Enlarged Egg Size Increases Offspring Fitness of a Frog Species on the Zhoushan Archipelago of China.

Egg size represents maternal investment and is an important life-history trait. It also is assumed to have an effect on offspring fitness. Life-history theory predicts that oviparous animals on islands will produce enlarged eggs because of increased maternal investment to improve offspring performance to facilitate intra-specific competition. The life-history theory, developed during the 1950s, provides a possible explanation for the "island rule", but this rule has seldom been tested. Although several studies have detected a positive relationship between egg size and offspring fitness, it is difficult to exclude the covarying effects on offspring performance, such as genetic variation and developmental plasticity; predictions made using the life-history theory on the islands have not been tested. In this study, we have evaluated the relationship between egg size and offspring fitness on 20 islands in the Zhoushan Archipelago and two nearby mainland sites. To exclude covarying effects, we compared larval performance among different egg sizes in three levels: among siblings within clutches, among clutches within populations, and among different islands. The results showed that frogs on most of the islands did produce enlarged eggs and that their larvae had improved larval fitness. Additionally, at all three levels, the offspring that evolved from enlarged eggs had increased offspring fitness. The results of this study indicate that, for the first time, the life-history theory predictions concerning egg size and offspring fitness are supported.