The way wear goes – phytolith-based wear on the dentine-enamel system in guinea pigs (Cavia porcellus)

The effect of phytoliths on tooth wear and function has been contested in studies of animal plant interactions. For herbivores whose occlusal chewing surface consists of enamel ridges in dentine tissue, the phytoliths might first erode the softer dentine, exposing the enamel ridges to different occlusal forces and thus leading to enamel wear. To test this hypothesis, we fed guinea pigs (Cavia porcellus; n=36 in 6 groups) for three weeks exclusively on dry or fresh forage of low (lucerne), moderate (fresh timothy grass) or very high (bamboo leaves) silica content representing corresponding levels of phytoliths. We quantified the effect of these treatments with measurements from micro CT scans. Tooth height indicated extreme wear of the bamboo diet that apparently brought maxillary incisors and molars close to the minimum required for functionality. There were negative relationships between a cheek tooth’s height and the depth of its dentine basin, corroborating the hypothesis that dentine erosion plays an important role in herbivore tooth wear. In spite of lower body mass, bamboo-fed animals had paradoxically longer cheek tooth rows, and larger occlusal surfaces. Because ever-growing teeth can only change in shape from the base upwards, this is a strong indication that failure to compensate for wear by dental height-growth additionally triggered general expansive growth of the tooth bases. The results suggest that enamel wear may occur in sequence after dentine wear and not the other way around, and illustrate a surprising plasticity in the reactivity of this rodent’s system that adjusts tooth growth to wear.

在动植物相互作用的相关研究中，植硅体（phytoliths）对牙齿磨损与功能的影响始终存在争议。对于咬合咀嚼面由牙本质组织内的釉质脊构成的植食性动物而言，植硅体可能首先侵蚀质地更柔软的牙本质，使釉质脊暴露于不同的咬合受力中，进而引发釉质磨损。为验证这一假说，我们将36只豚鼠（Cavia porcellus；分为6组，每组6只）连续三周仅喂食对应不同植硅体水平的干燥或新鲜饲草：硅含量低的紫花苜蓿、中等的新鲜梯牧草，以及极高的竹叶。我们通过显微CT扫描（micro CT scans）的测量数据量化了各处理组的效应。竹叶饲粮组豚鼠的牙齿高度显示出极端磨损，其上颌门齿与臼齿已近乎达到维持功能所需的最低高度。颊齿高度与牙本质窝深度呈负相关关系，这印证了牙本质侵蚀在植食性动物牙齿磨损中发挥重要作用的假说。尽管竹叶饲粮组豚鼠的体重更低，但它们的颊齿列却反常地更长，且咬合面面积更大。由于终生生长的牙齿仅能从牙基部向上发生形态变化，这一结果强烈表明：若无法通过牙齿高度生长补偿磨损，便会额外触发牙基部的整体扩张生长。研究结果提示，釉质磨损或许会在牙本质磨损之后依次发生，而非反之；同时也揭示了该啮齿类动物牙齿系统的惊人可塑性——其可通过调整牙齿生长以适应磨损。