Beavers and muskrats are semi-aquatic, generalist herbivorous rodents regarded as invasive in southern South America, with high impacts on the ecosystem. They share some morphological characters of the skull, but different body sizes and phylogenetic history, showing evident shape differences in young and adult skulls during the development in both species. Considering their similar ecological specializations, skull shape could be achieved through different or similar patterns of allometric growth during ontogeny. We analyzed quantitatively the ontogenetic series including 94 specimens of beavers and muskrats and performed multivariate and bivariate analyses considering 20 linear measurements. Our main results from the different approaches suggest high differences in the ontogenetic trajectories of beavers and muskrats, implying disparity in the muscular, functional, and structural conditions of the skull of both species. These differences reflect that although skulls might undergo similar mechanical stress, it is possible to reach a cranial morphology compatible with the similar behavioral and ecological specializations between both species from patterns of skull development that are markedly different.

海狸和麝鼠是半水生、通才的草食性啮齿动物，在南美洲南部被视为入侵动物，对生态系统具有重大影响。它们具有相同的头骨形态特征，但体型大小和系统发育历史不同，在两个物种的发育过程中显示出幼年和成年头骨的明显形状差异。考虑到它们相似的生态专业化, 头骨的形状可以通过个体发育过程中不同或相似的异速生长模式来实现。我们定量分析了个体发育系列，包括 94 个海狸和麝鼠标本，并进行了考虑 20 个线性测量的多变量和双变量分析。我们从不同方法中得出的主要结果表明，海狸和麝鼠的个体发育轨迹存在很大差异，这意味着这两个物种的头骨的肌肉、功能和结构状况存在差异。这些差异反映出，尽管头骨可能会承受相似的机械应力，但有可能从明显不同的头骨发育模式中获得与两个物种之间相似的行为和生态特化相兼容的颅骨形态。