastic responses induced by temperature, but caution is needed before implying that egg size per se is causal in influencing offspring traits.