Santana Sharlene E, Grossnickle David M, Sadier Alexa, Patterson Edward, Sears Karen E
Department of Biology, University of Washington, Seattle, WA.
Burke Museum of Natural History and Culture, University of Washington, Seattle, WA.
Integr Comp Biol. 2022 May 16. doi: 10.1093/icb/icac042.
The evolution of complex dentitions was a major innovation in mammals that facilitated the expansion into new dietary niches that imposed selection for tight form-function relationships. Teeth allow mammals to ingest and process food items by applying forces produced by a third-class lever system composed by the jaw adductors, the cranium, and the mandible. Physical laws determine changes in jaw adductor (biting) forces at different bite point locations along the mandible (outlever), thus individual teeth are expected to experience different mechanical regimes during feeding. If the mammal dentition exhibits functional adaptations to mandible feeding biomechanics, then teeth are expected to have evolved to develop mechanically-advantageous sizes, shapes, and positions. Here, we present bats as a model system to test this hypothesis and, more generally, for integrative studies of mammal dental diversity. We combine a field-collected dataset of bite forces along the tooth row with data on dental and mandible morphology across 30 bat species. We (1) describe, for the first time, bite force trends along the tooth row of bats, (2) use phylogenetic comparative methods to investigate relationships among bite force patterns, tooth and mandible morphology, and (3) hypothesize how these biting mechanics patterns may relate to the developmental processes controlling tooth formation. We find that bite force variation along the tooth row is consistent with predictions from lever mechanics models, with most species having the greatest bite force at the first lower molar. The cross-sectional shape of the mandible body is strongly associated with the position of maximum bite force along the tooth row, likely reflecting mandibular adaptations to varying stress patterns among species. Further, dental dietary adaptations seem to be related to bite force variation along molariform teeth, with insectivorous species exhibiting greater bite force more anteriorly, narrower teeth and mandibles, and frugivores/omnivores showing greater bite force more posteriorly, wider teeth and mandibles. As these craniodental traits are linked through development, dietary specialization appears to have shaped intrinsic mechanisms controlling traits relevant to feeding performance.
复杂齿系的进化是哺乳动物的一项重大创新，它促进了哺乳动物向新的饮食生态位扩张，而这又对紧密的形态-功能关系施加了选择压力。牙齿使哺乳动物能够通过运用由颌内收肌、颅骨和下颌骨组成的三级杠杆系统产生的力量来摄取和处理食物。物理定律决定了沿着下颌骨（外杠杆）不同咬点位置的颌内收肌（咬合力）的变化，因此，单个牙齿在进食过程中预计会经历不同的力学状态。如果哺乳动物的齿系表现出对下颌骨进食生物力学的功能适应性，那么牙齿预计已经进化出具有力学优势的大小、形状和位置。在这里，我们将蝙蝠作为一个模型系统来检验这一假设，更广泛地说，用于对哺乳动物牙齿多样性的综合研究。我们将实地收集的沿齿列的咬合力数据集与30种蝙蝠的牙齿和下颌骨形态数据相结合。我们（1）首次描述了蝙蝠齿列上的咬合力趋势，（2）使用系统发育比较方法来研究咬合力模式、牙齿和下颌骨形态之间的关系，以及（3）推测这些咬合力学模式可能如何与控制牙齿形成的发育过程相关。我们发现，沿齿列的咬合力变化与杠杆力学模型的预测一致，大多数物种在下颌第一磨牙处具有最大咬合力。下颌体的横截面形状与沿齿列最大咬合力的位置密切相关，这可能反映了下颌骨对不同物种间应力模式变化的适应性。此外，牙齿的饮食适应性似乎与沿臼齿状牙齿的咬合力变化有关，食虫物种在前部表现出更大的咬合力、更窄的牙齿和下颌骨，而食果/杂食物种在后部表现出更大的咬合力、更宽的牙齿和下颌骨。由于这些颅齿特征在发育过程中相互关联，饮食特化似乎塑造了控制与进食性能相关特征的内在机制。
