Using crowdsourced photographic records to explore geographical variation in colour polymorphism

Aim In wide-ranging taxa occupying broad climate gradients, alternative colour pattern variants (i.e. morphs) may emerge as an adaptive response to local climatic regimes. We explore the patterns of geographical segregation among colour morphs of the Lace monitor and determine whether morphs occupy divergent climatic niches along a mesic-arid gradient. Location Eastern and south-eastern Australia. Taxon Varanus varius. Methods We compiled a dataset of crowdsourced, georeferenced photographic records (n = 1637) of V. varius and scored each photographed lizard as one of two discrete colour pattern phenotypes-Lace morph (mostly dark with fine pale patterning) or Bell's morph (broad yellow and black bands). We then analysed the geographical segregation of colour morphs under an exploratory point-pattern process analysis framework. Finally, we quantified similarities and differences between each colour morphs' realised climatic niche in univariate and multivariate (i.e. Centroid shift, Overlap, Unfilling, and Expansion and n-dimensional hypervolume frameworks) environmental space. Results While both morphs co-occur over much of the species' range, there is a coastal-inland clinal transition in the relative densities of the morphs' occurrences-the Bell's morph becomes the more prevalent phenotype towards inland locations and is rare or absent towards coastal regions. Analysis of the realised climatic niches shows that the Bell's morph exploits areas that are relatively more arid, receive more solar irradiation and experience a broader range of thermal extremes than those occupied by the Lace morph. Main conclusions The mesic-arid transition zone in eastern Australia is a primary axis of environmental variation over which geographical variation in colour polymorphism has emerged in V. varius. More broadly, this study demonstrates how photographs from citizen science platforms can be used to explore geographical variation in colour polymorphisms without needing to conduct expensive, time-consuming field studies on wild organisms.