Spottier Targets Are Less Attractive to Tabanid Flies: On the Tabanid-Repellency of Spotty Fur Patterns

During blood-sucking, female members of the family Tabanidae transmit pathogens of serious diseases and annoy their host animals so strongly that they cannot graze, thus the health of the hosts is drastically reduced. Consequently, a tabanid-resistant coat with appropriate brightness, colour and pattern is advantageous for the host. Spotty coats are widespread among mammals, especially in cattle (Bos primigenius). In field experiments we studied the influence of the size and number of spots on the attractiveness of test surfaces to tabanids that are attracted to linearly polarized light. We measured the reflection-polarization characteristics of living cattle, spotty cattle coats and the used test surfaces. We show here that the smaller and the more numerous the spots, the less attractive the target (host) is to tabanids. We demonstrate that the attractiveness of spotty patterns to tabanids is also reduced if the target exhibits spottiness only in the angle of polarization pattern, while being homogeneous grey with a constant high degree of polarization. Tabanid flies respond strongly to linearly polarized light, and we show that bright and dark parts of cattle coats reflect light with different degrees and angles of polarization that in combination with dark spots on a bright coat surface disrupt the attractiveness to tabanids. This could be one of the possible evolutionary benefits that explains why spotty coat patterns are so widespread in mammals, especially in ungulates, many species of which are tabanid hosts.

在吸血行为中，虻科（Tabanidae）雌性个体不仅会传播多种严重疾病的病原体，还会对宿主动物造成极强的惊扰，致使宿主无法正常采食，进而显著降低宿主的健康水平。因此，具备适宜亮度、色彩与斑纹的抗虻被毛对宿主具有演化优势。斑点被毛在哺乳动物中广泛分布，尤以原牛（Bos primigenius，家牛的野生祖先）为典型代表。本研究通过野外实验，探究了斑纹尺寸与数量对受线偏振光（linearly polarized light）吸引的虻类的靶面吸引力的影响。我们测定了活体家牛、斑点家牛被毛以及实验所用靶面的反射偏振特性。研究结果表明，斑纹尺寸越小、数量越多，靶标（宿主）对虻类的吸引力便越低。我们还证实，若靶标仅在偏振角分布上呈现斑点特征，但其灰度均匀且偏振度始终维持在较高水平，其斑点图案对虻类的吸引力同样会被削弱。虻类对线偏振光具有强烈的响应，本研究显示家牛被毛的明暗区域反射的光线具有不同的偏振度与偏振角，而亮底色上的深色斑点恰好能够破坏这种偏振特征组合，从而降低对虻类的吸引力。这或许正是解释斑点被毛在哺乳动物中广泛存在的潜在演化优势之一——尤其在有蹄类动物中，其诸多类群均为虻类的宿主。