Developing echolocation: distinctive patterns in the ontogeny of the tympanoperiotic complex in baleen and toothed whales (Cetacea)

Cetaceans (baleen and toothed whales) present a unique set of adaptations for life in water. Among other abilities, the two living groups can hear and produce different sound frequencies: baleen whales use low frequencies primarily for communication, while toothed whales acquired the ability to echolocate using high frequency sounds. Both groups exhibit modifications to the morphology of the ear bones (tympanic bulla and periotic) that closely track their behaviour and ecology. While the evolution of sound reception in whales is being thoroughly investigated, the changes in prenatal development (ontogeny) that generate these disparate ear bone morphologies remain mostly unknown. In this study, we characterize the ontogeny of the ear bones in Cetacea by looking at the progression of ossification and associated changes in morphology using a combination of traditional measurements and an innovative landmark-free method to quantify shape on a newly assembled 3D dataset spanning the ontogeny and phylogeny of extant Cetacea. We found that the two groups of Cetacea share some aspects of inner ear ontogeny, such as a common growth trajectory of the periotic. However, differences in ossification, allometry and growth trajectory, particularly in the tympanic bone, reflect their divergent inner ear morphology and hearing abilities.

鲸类（Cetacea，涵盖须鲸与齿鲸两大支系）演化出诸多适配水生生活的独特适应性特征。现存两类鲸类具备差异化的声频感知与发声能力：须鲸主要依靠低频声波开展通讯，而齿鲸则演化出利用高频声波进行回声定位的能力。二者的耳骨——鼓泡（tympanic bulla）与围耳骨（periotic）——均发生了适应性形态改造，且改造模式与物种的行为习性及生态位高度契合。尽管学界已对鲸类声波感知的演化过程展开了充分研究，但催生二者耳骨形态差异的产前发育（个体发生ontogeny）机制仍鲜为人知。本研究依托全新构建的、覆盖现生鲸类个体发育与系统发育维度的三维数据集（3D dataset），结合传统形态测量法与创新的无标志点形状量化方法（landmark-free method），通过分析耳骨的骨化进程及伴随的形态变化，系统刻画了鲸类耳骨的个体发生过程。研究结果显示，两类鲸类的内耳个体发育存在部分共性，例如围耳骨具有共同的生长轨迹。然而二者在骨化模式、异速生长（allometry）规律及生长轨迹上的差异（尤以鼓泡最为显著），恰恰反映了它们在内耳形态与听觉能力上的分化。