Environmental effects on adult shell size in Cepaea hortensis

In natural populations of the land snail Cepaea hortensis there is often a negative association between shell size and population density. The usual interpretation of such associations, both in C. hortensis and other species, is that they represent an ecophenotypic effect that is part of a system of density‐dependent natality, rather than a genetically‐based effect that is produced through density‐related selection. However, this interpretation is usually based on supposition rather than evidence. Here we present the results of an experiment in which small juveniles collected from two sampling sites within a colony known to exhibit a negative size‐density association were grown to maturity under constant laboratory conditions. The highly significant inter‐site difference observed in the wild disappeared in the laboratory‐reared snails, indicating the direct influence of environmental factors on size. Because of the nature of the data‐sets from which density‐dependent natality is inferred, this result has a clearer connection with such density dependence than conventional within‐population heritability estimates.