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

Abstract

Zebras stripes cannot protect them from predators, Darwin concluded, and current consensus tends to support his view1,2. In principle, stripes could support crypsis or aposematism, could dazzle, confuse or disrupt predators perception3-8, yet no such effects are manifest in predator-prey interactions9-11. Instead, narrow stripes covering zebras head, neck, limbs and flanks are an effective deterrent to tabanids12, vectors for equine disease13,14. Accordingly, while other potential benefits, e.g., thermoregulation15,16 and intraspecific communication17, cannot be excluded, zebra stripes likely evolved primarily to deter parasites18-20. 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-deterrence12 and vary with hyaena threat18, 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. 1 HighlightsO_LIZebra stripes likely evolved to deter biting flies, but rump stripes are ill-suited to this. C_LIO_LIRump-stripes remain highly conspicuous when viewed at distance or in motion. C_LIO_LIComputational models and human observers judge rump stripes are most salient stripes. C_LIO_LISalient rump stripes drive predator attention to rear, hindering capture by predators. C_LIO_LIObserve this striking effect in moving zebra at: viscog.psychol.cam.ac.uk/resources-and-downloads C_LI