Data from: Architecture of the sperm whale forehead facilitates ramming combat

Herman Melville’s novel Moby Dick was inspired by historical instances in which large sperm whales (Physeter macrocephalus L.) sank 19th century whaling ships by ramming them with their foreheads. The immense forehead of sperm whales is possibly the largest, and one of the strangest, anatomical structures in the animal kingdom. It contains two large oil-filled compartments, known as the “spermaceti organ” and “junk”, that constitute up to one-quarter of body mass and extend one-third of the total length of the whale. Recognized as playing an important role in echolocation, previous studies have also attributed the complex structural configuration of the spermaceti organ and junk to acoustic sexual selection, acoustic prey debilitation, buoyancy control, and aggressive ramming. Of these additional suggested functions, ramming remains the most controversial, and the potential mechanical roles of the structural components of the spermaceti organ and junk in ramming remain untested. Here we explore the aggressive ramming hypothesis using a novel combination of structural engineering principles and probabilistic simulation to determine if the unique structure of the junk significantly reduces stress in the skull during quasi-static impact. Our analyses indicate that the connective tissue partitions in the junk reduce von Mises stresses across the skull and the load-redistribution functionality of the former is insensitive to tissue material parameters, the thickness of the partitions, and variations in the location and angle of the applied load. Absence of the connective tissue partitions increases skull stresses, particularly in the rostral aspect of the upper jaw, further hinting of the important role the architecture of the junk may play in ramming events. Our study also found that impact loads on the spermaceti organ generate lower skull stresses than an impact on the junk. Nevertheless, whilst an impact on the spermaceti organ would reduce skull stresses, it would also cause high compressive stresses on the anterior aspect of the organ and the connective tissue case, possibly making these structures more prone to failure. This outcome, coupled with the facts that the spermaceti organ houses sensitive and essential sonar producing structures and the rostral portion of junk, rather than the spermaceti organ, is frequently a site of significant scarring in mature males suggest that whales avoid impact with the spermaceti organ. Although the unique structure of the junk certainly serves multiple functions, our results are consistent with the hypothesis that the structure also evolved to function as a massive battering ram during male-male competition.

赫尔曼·梅尔维尔的小说《白鲸记》的创作灵感来源于历史事件：大型抹香鲸（Physeter macrocephalus L.）曾用前额撞击击沉19世纪的捕鲸船。抹香鲸硕大的前额可能是动物界中体积最大、形态最奇特的解剖结构之一，其内部包含两个大型充油腔室，分别为鲸蜡器（spermaceti organ）与junk结构（junk），这两个腔室的总重量可达体重的四分之一，延伸至鲸鱼体长的三分之一。学界公认该结构在回声定位（echolocation）中发挥重要作用，此前的研究还将鲸蜡器与junk结构的复杂构造归因于声学性选择（acoustic sexual selection）、声学致瘫猎物（acoustic prey debilitation）、浮力调控（buoyancy control）以及攻击性撞击（aggressive ramming）等功能。在上述提出的附加功能中，撞击假说仍最具争议，且鲸蜡器与junk结构的组成部件在撞击过程中的潜在力学作用尚未得到验证。本研究结合结构工程原理与概率模拟方法，对攻击性撞击假说展开探究，以明确junk结构的独特构造能否在准静态冲击（quasi-static impact）过程中显著降低颅骨所受应力。分析结果表明，junk结构内的结缔组织隔膜（connective tissue partitions）可降低颅骨整体的冯·米塞斯应力（von Mises stresses），且该结构的载荷再分配功能不受组织材料参数、隔膜厚度以及施加载荷的位置与角度变化的影响。若移除这些结缔组织隔膜，则会升高颅骨所受应力，尤其是在上颌吻侧区域（rostral aspect of the upper jaw），这进一步暗示junk结构的构造可能在撞击事件中发挥重要作用。本研究还发现，对鲸蜡器施加冲击载荷时，颅骨所受应力低于对junk结构施加冲击时的情况。不过，尽管对鲸蜡器施加冲击可降低颅骨应力，但会在该器官前部区域与结缔组织外壳处产生较高的压应力（compressive stresses），可能使这些结构更易发生破损。这一结果，结合鲸蜡器内存在敏感且不可或缺的声呐生成结构（sonar producing structures），以及成熟雄性抹香鲸的junk结构吻侧区域（而非鲸蜡器）常存在明显疤痕这两项事实，表明抹香鲸会规避对鲸蜡器施加冲击。尽管junk结构的独特构造显然具备多种功能，但本研究结果与该假说相符：该结构在演化过程中同时承担了雄性间竞争时的巨型撞击锤功能。