Distinguishing between ‘preference’ and ‘motivation’ in food choice: an example from insect oviposition

Abstract

Variation among individuals in acceptance of a second-ranked food resource could be caused either by variation in strength of ‘preference’ for the top-ranked over the second-ranked resource type, or by variation in the general readiness to feed. This distinction is important in models of diet evolution. Of the two traits, preference is the more likely to show additive genetic variation and hence be subject to evolutionary change. In the checkerspot butterfly, Euphydryas editha, changes in readiness to oviposit (abbreviated as oviposition ‘motivation’) mediate the relationship between host preference, host encounter rate, and realized diet, and thereby influence the strength of selection on preference. Definitions of acceptance, preference and motivation are given and applied to oviposition behaviour of herbivorous insects. From these definitions, operational measures of these traits are developed for E. editha. These operational measures are then used to investigate a difference between two E. editha populations in acceptance of a second-ranked host species. The results of this study show that this difference was caused partly by variation of preference and partly by variation of motivation. The difference in motivation probably stemmed from the absence of the most-preferred host from one of the study sites, resulting in prolonged search, reduced frequency of oviposition, and higher mean motivation at this site.