Novel mark recapture surveys reveal stable pigmentation patterns but age-related melanism in the common frog Rana temporaria

Variations in melanin-based pigmentation patterns are frequent evolutionary adaptations in vertebrates and allow the study of phenotypic diversity across populations as well as individual recognition. Age-related changes in such pigmentation patterns have been demonstrated for several species but remain rarely explored in adult amphibians. For the wide-ranging European common frog (Rana temporaria) dorsal melanism was discussed in the context of UV protection at high altitude or as thermal adaptation at high latitude, yet patterns remain unquantified across most of its range or in terms of individual variation over time. We investigated dorsal pigmentation patterns in a wild population of R. temporaria in a lowland site in England which was the focus of a 7 year high-intensity mark recapture programme. We collected dorsal photographs using phone cameras and analysed pigmentation patterns using visual classifications and ImageJ image processing software. Pigmentation was highly variable between individuals but overall: (i) the number of melanin-based spots was largely stable or decreased over time as enlarging spots sometimes merged but never disappeared; (ii) the total melanin-based pigmented area increased significantly over time on average, albeit not for all individuals; and (iii) pigmentation was similar in males compared to females although the darkest individuals tended to be males. Our analysis reveals important patterns in pigmentation change and stability in this widespread species but the mechanisms and specific drivers remain unknown.