Toothed whales (Odontoceti, Cetacea) are well-known for their ability to produce complex vocalizations, to use tools, to possess self-recognition, and for their extreme behavioural plasticity. The toothed whale intelligence is said to compete with that of primates, so does their extremely large brain to body size ratio. Common explanations for the acquisition of such large brains over the evolutionary time (encephalization) in toothed whales range from their demanding, complex social lives, to their feeding habits, to echolocation. Yet, several studies found no macroevolutionary trend in Odontoceti encephalization, which casts doubts on its selective advantage. We applied a recently developed phylogenetic comparative method to study macroevolutionary trends in relative brain size (RBS) and brain size evolutionary rates in cetaceans, comparing toothed whales to the other cetaceans and contrasting groups of species as ascribed to different feeding categories. We found that cetaceans as a whole followed a trend for increased encephalization over time, starting from small-brained archaeocete ancestors. Toothed whales do not show this same trend in RBS but have possessed larger RBS than any other cetacean ever since the beginning of their existence. The rate of RBS evolution in Odontoceti is significantly slower than in other Cetacea and slower than the rate of Odontoceti body size evolution. These results suggest that toothed whales’ history is characterized by high and conservative relative encephalization. Feeding lifestyle does not explain these patterns, while the appearance of echolocation within stem group Odontoceti remains a viable candidate for them.

中文翻译：

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齿鲸的宏观进化异常的相对大脑大小

齿鲸（Odontoceti，Cetacea）以产生复杂声音，使用工具，具有自我认知能力以及极高的行为可塑性而闻名。据说，齿鲸的智力可与灵长类竞争，而它们极大的大脑与身体的大小比也是如此。齿鲸在进化时间（脑性化）中获得如此大的大脑的常见解释，从要求苛刻，复杂的社会生活，饮食习惯到回声定位。然而，几项研究发现，牙牙本质脑组织没有大的进化趋势，这对其选择优势产生了怀疑。我们应用了最近开发的系统发育比较方法来研究鲸类相对大脑大小（RBS）和大脑大小进化率的宏观进化趋势，将带齿鲸与其他鲸类进行比较，并根据不同的喂养类别对物种进行对比。我们发现，从小脑的古细菌祖先开始，整个鲸类都遵循随时间而增加的脑化趋势。齿鲸在苏格兰皇家银行中并没有表现出同样的趋势，但是自其开始存在以来，拥有比其他鲸类鲸更大的苏格兰皇家银行。Odontoceti中RBS进化的速度明显慢于其他鲸类，并且比Odontoceti体型进化的速度慢。这些结果表明，齿鲸的历史特征是相对偏高和保守。喂养方式不能解释这些模式，而茎组牙形刺中回声定位的出现仍然是可行的选择。