Inversely related aposematic traits: reduced conspicuousness evolves with increased toxicity in a polymorphic poison-dart frog.

Prevailing theory contends that aposematic coloration evolves in tandem with toxicity so that the evolution of increased toxicity will accompany the evolution of greater conspicuousness. Although variation in aposematic coloration within single species should be selectively constrained, because individuals varying from a predator-recognized warning signal will incur greater risk of predation, several species of poison-dart frogs display remarkable phenotypic variation. This variation may have evolved to match different levels of toxicity, and these species provide excellent opportunities to examine the evolution of aposematic coloration. Here, I test whether increased conspicuousness in the granular poison-dart frog evolved in tandem with increased toxicity. Contrary to classical predictions, toxicity assays, spectral reflectance measurements, and phylogenetic reconstruction reveal that the less conspicuous color morphs are actually significantly more toxic than the brightest, most conspicuous phenotypes and that the more toxic, less-conspicuous form evolved from a less toxic, more conspicuous ancestor. Through gas chromatography--mass spectrometry analysis of toxin profiles, I traced the increase in toxicity in the less-conspicuous populations to an acquisition of specific alkaloids, some of which are proven convulsants. These results challenge the tenet that increased conspicuousness always evolves with increased toxicity and support the idea that once aposematism has been established in a species, phenotypic variation may evolve from brightness and toxicity becoming decoupled.

Wang IJ 《-》

Rapid color evolution in an aposematic species: a phylogenetic analysis of color variation in the strikingly polymorphic strawberry poison-dart frog.

Wang IJ ，Shaffer HB 《EVOLUTION》

Poison frog colors are honest signals of toxicity, particularly for bird predators.

Antipredator defenses and warning signals typically evolve in concert. However, the extensive variation across taxa in both these components of predator deterrence and the relationship between them are poorly understood. Here we test whether there is a predictive relationship between visual conspicuousness and toxicity levels across 10 populations of the color-polymorphic strawberry poison frog, Dendrobates pumilio. Using a mouse-based toxicity assay, we find extreme variation in toxicity between frog populations. This variation is significantly positively correlated with frog coloration brightness, a viewer-independent measure of visual conspicuousness (i.e., total reflectance flux). We also examine conspicuousness from the view of three potential predator taxa, as well as conspecific frogs, using taxon-specific visual detection models and three natural background substrates. We find very strong positive relationships between frog toxicity and conspicuousness for bird-specific perceptual models. Weaker but still positive correlations are found for crab and D. pumilio conspecific visual perception, while frog coloration as viewed by snakes is not related to toxicity. These results suggest that poison frog colors can be honest signals of prey unpalatability to predators and that birds in particular may exert selection on aposematic signal design.

Maan ME ，Cummings ME 《-》

Rapid population divergence linked with co-variation between coloration and sexual display in strawberry poison frogs.

The likelihood of speciation is assumed to increase when sexually selected traits diverge together with ecologically important traits. According to sexual selection theory, the evolution of exaggerated display behavior is driven by increased mating success, but limited by natural selection, for example, through predation. However, the evolution of aposematic coloration (i.e., an ecologically important trait) could relieve the evolution of exaggerated display behavior from the bound of predation, resulting in joint divergence in aposematic coloration and sexual display behavior between populations. We tested this idea by examining conspicuousness, using color contrasts between individuals and their native backgrounds, and sexual display of 118 males from genetically diverged populations of the Strawberry poison frog, Dendrobates pumilio. Our results show that the level of conspicuousness of the population predicts the sexual display behavior of males. Males from conspicuous populations used more exposed calling sites. We argue that changes in aposematic coloration may rapidly cause not only postmating isolation due to poorly adapted hybrids, but also premating isolation through shifts in mating behaviors.

Rudh A ，Rogell B ，Håstad O ，Qvarnström A ... -  《-》

An Analysis of Predator Selection to Affect Aposematic Coloration in a Poison Frog Species.

Dreher CE ，Cummings ME ，Pröhl H 《PLoS One》