Beyond beauty, colour serves important functions for frogs and toads around the world

Frogs and toads come in a wide range of colours. However, these colours are not only beautiful, but also fulfil certain functions. In addition to protection from predators, colour lightness in particular can play an important role in their lives!
Beyond beauty, colour serves important functions for frogs and toads around the world

Over 8,000 frog and toad species hop and croak around the globe. They are masters of adaptation and have a range of unique survival strategies. However, according to the IUCN Red List, amphibians are the second most threatened group on our planet and, undisputedly the most threatened vertebrate class with 40.7 % of threatened species. Ongoing habitat loss, climate change and the pandemic chytrid disease are some of the main drivers that contribute to the threat of amphibians. However, why some species are more vulnerable than others remains largely unknown.

Frogs and toads (the amphibian order Anura) are not always brown and green, but exhibit an incredible range of colours, patterns and even textures that did not develop by chance (Figure 1). Some species blend in seamlessly with their surroundings to not be detected, while others use conspicuous colours and patterns (aposematism) as a warning signal to shout: “Don’t touch me, I’m poisonous and inedible!” But colour does not only function as visual signal as we show.

Figure 1: Frogs from all over the world, in a variety of patterns and colours: Species from the domestic terrarium, as well as France, Ecuador, Sri Lanka and Vietnam

Beyond camouflage and aposematism, colour and particularly the melanisation measured as lightness plays a crucial role in regulating the body temperature. Colour-based thermoregulation is increasingly supported for insect groups, which, like anurans, depend on absorbing sunlight to heat up and maintain their body temperature. The relationship between colour lightness, physiology and distribution is conceptualized in the thermal melanism hypothesis stating that darker coloured organisms are favoured to live in colder regions as they heat up faster, whereas lighter coloured ones benefit from the prevention of overheating in warmer regions. In addition, a darker skin can enhance the protection of detrimental UVB radiation and the tougher walls of melanised skin cells can provide an advantage against the penetration by bacteria and fungi (Figure 2). However, our current knowledge of the ecological importance of colour lightness is mainly based on studies in colder areas such as Europe and North America and the physiological implications of colour variation in anurans remained thus far unexplored.

These considerations set the starting point for our endeavour to explore the functions of colour in anurans. I am interested in the colourful world of animals since my earliest studies on leaf beetles and continued to delve into their ecological significance in anurans throughout my Master’s thesis. The latter - a study on African anurans - served as a pilot project to test whether the functions of colour, that have already been demonstrated for insects, are also relevant for this large vertebrate group and indeed globally relevant. But how can you get a comprehensive picture of species’ colour lightness variation without travelling the globe with your measurement set up in the backpack?

Figure 2: The measurement of colour data, underlying ecological functions and the discovery of global patterns of colour lightness in frogs and toads (copyrights for the image of Hyperolius viridiflavus are held by Alan Channing).

Previous studies on colour variation have used standardized images from literature or imaged museum specimens, for anurans such sources were lacking and revisiting museum collections would not have helped as amphibian specimens fade in colour after their death. However, extensive literature such as "Field Guide to the Frogs and Other Amphibians of Africa" by Alan Channing and Mark-Oliver Rödel, the book where it all started with, provide an astonishingly accurate and complete overview of species across the world. On the initiative of Stefan Pinkert, we set out to leverage this potential assessing the colours and their coverage based on a predefined scale (Figure 2), similar to the RAL-charts many probably already used in their everyday life. Our focus on colour lightness and not colourfulness of species reduces the dimensions of colour variation to a proxy most relevant for the thermoregulatory and protective functions of colour.

Motivated by the promising results of the pilot study and supported by the huge herpetological literature and photo collection of our colleague Mark-Oliver Rödel at the Natural History Museum in Berlin, we scaled up our efforts to what represents almost five thousand individuals from 27 books covering 41% of all anuran species. Our study documents for the first time the importance of colour-based thermoregulation, UV-protection, and pathogen resistance in anurans. We showed that all three functions shape the distribution of anurans in concert. Thermoregulation and UV-protection was generally of greater importance in temperate regions, and pathogen resistance was of greater importance in the tropics (Figure 2). In anurans, colour lightness appears to be the main thermoregulatory adaptation in this taxon. Moreover, our support for colour-based pathogen resistance suggests colour lightness as a major predictor of the threat of anurans in the face of massive population declines caused by pathogens (e.g. chytrid disease).

These new findings motivate the integration of colour lightness and its relationship to the environmental conditions in which species occur, in order to understand and predict threats to species and their biological response to climate change.

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Vertebrate Zoology
Life Sciences > Biological Sciences > Zoology > Vertebrate Zoology
Life Sciences > Biological Sciences > Ecology
Life Sciences > Biological Sciences > Ecology > Macroecology
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