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Size-dependent colouration balances conspicuous aposematism and camouflage.
Barnett JB
,
Yeager J
,
McEwen BL
,
Kinley I
,
Anderson HM
,
Guevara J
.
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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 (Lithodytes lineatus, 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).
FIGURE 1
Study system. Top. Examples of convergent evolution in conspicuous colouration between Lithodytes lineatus (Leptodactylidae) and two poison frogs (Dendrobatidae), Phyllobates lugubris (allopatric) and Dendrobates tinctorius (sympatric): (a) = L. lineatus, (b) = P. lugubris, and (c) = D. tinctorius. Bottom. Size dependent colouring found in Lithodytes lineatus: (d) = small (19 mm), (e) = medium (38 mm) and (f) = large (58 mm). Photo credits: (a) = Martin Hinojosa, (b & c) = James B. Barnett and (d–f) = all authors.
FIGURE 2
Visual modelling. Left: Spectrophotometry reflectance curves (300–700 nm) from (a) the dorsum, stripe and spots of a mid-sized (39 mm) Lithodytes lineatus and (b) four colour classes of forest floor leaves (inset squares show exaggerated colours to differentiate plotted lines). There was minimal UV reflectance (300–400 nm) from the frog and from the leaf litter background. Middle and right: Internal pattern contrast represented as (c) chromatic and (d) achromatic contrast between the dorsum and the stripe, and (e) chromatic and (f) achromatic contrast between the dorsum and the spots. The solid black lines represent the estimated GAM smoothing curves, the grey shading represents 95% confidence intervals, and the horizontal dashed line represents the visual discrimination threshold (3 JND). There was no significant effect of body size on chromatic contrast, however, achromatic contrast changed with body size. The achromatic contrast of the stripes peaked in mid-sized individuals, whereas the achromatic contrast of the spots decreased with increasing body size.
FIGURE 3
Detection experiments. Top-left: The 12 colour treatments used in both experiments (S = small (<20 mm), M = medium (30–40 mm), L = large (>50 mm) | A = dorsal, B = dorsal and spot, C = dorsal and stripe, D = dorsal, stripe, and spot). Top-right: An example of the experimental stimuli (treatment MD at 35 mm on the leaf litter background—highlighted by white circle). Bottom. Response time (seconds; means ±95% CI from the model) for each experiment (green = S, orange = M, black = L). Silhouettes indicate relative size between S, M, and L treatments. In experiment 1 (left) all treatments were presented at a standardized size (35 mm), and in experiment 2 (right) each size class was presented at its natural size (S = 20 mm, M = 35 mm and L = 50 mm).