Sex-specific shifts in morphology and diet in a frog after 50 years of anthropogenic habitat fragmentation

Aim: Phenotypic shifts are commonly observed when animals face insular habitat change and may reflect ongoing stresses on individuals. However, the generality and the driving processes of this 'island rule' remain equivocal, notably in amphibians. Here, we investigate both morphological and dietary shifts in a frog using a mosaic of human-created islands to assess the potential operating mechanisms underlying these phenotypic responses.
Location: Thousand Island Lake, China.
Taxon: The Chinese piebald odorous frog, Odorrana schmackeri.
Results: We found insular dwarfism in female but not male frogs. Meanwhile, insular females also had smaller gape widths after accounting for snout-vent lengths (SVLs) than mainland females. According to SEMs, resource availability had a direct positive effect on body size. Finally, diet composition differed between island and mainland populations, but only in females. Males and females on islands exhibited greater overlaps in diet.
Main conclusions: In contrast with most studies in amphibians, we found insular dwarfism rather than gigantism in females. The smaller gape width after accounting for SVL in insular females suggests potential changes in prey utilization or food availability on these human-created islands. This notion is further supported by the differentiation of diet composition between island and mainland females. The higher diet overlap between sexes implies stronger intersexual competition for food resources after habitat fragmentation. Overall, we found shifts in morphology and diet in frogs which may implicate the influence of habitat fragmentation and underscores the need to consider intersexual differences when assessing responses of species to anthropogenic disturbances.

研究目的：当动物面临岛屿生境变化时，通常可观测到表型转变（phenotypic shifts），这类转变或可反映个体正承受的持续胁迫。然而，这一岛屿法则（island rule）的普遍性及其驱动机制仍尚不明确，在两栖动物类群中尤为如此。本研究以中国花斑臭蛙（Odorrana schmackeri）为对象，依托人工营造的岛屿镶嵌生境，探究其形态与食性的转变，以此解析这类表型响应背后的潜在作用机制。
研究地点：中国千岛湖。
研究类群：中国花斑臭蛙（Odorrana schmackeri）。
研究结果：我们仅在雌性蛙类中发现了岛屿矮化现象，雄性未呈现该特征。同时，在校正吻肛长（snout-vent lengths，SVL）后，岛屿种群雌性的口裂宽度仍显著小于大陆种群雌性。通过结构方程模型（Structural Equation Models）分析可知，资源可获得性对体型具有直接正向作用。最后，岛屿与大陆种群的食性组成仅在雌性个体间存在显著差异；岛屿种群内的雌雄个体食性重叠度更高。
主要结论：与多数两栖动物相关研究结果相悖，我们在雌性个体中发现的是岛屿矮化现象而非体型巨型化。岛屿种群雌性在校正吻肛长后仍具有更小的口裂宽度，这提示人工营造的岛屿上猎物利用策略或食物可获得性可能发生了改变。岛屿与大陆种群雌性间的食性组成差异进一步佐证了这一推论。种群内雌雄个体更高的食性重叠度，意味着生境破碎化后雌雄个体间存在更激烈的食物资源竞争。综上，我们在蛙类中观测到的形态与食性转变，可能与生境破碎化的影响相关；该研究同时强调，在评估物种对人为干扰的响应时，需纳入雌雄个体间的差异维度。