Froglife have been running an enquiry line and an email advice service for a very long time now and some issues have been reappearing regularly over the years. Most of those have been incorporated in the FAQs on the website specifically to facilitate the rapid dissemination of information to people searching for it. However, other topics are more complex and there is no short answer to some questions. For example: Do amphibians change their colour and if so what is prompting that change? Is it an adaptation to match the background colour (i.e. a toad is brown on brown soil but becomes dark grey when moving onto a gravel substrate to blend in), is it a seasonal change (lighter in spring but dark in autumn), is it an indication of disease or stress or is it when they are breeding?
It has been well described how colour can change in many amphibian species due to environmental conditions such as humidity and especially temperature and often frogs and toads or even salamander larvae are much darker when their body temperature is low. The common toad (Bufo bufo) however can have significant variation in dominant colour depending on age and some salamander larvae become lighter as they age, probably because early in the season there is a clear advantage to be darker and warm up quicker in cold water but later on dark larvae are at higher risk of predation. There is equally some evidence that in some species, including the common toad there is a certain level of adaptation of the colour depending on the predominant substrate in the area where they live. Probably toads that stand out (brown toads in dark grey soils) are more in danger of predation and as such there is a level of predation selection. Complex mechanisms are involved in relation to the colour change in most amphibians, including the common frog (Rana temporaria) in relation to the background colour. Confusingly, tree frogs in Europe are known to change their colour dramatically between dark grey in early morning to bright green or light grey later by mid-day yet neither of these species is particularly adapt at changing their colour to match that of their environment. Similar species of treefrogs from North America however do change to adapt to their background.
Some amphibians also dramatically change colour during breeding, with the Moor frog (Rana arvalis) in continental Europe as the best example. Males become UV-blue during breeding and create an amazing visual spectacle at ponds and marshes where they congregate. However, the reasons for this colour change have remained relatively unclear given that the vast majority of social interactions between amphibians are auditory (males sing during the breeding season to attract a high density of females in a small patch). Recent research indicates that while females remain brown during breeding the males change colour at that time not just in the human visible spectrum but also in UV spectrum but neither intensity nor wave length of this blue colour are correlated with body condition of the males. As such the most plausible explanation is that the colour change might serve to communicate male-male competitive advantages or potentially to showcase the fact that they are indeed males and therefore to avoid potentially energetically expensive mistakes when males try to get in amplexus with other males due to confusion in a small area, full of frogs.
Colour change in amphibians is a highly complex and variable topic, with numerous species-specific adaptations. So the question Why do amphibians change colour? has a myriad answers and can vary to involve different species, life stages, predation pressure, ambient conditions, especially temperature, stress, UV light and intra-sexual communication between males.
References:
King, R. B., Hauff, S., & Phillips, J. B. (1994). Physiological color change in the green treefrog: responses to background brightness and temperature. Copeia, 422-432.
Ries C., Spaethe J., Sztatecsny M., Strondl C & Hodl W.(2008) Turning blue and ultraviolet: sex-specific colour change during the mating season in the Balkan moor frog Journal of Zoology 276, 229–236
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