A design for multifactorial choice experiments: an example using microhabitat selection by sea slugs Onchidoris bilamellata (L.)

Abstract

When animals make choices (selecting foods, habitats, mates, etc.), they must assess multiple factors, often simultaneously. Moreover, certain factors have more weight in the choice-making process than others. Typically, however, studies of animal choice consider the effect of only one factor (i.e. different levels of a single factor). In this paper, we present a design for multifactorial choice experiments, in which animals are offered a choice between different levels of two or more factors. The advantages of the multifactorial choice design are that interactions between different factors affecting choice can be explicitly examined and the relative importance of several factors can be assessed. We illustrate the use of the design in a laboratory experiment to examine the effect of light intensity and texture of substratum on selection of microhabitat by sea slugs ( Onchidoris bilamellata (L.)). In this experiment, sea slugs preferred darkness over light, irrespective of the texture. They also preferred rough substratum over smooth substratum, but only when they were in the dark. In light, sea slugs did not show a preference between textures. Thus, the analysis showed: (1) preferences for different levels of one factor (light intensity) overrode preferences for different levels of the other factor, and (2) there was a significant interaction between factors (such that preferences for texture depended on light intensity).