Why do snails have hairs? A Bayesian inference of character evolution
1 Abteilung Ökologie & Evolution, J.W. Goethe-Universität, BioCampus Siesmayerstraße, 60054 Frankfurt/Main, Germany
2 Deparment of Zoology, Charles University, Viniènà 7, 128 44 Praha 2, Czech Republic
3 Department of Biology, University of Konstanz, Postbox 5560 M618, 78457 Konstanz, Germany
4 Département d'Ecologie et Evolution, Université de Lausanne, Bâtiment de Biologie, Dorigny, 1015 Lausanne, Switzerland
BMC Evolutionary Biology 2005, 5:59 doi:10.1186/1471-2148-5-59Published: 4 November 2005
Costly structures need to represent an adaptive advantage in order to be maintained over evolutionary times. Contrary to many other conspicuous shell ornamentations of gastropods, the haired shells of several Stylommatophoran land snails still lack a convincing adaptive explanation. In the present study, we analysed the correlation between the presence/absence of hairs and habitat conditions in the genus Trochulus in a Bayesian framework of character evolution.
Haired shells appeared to be the ancestral character state, a feature most probably lost three times independently. These losses were correlated with a shift from humid to dry habitats, indicating an adaptive function of hairs in moist environments. It had been previously hypothesised that these costly protein structures of the outer shell layer facilitate the locomotion in moist habitats. Our experiments, on the contrary, showed an increased adherence of haired shells to wet surfaces.
We propose the hypothesis that the possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high. The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts.