From crypsis to masquerade: Ontogeny changes the colour defences of a crab spider hiding as bird droppings

Selection imposed by visually hunting predators has driven the evolution of colour‐based antipredator defence strategies such as crypsis, masquerade, mimicry and aposematism. Individuals of many animals are generally considered to rely on a single type of defence strategy, but individuals of some species use multiple colour‐based defences. Many animals switch between colour‐based defences against visually hunting predators during ontogeny. However, why this occurs remains poorly understood. The crab spider Phrynarachne ceylonica is an often‐cited example of a bird dropping masquerade. It has recently been demonstrated that P. ceylonica crab spiders gain protection from their predators by being misidentified as bird droppings by their predators. P. ceylonica females show an ontogenetic shift in colour defences: early instars possess a dark and cryptic form, while at later instars and as adults, the spiders resemble bird droppings. We hypothesised that this shift may be driven by differential changes in predation risk of two defence strategies with increasing body size due to ontogeny. We tested this hypothesis by presenting naïve domestic chicks with 3D printed artificial spiders of two different sizes (small, large) and two colours (dark, bird dropping‐like), and determined whether larger bird dropping‐like spiders are more readily found and attacked than cryptic forms by chicks. We found that small cryptic spiders were more difficult to detect than small bird dropping masquerading spiders, but large cryptic spiders were attacked much more quickly and more frequently than large bird dropping masquerading spiders. Increasing predation pressure on larger, cryptic spiders during ontogeny suggests that switching to bird dropping masquerade may be a more effective defence as spiders increase in size. We thus conclude that the ontogenetic shift from crypsis to masquerade is adaptive. 摘要 在依靠视觉捕食的捕食者所施加的强大选择压力下，很多动物演化出了诸多基于颜色的防御对策，比如隐蔽色、伪装、拟态和警戒色等。过去通常认为，很多动物依赖于一种一成不变的、单一类型的防御对策，但越来越多的证据显示有些物种会采用多种基于颜色的防御对策。例如，很多动物在其个体发育过程中，会在不同防御对策之间进行转换。但是，动物在个体发育过程中转换不同防御对策的原因仍然不清楚。 锡兰瘤蟹蛛的鸟粪伪装常被演化生物学家作为经典案例引用，因为它们从外观上会被捕食者误认为是鸟粪，进而躲避捕食者的攻击。同时，雌性锡兰瘤蟹蛛在其个体发育过程中也会发生体色的转换，即基于颜色防御的转变：低龄幼蛛体色多为较暗的隐蔽色，而从四龄幼蛛开始到成熟雌蛛，其体色均为与鸟粪极为相似的伪装色。因此，我们推测这两种不同的防御策略可能是随着个体发育，体型增大，被捕食的压力增加而产生的变化。 为了验证这个假设，本研究以小鸡作为捕食者，以用3D打印技术打印出的不同体型（小和大）、不同体色（暗色和鸟粪色）的蜘蛛模型为猎物，进行了一系列的小鸡捕食实验，以探究体型大的鸟粪色蜘蛛模型是否会比体型小的暗色蜘蛛模型更易被小鸡发现，且更易受到攻击。实验结果显示，小鸡很难发现体型小、暗色的蜘蛛模型，但对于体型大的蜘蛛模型来说，暗色个体比鸟粪色个体遭到小鸡更快、更多地攻击。 实验证明锡兰瘤蟹蛛随着个体发育，其体型不断增大，鸟