The grove snail and its bands

Cepaea nemoralis, the grove snail, is the snail of textbook color polymorphism. The shell may be yellow, pink, or brown, and it may carry from zero to five dark spiral bands. A close relative, Cepaea hortensis, the white-lipped snail, overlaps it in color and pattern. The lip of an adult nemoralis shell is usually dark. The lip of hortensis is usually pale. That single character is a better field mark than the color of the rest of the shell, which will mislead you on purpose.

The variation is inherited, and it is visible to birds. Song thrushes break Cepaea on stones, leaving an anvil scattered with pieces. Research running from the middle of the twentieth century to the present has asked whether thrushes take the shells that contrast with a given background. The rough answer is yes, with enough exceptions to keep the argument alive. Habitat, climate, and the history of a particular colony all shift the frequencies. Yellow unbanded shells are often commoner in open grass. Darker and banded shells are often commoner in woods. “Often” is the correct strength of the claim. One hedge can refuse the pattern.

For a walker the lesson is immediate. Five Cepaea from one nettle patch can look like a drawer of different species. Turn each adult so the lip faces you. Note whether the ground was a sunny verge or a shaded ditch. You are repeating, at the scale of a footpath, a comparison that helped biologists treat visible variation as something selection could actually see.

A photograph of those upturned lips, shells then set back in the vegetation, holds the comparison. A collected series does not improve it. If you find the anvil stone, with fresh pieces and old white fragments together, you have found the predator’s record. It can be read without adding a specimen to it.