The island rule in the Brazilian frog Phyllodytes luteolus (Anura: Hylidae): incipient gigantism?

ABSTRACT The island rule suggests that, when mainland animals are isolated on islands, large animals tend to become smaller, while small animals tend to become larger. A small frog in eastern Brazil, Phyllodytes luteolus (Wied-Neuwied, 1824), is widely distributed in association with bromeliads. At the end of the last glaciation, parts of the mainland became islands due to rising sea levels, thereby isolating frog populations on these islands. If the island rule holds, we predicted that frogs on islands would tend to be larger than frogs on the mainland. We compared sizes (weight and length) of 30 randomly selected male frogs from the mainland with 30 from an island in the state of Espírito Santo, Brazil. We also sampled population density on the island and mainland because concurrent with changing sizes, depending on the causal relationship, density may also change. As predicted, island frogs tended to be larger (both in snout-vent length and weight) and were much more abundant. While not specifically addressed in this study, the absence of predators and interspecific competitors may explain both of these trends.

摘要 岛屿法则（island rule）提出，当大陆动物被隔离于岛屿之上时，大体型类群往往趋于小型化，而小体型类群则趋于大型化。巴西东部的小型蛙类黄顶雨滨蛙（*Phyllodytes luteolus* (Wied-Neuwied, 1824)）与凤梨科植物（bromeliads）伴生，分布范围广泛。末次冰期结束之际，海平面上升致使部分大陆区域演变为岛屿，由此将栖息于这些岛屿上的蛙类种群与大陆种群相互隔离。若岛屿法则成立，我们预测岛屿上的蛙类比大陆个体更大。我们选取了大陆地区随机挑选的30只雄性蛙类，与巴西圣埃斯皮里图州一座岛屿上的30只雄性蛙类，对二者的体重及吻肛长两项体型指标进行了比较。此外，我们还对该岛屿与对应大陆区域的种群密度开展了采样调查：鉴于体型变化的同时，依据其因果作用路径的不同，种群密度亦可能发生相应改变。研究结果与预测一致：岛屿上的蛙类体型更大（吻肛长与体重均如此），且种群丰度显著更高。尽管本研究未针对该问题展开专门探讨，但天敌与种间竞争者的缺失，或可解释上述两项观测趋势。