Abstract
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.
Highlights
Variation in egg size is widespread in oviparous animals
The results indicated that island characteristics, represented by island size and inherent isolation, had an effect on the rice frog’s maternal investment: stronger on small islands than on big islands and the mainland
The results indicated that rice frog offspring on the islands had a higher level of fitness than those on the mainland
Summary
Variation in egg size is widespread in oviparous animals. The maternal investment, genetic variation, and developmental plasticity were proposed as the causes of this variation, and were thought to have an impact on offspring fitness[6,7,8,9]. The rice frogs present on the islands provide an ideal chance to investigate the effect of maternal investment on egg size variation and offspring fitness. The rice frog is a small grey anuran with a wide distribution in China[25], and it is one of the most abundant frog species in the study area[17,26] They start breeding in late April and continue to late August. With found clutches and a short hatching time, rice frogs provide an ideal opportunity to test the effects of egg size on offspring fitness. In this paper, according to the life-history theory and the characteristics of rice frogs, we tried to answer the following questions: (1) does the rice frog on the island have higher maternal investment and offspring fitness; and (2) does the enlarged egg size cause high larval fitness for this species on the islands?
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