To test the
possibility of differential selection of colour morphs on different backgrounds,
colour proportions of juvenile and adult shells were compared on trees with
abundant foliage and on those with sparse foliage at the levels occupied by the
snails. Selection by the morphs of the background against which they are most
cryptic may promote polymorphism (Endler, 1978), so the distribution of shell
colours on bark and leaf substrates within trees was investigated.
Shell
coloration affects the thermal properties of shells, and climatic selection has been
suggested to operate in some pulmonates (reviewed by Cain, 1983).
Accordingly, shell colour frequencies were compared on trees in shaded and
sunny positions, and monthly records of colour frequencies on two trees over a
13-month period were examined for seasonal trends.