American Society of Naturalists

Evan Twomey, Morgan Kain, Myriam Claeys, Kyle Summers, Santiago Castroviejo-Fisher, and Ines Van Bocxlaer (May 2020)

Color in poison frogs (Dendrobatidae) is mostly structural

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Skin nanostructures are the key to poison dart frog color mimicry

Mimicry is an adaptation where one species evolves color similarity to another species to avoid predation. Often, a single species evolves resemblance to multiple species, leading to color variation in the mimic species. How these shared colors evolve among distant relatives and how a single species is capable of evolving a suite of colors to matching multiple species is not well understood. We studied the mechanisms underlying color mimicry in the Peruvian poison dart frog species Ranitomeya imitator, and found that color variation and mimicry is largely controlled by the thickness of nanocrystals in the skin. These crystals are responsible for the structural color of the skin, which results in highly reflective coloration that may be important for producting effective warning coloration to avoid predation. Other species of poison dart frogs, including the species mimicked by Ranitomeya imitator as well as other poison dart frogs not involved in mimicry, also regulate color through structural mechanisms. These results demonstrate that much of the color diversity seen across poison frogs, including mimetic colors, is controlled by a single trait: the thickness of reflective nanocrystals in the skin. Mimicry appears to have evolved from a common set of color mechanisms found across poison dart frogs, rather than from novel color mechanisms having evolved ‘from scratch’ in the mimetic species. These results add further weight to the idea that convergent evolution often occurs through parallel evolution within a shared set of developmental pathways rather than the evolution of completely novel structures.

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Abstract

In animals, bright colors often evolve to mimic other species when a resemblance is selectively favored. Understanding the proximate mechanisms underlying such color mimicry can give insights into how mimicry evolves, for example, whether color convergence evolves from a shared set of mechanisms or through the evolution of novel color production mechanisms. We studied color production mechanisms in poison frogs (Dendrobatidae), focusing on the mimicry complex of Ranitomeya imitator. Using reflectance spectrometry, skin pigment analysis, electron microscopy, and color modeling, we found that the bright colors of these frogs, both within and outside the mimicry complex, are largely structural and produced by iridophores, but that color production depends crucially on interactions with pigments. Color variation and mimicry is regulated predominantly by iridophore platelet thickness and, to a lesser extent, concentration of the red pteridine pigment drosopterin. Compared to each of the four morphs of model species which it resembles, R. imitator displays greater variation in both structural and pigmentary mechanisms, which may have facilitated phenotypic divergence in this species. Analyses of non-mimetic dendrobatids in other genera demonstrate that these mechanisms are widespread within the family, and that poison frogs share a complex physiological “color palette” that can produce diverse and highly reflective colors.