A risky defence by a spider using conspicuous decoys resembling itself in appearance

One Darwinian puzzle is that many prey are morphologically or behaviourally conspicuous. Being conspicuous seems detrimental to prey with limited antipredator ability, because conspicuousness is likely to increase detection by predators. Theory predicts that if conspicuousness effectively redirects predator attacks and decreases hunting success, the benefits of enhanced survival can outweigh the cost of conspicuousness. We investigated a conspicuous antipredator behaviour in the orb spider, Cyclosa mulmeinensis, which adds decorations composed of prey pellets and eggsacs to its web. We calculated colour contrasts of spiders against prey pellets and eggsacs and recorded predators' responses to spiders on webs in the field. Webs with more decorations suffered increased numbers of wasp attacks. However because these decorations resembled spiders in size and colour, they functioned as decoys to distract predators and became the focus of predator attacks. By quantifying the costs and benefits of Cyclosa spiders' web-decorating behaviour in a natural setting, we found that this conspicuous antipredator display would enhance overall survival and was adaptive for this vulnerable prey.