What the coloring of frogs and toads reveals

Tropical tree frog

The tree frog Boana boans is found in the rainforests of Central and South America. © Mark-Oliver Rödel

Whether grass green, inconspicuous brown or bright orange: frogs and toads come in many different colors. A study of over 3,000 species now shows that the appearance of these amphibians not only determines how conspicuous or camouflaged they are in their respective habitat, but that it also has a far-reaching influence on their ability to survive. Accordingly, lighter-colored animals have a higher heat tolerance, but are also more susceptible to UVB radiation and certain pathogens. From the research team's perspective, the results could help assess the resilience of various frogs and toads to climate change.

For frogs and toads, their respective coloring is important in order to avoid predators. Some species use bright colors to warn potential predators that they are poisonous. Others adapt so well to their surroundings with camouflage colors that they can hardly be seen between leaves, on trees or in the mud. Previous studies have already suggested that the coloring also fulfills other functions. For example, darker colors absorb more light, which can allow cold-blooded amphibians to warm up more quickly in the sun.

Color influences temperature regulation

A team led by Ricarda Laumeier from the Philipps University of Marburg has now examined in more detail what influence body coloration has on frogs and toads. To do this, the researchers collected data from 3,059 species of frogs and toads worldwide - almost half of all species from the order Anura. A geographical-climatic connection emerged: “Our study shows that the brightness of the color is consistently positively influenced by the temperature is influenced,” reports the team. The warmer a region is, the brighter the frogs and toads that live there are.

One possible reason for this is the effect of colors on light reflection: a light color reflects most of the incident sunlight and is therefore associated with increased heat tolerance, while a darker color absorbs more radiation and enables the sun's heat to be optimally used in regions with lower solar intensity to exploit. The data also showed that color brightness is particularly similar between closely related species and is apparently related to the thermoregulation of amphibians. “This suggests that the evolution of color brightness has favored the colonization of temperate climate zones by some closely related lineages of frogs,” the researchers write.

Advantages for darker colored species

At the same time, the Laumeiers and their team also noticed an opposite influence: Similar to humans, light-skinned frogs and toads are more susceptible to sunburn than darker-colored specimens. Particularly strong UV-B radiation therefore favors species with darker skin. There is also another advantage of darker colors: Apparently, light-colored frog and toad species are more susceptible to certain pathogens such as the chytrid fungus (Batrachochytrium dendrobatidis), which is rampant among amphibians in a pandemic and has already wiped out several populations and even entire species.

“Infections with pathogens, especially with the chytrid fungus, are considered to be one of the main causes of the decline in amphibian populations,” explain the researchers. “We combined detailed data on chytridiomycosis infections with our data on the coloration of the different species and found that the severity of infections was higher in lighter-colored species.” This is particularly true in warm, humid regions with particularly productive ecosystems, for example in Ecuador , Madagascar and Peru. Although the temperatures here would actually favor lighter species, species with darker colors are mainly found in these regions.

“The relative influence of pathogen resistance on coloration is higher in the tropics, while in temperate regions the importance of thermoregulation predominates,” explains the research team. Since climate change is causing temperatures to increase even in temperate regions and at the same time pathogens can spread more easily worldwide, the researchers believe the new results could help to better predict how resilient certain frog species are to the changes.

Source: Ricarda Laumeier (Philipps University of Marburg) et al., Nature Communications, doi: 10.1038/s41467-023-43729-7

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