How the Zebra got its Rump Stripes: Salience at Distance and in Motion

Zebras’ stripes cannot protect them from predators, Darwin concluded, and current consensus tends to support his view[1][1],[2][2]. In principle, stripes could support crypsis or aposematism, could dazzle, confuse or disrupt predators’ perception[3][3]–[8][4], yet no such effects are manifest in predator-prey interactions[9][5]–[11][6]. Instead, narrow stripes covering zebras’ head, neck, limbs and flanks are an effective deterrent to tabanids[12][7], vectors for equine disease[13][8],[14][9]. Accordingly, while other potential benefits, e.g., thermoregulation[15][10],[16][11] and intraspecific communication[17][12], cannot be excluded, zebra stripes likely evolved primarily to deter parasites[18][13]–[20][14]. Rump stripes, however, do not fit this, or any extant view. Typically horizontal and broader in sub-species with width variation, they are ill-suited to crypsis or parasite-deterrence[12][7] and vary with hyaena threat[18][13], perhaps shaped by an additional selective pressure. We observed that rump (and rear-flank) stripes remain highly conspicuous when viewed in motion or at distance, while other stripes do not. To study this striking effect, we filtered images of zebra to simulate acuity limitations in lion and hyaena photopic and mesopic vision. For mountain zebra and plains zebra without shadow striping, rump stripes were the most conspicuous image regions according to computational salience models, corroborated by human observers’ judgements of maximally attention-capturing image locations, which were strongly biased toward the rear. By hijacking exogenous attention mechanisms to force predator attention to the rear, salient rump stripes confer benefits to zebra, estimated here in pursuit simulations. Benefits of rump stripe salience may counteract anti-parasite benefits and costs of conspicuity to shape rump and shadow stripe variation. Highlights ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-8 [5]: #ref-9 [6]: #ref-11 [7]: #ref-12 [8]: #ref-13 [9]: #ref-14 [10]: #ref-15 [11]: #ref-16 [12]: #ref-17 [13]: #ref-18 [14]: #ref-20