Data from: High frequency echolocation, ear morphology, and the marine–freshwater transition: a comparative study of extant and extinct toothed whales

This study compares the bony ear morphology of freshwater and marine odontocetes (toothed whales). Odontocetes are unique among marine mammals in two important respects: 1) they use echolocation; 2) at least three lineages have independently evolved obligate freshwater habits from marine ancestries. Freshwater odontocetes include the so-called “river dolphins,” a paraphyletic group that each evolved convergent external morphological characters that distinguish them from oceanic dolphins (Delphinoidea). In addition to their convergent external morphology, “river dolphins” all have echolocation that use one peak (narrow-band) frequency around 100 kHz, compared to oceanic delphinoids which use a two peak (bimodal) frequency ranging from 40 to 140 kHz. The differences in echolocation suggest that the sensory systems responsible for detecting these different sound frequencies should also differ, although quantitative assessments of the cetacean hearing system remain understudied and taxonomically undersampled. To test if ear bone morphology reflects underlying environmentally driven differences in echolocation ability, we assembled a dataset of odontocete periotics (n = 114) from extant and fossil species. We examined 18 external and three internal linear periotic measurements, the latter of which were examined using cone-beam scanning tomography. Results from multivariate canonical ordination analyses show that periotic height, periotic thickness and pars cochlearis width collectively explain the largest amount of interspecific variation in our dataset. Because these particular ear bone measurements correspond to acoustic hearing ranges, we propose that they are also proxies for environmental preference (i.e., marine, freshwater and intermediate habitats) and may be useful for deciphering environmental preferences of extinct odontocetes.本研究对比了淡水与海洋齿鲸（odontocetes，toothed whales）的骨性耳形态结构。齿鲸在海洋哺乳动物中具备两项独特特征：其一，它们具备回声定位（echolocation）能力；其二，至少有三个演化支系从海洋祖先独立演化出专性淡水栖息习性。淡水齿鲸类包含所谓的“河豚类”，这是一个并系群，其各个分支均演化出了区别于海洋海豚总科（Delphinoidea）的趋同外部形态特征。除趋同的外部形态特征外，“河豚类”的回声定位系统均采用约100千赫的单峰窄带（narrow-band）频率；而海洋海豚总科类群则采用40至140千赫的双峰（bimodal）频率。回声定位的差异提示，负责检测不同声波频率的感觉系统理应存在差异，但目前针对鲸类听觉系统的定量评估研究仍较为匮乏，且分类学采样不足。为验证耳骨形态是否能反映回声定位能力受环境驱动的差异，我们收集了现生及化石物种的齿鲸耳围骨（periotic）样本共114份（n=114）。我们对18项外部耳围骨线性测量指标与3项内部指标进行了分析，其中内部指标通过锥形束扫描断层成像（cone-beam scanning tomography）获取。多元典范排序分析结果显示，耳围骨高度、耳围骨厚度以及耳蜗部（pars cochlearis）宽度这三项指标，共同解释了本数据集内最大的种间变异量。由于这些特定的耳骨测量指标与听觉声学频率范围直接相关，我们认为它们可作为环境偏好（即海洋、淡水及过渡生境）的替代指标，同时也可用于推断已灭绝齿鲸类的栖息环境偏好。