Colors of Fruit Displays of Bird-dispersed Plants in Two Tropical Forests

Fruit-eating birds are the primary seed dispersers for many plant species in various habitats (Ridley 1930; van der PijI1969). Because seed dispersal is important for plants (Howe and Smallwood 1982), the criteria that birds use in choosing fruits should directly affect the reproductive success of plants that depend on avian seed dispersal. Any fruit trait that enhances the likelihood that an appropriate seed disperser will consume the fruit should be favored (Jenkins 1969; Snow 1971). Traits such as nutritional value, seed-to-fruit ratios (Howe and Vande Kerckhove 1980), taste (Sorensen 1983), time of ripening (Wheelwright 1983), and spatial display (Denslow and Moermond 1982) have been demonstrated to influence birds' selection of fruits. Until recently (Stiles 1982; Willson and Thompson 1982) there has been little research on fruit colors and their significance for seed dispersal. With their excellent visual acuity and well-developed color vision (Jacobs 1981), birds probably use color to find and recognize fruits. If birds do notice and choose among fruits on the basis of color, existing differences in fruit colors between plant species may be the evolutionary result of differential selection by birds for increased fruit conspicuousness or attractiveness. The conspicuousness of a plant's fruit display is likely to be a compromise between conflicting selection pressures. If plants benefit from the rapid removal of the greatest number of their fruits by appropriate seed dispersers (McKey 1975), natural selection should favor prominent fruit displays. Such displays may have associated costs, however (Snow 1971). Energy is invested in producing the display; frequently, accessory structures are enlarged and contrastingly colored, as in the bracts of Heisteria species (Olacaceae). Moreover, prominent fruit displays may attract seed predators and inappropriate dispersers as well as appropriate dispersers. Birds probably differ in their visual sensitivity and the way in which they find individual fruit trees; many bird species may encounter fruiting trees opportunistically, locating bright displays more readily than cryptic dis-