Dark frogs like it cool

Frogs and toads have darker colours the colder it is in their habitat, the more pathogens threaten them there and the more they are exposed to ultraviolet radiation. Researchers from the Philipps University of Marburg, the Museum für Naturkunde Berlin and the University of Erfurt found this out by comparing data from more than 40 percent of all known frog and toad species. The research group reports on their results in the scientific magazine Nature Communications.

Bright warning tones, inconspicuous camouflage costumes – frogs and toads display an impressive variety of colours. The colouration also depends on external conditions: the shape of the animals influences their geographic distribution and vice versa. "In view of climate change, those processes that are based on external characteristics and underlie the distribution of species are increasingly coming into scientific focus," says lead author Ricarda Laumeier from the Department of Animal Ecology at Philipps-Universität Marburg.

Ecology knows various rules according to which colouring affects the spatial distribution of cold-blooded animals that do not keep their body temperature constant. "Darker coloured species have an advantage in cold environments because darker bodies heat up faster than lighter ones," explains Laumeier. "Darker colours also provide better protection against ultraviolet radiation. It is also assumed that darker species have better protection in warm and humid conditions enjoy greater protection against the penetration of pathogens such as fungi and bacteria, which thrive particularly well there." A warmer environment, on the other hand, offers advantages for lighter coloured species as they reflect light better and thus avoid overheating.

"How extensive the significance of these functions is for ectothermic species has so far remained unclear," adds Laumeier's colleague Dr. Stefan Pinkert, another lead author of the study; "Previous studies focused primarily on European and North American species. In order to find out whether this is a regularity, we examined on a global scale whether the current distribution of frog urchins and the composition of communities can be explained by physiological functions of colouration."

The team chose the animal group of frogs or anurans, i.e. frogs and toads. "In comparison to other cold-blooded groups, distribution information and data on pathogen infestation are uniquely rich and complete for anurans. This means that these connections can be tested so comprehensively for the first time," explains Pinkert.

The research group compared data from 3,059 species, or 41 percent of all known frogs and toads. "It was confirmed that the brightness decreases continuously with decreasing temperature of the habitat and warm, humid conditions, but also with increasing ultraviolet radiation," reports Laumeier.

The results also show that closely related species are similar in terms of the brightness of their colouring. "Together with the distribution of certain families, this suggests that the evolution of colour brightness has favored the colonisation of temperate climates by a few, closely related lineages of frog urchins," concludes Pinkert.

Dr. Mark-Oliver Rödel, curator of herpetology at the Museum für Naturkunde Berlin, also points out that global warming and invasive pathogens are contributing to the decline of amphibians around the world. The results presented could help predict which frog and toad species are particularly at risk and where they live.

Publication

Ricarda Laumeier & al.: The global importance and interplay of colour-based protective and thermoregulatory functions in frogs, Nature Communications 2023, DOI: https://doi.org/10.1038/s41467-023-43729-7