Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses

Averting attack by biting flies is increasingly regarded as the evolutionary driver of zebra stripes, although the precise mechanism by which stripes ameliorate attack by ectoparasites is unknown. We examined the behaviour of tabanids (horse flies) in the vicinity of captive plains zebras and uniformly coloured domestic horses living on a horse farm in Britain. Observations showed that fewer tabanids landed on zebras than on horses per unit time, although rates of tabanid circling around or briefly touching zebra and horse pelage did not differ. In an experiment in which horses sequentially wore cloth coats of different colours, those wearing a striped pattern suffered far lower rates of tabanid touching and landing on coats than the same horses wearing black or white, yet there were no differences in attack rates to their naked heads. In separate, detailed video analyses, tabanids approached zebras faster and failed to decelerate before contacting zebras, and proportionately more tabanids simply touched rather than landed on zebra pelage in comparison to horses. Taken together, these findings indicate that, up close, striped surfaces prevented flies from making a controlled landing but did not influence tabanid behaviour at a distance. To counteract flies, zebras swished their tails and ran away from fly nuisance whereas horses showed higher rates of skin twitching. As a consequence of zebras’ striping, very few tabanids successfully landed on zebras and, as a result of zebras’ changeable behaviour, few stayed a long time, or probed for blood.规避吸血蝇类的攻击，日益被视作斑马条纹的进化驱动力，但其条纹减轻体外寄生虫（ectoparasites）侵扰的确切机制仍不明晰。我们于英国一处马场，对圈养平原斑马（plains zebras）与纯色家养马（domestic horses）周边的虻（tabanids，horse flies，即马蝇）行为展开观测。结果显示，单位时间内降落至斑马被毛（pelage）的虻数量少于家马，但虻在斑马与马的被毛周围盘旋或短暂触碰的频次并无显著差异。在一项让马匹依次穿戴不同颜色布制外套（cloth coats）的实验中，穿戴条纹图案外套的马匹，其外套被虻触碰与降落的频次远低于穿戴纯黑或纯白外套的同一匹家马，但二者裸露头部（naked heads）受到的攻击频次并无差异。在独立开展的详细视频分析（video analyses）中，虻接近斑马的速度更快，且在接触斑马前未完成减速（decelerate）；相较家马，更多虻仅触碰斑马被毛而未实现可控降落（controlled landing）。综合上述结果可知，在近距离范围内，条纹表面可阻碍蝇类完成可控降落，但不会影响虻在远距离的行为。为抵御蝇类，斑马会甩动尾巴（swished their tails）并逃离蝇群的骚扰（fly nuisance），而家马则表现出更高频率的皮肤抽搐（skin twitching）。得益于斑马的条纹特征，极少有虻能成功降落至斑马体表；又因斑马的灵活避险行为，鲜有虻能长时间停留或刺吸血液（probed for blood）。