Three-dimensional crevice structure affects retreat site selection by reptiles

When selecting retreat sites, rock-dwelling animals assess thermal and hydric properties of the rocks under which they shelter. Another obvious, but harder to measure, factor that may affect retreat site selection is the size and structure of the three-dimensional crevice beneath the rock. We developed a novel technique to quantify crevice structure beneath rocks and compared crevices used by snakes and lizards with unused crevices. Multiple attributes of crevice structure such as the height of the crevice above the substrate, the area of the crevice suitable for fitting the entire body of the animal, the degree of closure of the crevice and the amount of fragmentation within the crevice affected the suitability of a crevice as a retreat site. Therefore, crevice structural attributes, not simply size and thickness of the overlying rock, may be important factors driving retreat site selection by ectotherms. We also found that crevice structure directly alters thermal regimes, and may be more important than rock thickness in this respect. Understanding the characteristics of the space actually occupied by animals can yield a greater understanding of habitat selection, and our novel technique for quantifying crevice geometry should be readily transferable to other systems. © 2008 The Association for the Study of Animal Behaviour