Seasonal Variation in the Quality of Fruits and Diffuse Coevolution Between Plants and Avian Dispersers

The hypothesis is formulated that, among vertebrate—dispersed plants, species ripening fruits at different times of year should differ in the nutritional properties of their fruits in such a way as to match the seasonally changing demands of their major dispersers. This was tested for a sample of 62 species of southern Spanish Bird—dispersed plants, and results fully conform to expectations. Species ripening fruits during the dry mediterranean summer, when water demands of dispersers are highest, have the most watery fruits. Species producing fruits in winter, when energy needs of avian frugivores are at a maximum, possess fruits with the highest lipid content. No marked seasonal trend in protein yield of fruits was found, which is consistent with the fact that protein demands of avian dispersers appear to be fairly constant during the summer—through—winter period considered. Coupling between fruit quality and frugivores' needs is shown to be most likely related to coevolution between plants and birds, and not to fortuitous coincidence over time of fruit quality and disperser needs. The significance of highly rewarding winter fruits for the evolution of some physiological and behavioral traits among overwintering dispersers points to the existence of a closely coevolved system involving the latter and the assemblage of winter—ripening mediterranean evergreens. It is concluded that the seasonal gradient in plant—bird coevolutionary adjustment has been concurrently brought about by (1) seasonally changing demands of dispersers, and (2) the differential coevolutionary potentials open to the plant—bird system through changing spatio—temporal asymmetry in relationships between vegetation and avifauna.