Supplementary Material for: Comparison of Dolphins' Body and Brain Measurements with Four Other Groups of Cetaceans Reveals Great Diversity

We compared mature dolphins with 4 other groupings of mature cetaceans. With a large data set, we found great brain diversity among 5 different taxonomic groupings. The dolphins in our data set ranged in body mass from about 40 to 6,750 kg and in brain mass from 0.4 to 9.3 kg. Dolphin body length ranged from 1.3 to 7.6 m. In our combined data set from the 4 other groups of cetaceans, body mass ranged from about 20 to 120,000 kg and brain mass from about 0.2 to 9.2 kg, while body length varied from 1.21 to 26.8 m. Not all cetaceans have large brains relative to their body size. A few dolphins near human body size have human-sized brains. On the other hand, the absolute brain mass of some other cetaceans is only one-sixth as large. We found that brain volume relative to body mass decreases from Delphinidae to a group of Phocoenidae and Monodontidae, to a group of other odontocetes, to Balaenopteroidea, and finally to Balaenidae. We also found the same general trend when we compared brain volume relative to body length, except that the Delphinidae and Phocoenidae-Monodontidae groups do not differ significantly. The Balaenidae have the smallest relative brain mass and the lowest cerebral cortex surface area. Brain parts also vary. Relative to body mass and to body length, dolphins also have the largest cerebellums. Cortex surface area is isometric with brain size when we exclude the Balaenidae. Our data show that the brains of Balaenidae are less convoluted than those of the other cetaceans measured. Large vascular networks inside the cranial vault may help to maintain brain temperature, and these nonbrain tissues increase in volume with body mass and with body length ranging from 8 to 65% of the endocranial volume. Because endocranial vascular networks and other adnexa, such as the tentorium cerebelli, vary so much in different species, brain size measures from endocasts of some extinct cetaceans may be overestimates. Our regression of body length on endocranial adnexa might be used for better estimates of brain volume from endocasts or from endocranial volume of living species or extinct cetaceans.

本研究将成熟海豚与其余4类成熟鲸类开展对比分析。依托大规模数据集，我们发现5个不同分类类群的鲸类大脑存在显著多样性。本数据集涵盖的海豚个体体质量范围约为40~6750 kg，脑质量范围为0.4~9.3 kg，体长范围为1.3~7.6 m。其余4类鲸类的合并数据集显示，其体质量范围约为20~120000 kg，脑质量范围为0.2~9.2 kg，体长范围为1.21~26.8 m。并非所有鲸类的脑质量相对于体质量都处于较高水平：部分体型接近人类的海豚拥有与人类相当的脑质量，而部分其他鲸类的绝对脑质量仅为人类的1/6。我们发现，相对于体质量的脑体积从海豚科（Delphinidae）、鼠海豚科与一角鲸科（Phocoenidae and Monodontidae）类群、其他齿鲸类群、鳁鲸超科（Balaenopteroidea）最终降为露脊鲸科（Balaenidae）。在以体长为参照对比脑体积时，我们也观察到了相同的总体趋势，仅海豚科与鼠海豚科-一角鲸科类群间无显著差异。露脊鲸科的相对脑质量与大脑皮层表面积均为所有类群中最低。脑各分区的占比同样存在差异：相对于体质量与体长，海豚的小脑体积为所有类群中最大。若排除露脊鲸科类群，大脑皮层表面积与脑大小呈等距生长关系。本研究数据显示，露脊鲸科的大脑褶皱程度低于其他被测鲸类。颅腔内的大型血管网络或有助于维持脑部温度，这类非脑组织的体积随体质量与体长增长，占颅内体积的比例范围为8%~65%。由于不同物种的颅内血管网络及其他附属结构（如小脑幕（tentorium cerebelli））存在显著差异，基于部分已灭绝鲸类颅内模估算的脑尺寸可能存在高估。我们建立的体长与颅内附属结构体积的回归模型，或可用于基于颅内模或现存、已灭绝鲸类颅内体积更精准地估算脑体积。