Data for: Size-dependent colouration balances conspicuous aposematism and camouflage

Colour is an important component of many different defensive strategies, but signal efficacy and detectability will also depend on the size of the coloured structures, and how pattern size interacts with the background. Consequently, size-dependent changes in colouration are common among many different species as juveniles and adults frequently use colour for different purposes in different environmental contexts. A widespread strategy in many species is switching from crypsis to conspicuous aposematic signalling as increasing body size can reduce the efficacy of camouflage, while other antipredator defences may strengthen. Curiously, despite being chemically defended, the gold-striped frog (<i>Lithodytes lineatus</i>, Leptodactylidae) appears to do the opposite, with bright yellow stripes found in smaller individuals whereas larger frogs exhibit dull brown stripes. Here, we investigated whether size-dependent differences in colour support distinct defensive strategies. We first used visual modelling of potential predators to assess how colour contrast varied among frogs of different sizes. We found that contrast peaked in mid-sized individuals while the largest individuals had the least contrasting patterns. We then used two detection experiments with human participants to evaluate how colour and body size affected overall detectability. These experiments revealed that larger body sizes were easier to detect, but that the colours of smaller frogs were more detectable than those of larger frogs. Taken together our data support the hypothesis that the primary defensive strategy changes from conspicuous aposematism to camouflage with increasing size, implying size-dependent differences in the efficacy of defensive colouration. We discuss our data in relation to theories of size-dependent aposematism and evaluate the evidence for and against a possible size-dependent mimicry complex with sympatric poison frogs (Dendrobatidae).

颜色是诸多防御策略的核心组成部分，但信号的有效性与可检测性同时取决于有色结构的尺寸，以及图案尺寸与背景环境的相互作用。因此，体型依赖的体色变化在众多物种中十分普遍——幼体与成体在不同环境背景下，往往出于不同的目的使用体色信号。许多物种普遍采用的一类策略是从隐蔽色（crypsis）转向醒目的警戒色信号（aposematic signalling）：随着体型增大，伪装的效能会逐渐降低，而其他抗捕食防御机制则可能得到增强。令人好奇的是，尽管金纹蛙（*Lithodytes lineatus*，细趾蟾科Leptodactylidae）本身具备化学防御能力，其体色变化却恰好相反：体型较小的个体拥有亮黄色条纹，而体型较大的个体则呈现暗淡的棕色条纹。本研究旨在探究体色的体型差异是否对应不同的防御策略。我们首先通过潜在捕食者的视觉模型，评估不同体型青蛙的体色对比度差异。结果发现，体型中等的个体对比度最高，而体型最大的个体其图案对比度最低。随后，我们开展了两项人类受试者检测实验，以评估体色与体型对整体可检测性的影响。实验结果显示，体型更大的个体更容易被察觉，但小型青蛙的体色比大型青蛙的体色更易被识别。综上，我们的数据支持下述假说：随着体型增长，主要防御策略会从醒目的警戒色（aposematism）转变为伪装，这表明体色防御的效能存在体型依赖性差异。我们结合体型依赖的警戒色理论对本研究数据展开了讨论，并评估了该蛙与同域分布的箭毒蛙（Dendrobatidae）可能形成体型依赖拟态复合体的相关支持与反对证据。