Shift in investment between sexually selected traits: tarnishing of the silver spoon

Studies of resource allocation strategies have concentrated on the influence of natural selection on the evolution of life history traits. To a lesser degree, the effects of trade-offs between natural and sexual selection on the evolution of allocation strategies have also been considered. Trade-offs between sexually selected traits that are important to females but that appear to differ in cost, however, have not been considered. Female green swordtails, Xiphophorus helleri, prefer males with longer swords to males with shorter swords, and in this study they demonstrated a preference for larger males to smaller males. Furthermore, sexually mature males invested differentially in body and sword growth depending on resource availability; males that had an unlimited amount of food invested in both body and sword growth, but males shifted to a food-restricted regime halted investment in body growth and invested only in sword growth. These results suggest that males shift their pattern of investment in two sexually selected traits when food becomes restricted. In general, variable environmental conditions may favour such conditional investment strategies in species in which there is more than one preferred male trait and the costs of the traits differ. (C) 1998 The Association for the Study of Animal Behaviour.