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，通常会观测到表型偏移现象，此类偏移或可反映个体正承受的生存压力。然而，这一“岛屿法则”的普遍性及其驱动过程仍存在争议，在两栖动物类群中尤为显著。本研究依托人工构建的千岛湖群岛生境，针对一种蛙类的形态与食性变化展开探究，以评估介导此类表型响应的潜在作用机制。
研究地点：中国千岛湖。
研究类群：中国花臭蛙（Odorrana schmackeri）。
研究结果：我们仅在雌性蛙类中发现了岛屿侏儒症。在校正吻肛长（snout-vent lengths, SVLs）后，岛屿种群雌性的口裂宽度也较大陆种群雌性更小。通过结构方程模型（Structural Equation Models, SEMs）分析发现，资源可获得性对体型具有直接正向影响。最后，岛屿与大陆种群的食性组成存在差异，但该差异仅存在于雌性个体中；岛屿种群雌雄个体间的食性重叠度更高。
主要结论：与多数两栖动物相关研究结果相悖，我们在雌性个体中观测到的是岛屿侏儒症而非岛屿巨症。岛屿种群雌性在校正吻肛长后口裂宽度更小，这提示人工创建的岛屿上猎物利用方式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分化进一步佐证了这一观点。雌雄个体间更高的食性重叠度则意味着生境破碎化后，两性间为食物资源产生的竞争更为激烈。综上，我们在蛙类中观测到的形态与食性偏移，或暗示生境破碎化的影响，同时也凸显了在评估物种对人为干扰的响应时，考虑雌雄个体间差异的必要性。