Niche Differentiation and Spatial Segregation of Melanistic and Non-Melanistic Colour Morphs in a Widespread Palearctic Reptile

Abstract

Aim

This study investigates the spatial distribution and climatic niche differentiation between melanistic and non-melanistic colour morphs of the common adder (Vipera berus) across the species range. We aim to elucidate the role of environmental factors in shaping the geographic and ecological variation in colour polymorphism within this species.

Location

The Palearctic region.

Methods

We first compiled georeferenced occurrence records from citizen science databases to map the distribution of the common adder's melanistic and non-melanistic colour morphs (V. berus). Then, spatial point-pattern analyses were employed to assess the geographic segregation of the morphs. Climatic niche differentiation was quantified using both univariate density profiles and multivariate approaches. Niche equivalency and similarity tests were performed to determine the extent of overlap and divergence between the morphs' climatic niches.

Results

The results reveal significant spatial segregation between melanistic and non-melanistic morphs, with melanistic individuals predominantly occurring in colder, more arid regions. Despite considerable geographic overlap, niche analyses indicate a marked climatic differentiation, with the melanistic morph occupying a narrower and more environmentally extreme niche compared to the non-melanistic morph. Niche similarity tests confirm that while the niches are distinct, they are more similar than would be expected by chance.

Main Conclusions

The findings suggest that the observed geographic and ecological patterns of colour polymorphism in V. berus are shaped by climatic factors via local adaptation. The differential distribution of colour morphs likely reflects adaptive responses to temperature, precipitation and habitat structure, further refined by ecological mechanisms such as predation pressures and thermoregulatory trade-offs. These results provide a complementary perspective on adder polymorphism, supporting niche specialisation under climatic constraints, and highlighting the value of citizen science data in capturing large-scale phenotypic variation.