Angus Davison, Daniel Ramos Gonzalez

One of the emerging strengths of working with the land snail genus Cepaea is that historical collections can be compared against modern day samples, for instance to understand the impact of changing climate and habitat upon shell morph frequencies. However, one potential limitation is that prior studies scored shell ground colour by eye, usually in the field, into three discrete colours yellow, pink or brown. This incurs both potential error and bias in comparative surveys. In this study, we therefore aimed to use a quantitative method to score shell colour, and evaluated it by comparing patterns of C. nemoralis shell colour polymorphism, using both methods on present day samples, and against historical data gathered in the 1960s using the traditional method. The Central Pyrenees were used as an exemplar, because previous intensive surveys sometimes showed sharp discontinuities of morph frequencies within and between valleys. Moreover, selective factors, such as climate or the human impact in the Pyrenees, have significantly changed since 1960s. The main finding was that while quantitative measures of shell colour reduced the possibility of error, and standardised the procedure, the same altitudinal trends were recovered, irrespective of the method. There was remarkable stability in the local shell patterns over five decades, with the exception of one valley that has been subject to increased human activity. Therefore, although subject to potential error, human-scoring of snail colour data remains valuable, especially if persons have appropriate training. In comparison, while there are benefits in taking quantitative measures of colour in the laboratory, there are also several practical disadvantages, mainly in terms of throughput and accessibility. In the future, we anticipate that both methods may be combined, for example, using automated measures of colour taken from photos generated by citizen scientists conducting field surveys.