Why so Toxic: How Coloration and Toxicity Evolve

By: Hailee Arena, Caitlin Swanson, Elizabeth Cravinho, and Matthew Healy (Stonehill College, BIO323: Evolution, Spring 2023)

Just like your ex, being toxic is not always obvious! Did you know that frogs can display warning signals that indicate their toxicity, known as aposematism? Aposematism is a strategy employed by organisms to advertise toxicity to deter predators through color patterns. While this is a common trait, conspicuous coloration does not always signal toxicity. What causes coloration to evolve? Can toxicity depend on other factors, such as body size or time of activity? Due to this knowledge gap between conspicuousness and toxicity, Drs. Roberts, Stuart-Fox, and Medina, from the University of Melbourne, conducted research titled, The evolution of conspicuousness in frogs: When to signal toxicity?, as published in the Journal of Evolutionary Biology. The researchers collected data on various frog species to determine whether toxicity is linked to coloration. They examined the chemical defenses in these species and sought to identify whether body size and daytime (diurnal) activity could be linked to conspicuousness in chemically defended frogs. Ultimately, the researchers hypothesized that the conspicuous coloration in diurnal species is directly related to toxicity.

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Figure 1 from Roberts et al. (2022). Phylogenetic tree of the frogs included in this study. The red lines indicate a chemically defended species, and the black lines indicate a non-chemically defended species. Each red or black line indicates the measured internal chromatic contrast.

Researchers began data collection by compiling sets of photographs, depicting both dorsal (back) and ventral (belly) views of frogs (order Anura). Images of 594 different frog species were obtained via iNaturalist, Cal Photos, and Google Images, with more than half being chemically defended species. Species identification was then confirmed by AmphibiaWeb. Further, in regions free of lighting artifacts, researchers utilized computer software to determine the intensity of red (R), green (G) and blue (B) coloring on the frogs. The RGB values were used to calculate the internal contrast between two colors on the frog’s body and the background contrast between the frog and its habitat (brown or green), which were used to compute chromatic (color) contrast and luminance (brightness) contrast. These values helped quantify the strength of frog warning signals. In addition, the body size and activity time were obtained from global databases. The frog species were categorized as either nocturnal, diurnal, or “no data”. A species that exhibited both nocturnal and diurnal traits was categorized as diurnal. The average body size of each species was extracted from previous research by Womack and Bell (2020).

The results of the study revealed that RGB color variables are strongly correlated with chemical defenses. Frogs that were diurnal and chemically defended had higher internal chromatic contrast (dorsal and ventral) against brown backgrounds. Furthermore, the researchers observed a higher dorsal and ventral internal chromatic contrast in chemically defended frogs when body size was smaller. However, they observed no relationship between body size and conspicuous coloration in species that are not chemically defended, which signifies conspicuousness may depend on chemical defense.

Figure 2 from Roberts et al. (2022). (a) Color contrast between the dorsal and ventral sides of the frogs’ bodies. (b) The relationships between conspicuous coloration and chemical defense, body size, and time of activity.

So, when do frogs evolve to signal their toxicity? The scientists concluded that when frogs are small and active during the day, they are more likely to evolve these colorations. Brighter-colored frogs are more vulnerable to predation, as they are likely to be spotted by predators during the day. Frogs with larger body size are seen to have cryptic (camouflage) coloration when chemically defended. The larger body size makes them vulnerable to predators, thus conspicuous coloration would not benefit these species. In fact, the warning signal would not ward off predators but attract them since the high source of energy, due to larger body mass, outweighs the risks of toxicity.

Ultimately, aposematism was found to be in chemically defended frogs with smaller body sizes and is more associated with diurnal species. Furthermore, frogs with the largest body size are most vulnerable to predation, thus driving natural selection to favor the evolution of warning signals in smaller frog species. From these findings, the researchers are hopeful that body size and time of activity may be applicable to additional species when determining the evolution of conspicuousness.

And remember while these frogs may be toxic, they won’t gaslight you at an IHOP.

Article: Roberts, S.M., D. Stuart-Fox, & I. Medina. 2022. The evolution of conspicuousness in frogs: When to signal toxicity? Journal of Evolutionary Biology 35: 1455–1464. https://doi.org/10.1111/jeb.14092

Womack, M.C., & R.C. Bell. 2020. Two-hundred million years of anuran body-size evolution in relation to geography, ecology and life history. Journal of Evolutionary Biology 33: 1417–1432.

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