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）指出，当大陆动物被隔离于岛屿环境时，大体型类群往往趋向体型缩小，而小体型类群则倾向于体型增大。巴西东部的小型蛙类黄侧雨蛙（Phyllodytes luteolus, Wied-Neuwied, 1824）广泛分布于与凤梨科植物伴生的生境中。末次冰期结束之际，海平面上升导致部分陆地演变为岛屿，由此将该蛙类的种群隔离于这些岛屿之上。若岛屿法则成立，我们预测岛屿种群的蛙类体型应大于大陆种群。为此，我们分别随机选取了大陆种群的30只雄性蛙与巴西圣埃斯皮里图州某岛屿种群的30只雄性蛙，对其体型（体重和体长）开展了对比分析。此外，我们还对岛屿与大陆的种群密度进行了采样调查——鉴于体型变化存在潜在因果关联，种群密度或也将随之改变。正如预测所示，岛屿种群的蛙类在吻肛长（snout-vent length）与体重上均表现出更大的体型，且种群丰度显著更高。尽管本研究未专门针对这一现象展开深入探讨，但捕食者与种间竞争者的缺失或可解释上述两项观测结果。