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).

色彩是诸多防御策略的核心组成部分，但其信号功效与可探测性同时取决于有色结构的尺寸，以及斑纹大小与背景环境的相互作用。因此，诸多物种普遍存在体色的尺寸依赖性变化，因为幼体与成体往往在不同环境背景下出于不同目的使用体色信号。诸多物种普遍采用的一类防御策略为：随体型增长从隐蔽色（crypsis）转向醒目的警戒色信号（aposematic signalling）——因为体型增大可降低保护色的防御功效，而其他反捕食防御机制则会随之增强。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，而体型较大的个体条纹则呈黯淡的棕褐色。本研究旨在探究体色的尺寸依赖性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建模（visual modelling），评估不同体型蛙类的色彩对比度差异，结果发现对比度在中等体型个体中达到峰值，而体型最大的个体其斑纹对比度最低。随后我们开展了两项人类受试者检测实验，以评估体色与体型如何影响整体可探测性。实验结果显示，体型更大的蛙类更易被探测，但小型蛙的体色比大型蛙的体色更具可探测性。综合来看，我们的数据支持以下假说：随着体型增长，主要防御策略会从醒目的警戒色转向保护色，这意味着防御体色的功效存在尺寸依赖性差异。最后，我们结合体色尺寸依赖性警戒色相关理论解读本研究数据，并评估了该蛙类与同域分布（sympatric）箭毒蛙科（Dendrobatidae）可能存在尺寸依赖性拟态复合体的相关证据。