Fruit preference criteria by Avian Frugivores: their implications for the evolution of clutch size in Solanum pubescens

Abstract

Solanum pubescens is dispersed by red-vented bulbuls, which forage on its fruits and defecate the seeds. We tested three alternative fruit-preference criteria proposed to be used by the avian frugivores, namely that based on the absolute pulp content, pulp-to-seed weight ratio, and the benefit-to-cost ratio of handling the fruits, each with entirely different consequences on the evolution of clutch size in plants. Both in the field and laboratory, red-vented bulbuls were found to base their preference on the benefit-to-cost ratio of harvesting the fruits and not on the other two criteria. This preference criterion also appears to have shaped the clutch size in Solanum; seed number per fruit was normally distributed around the mode corresponding to the fruit sizes preferred by birds. Based on our results we argue that avian frugivores prefer fruits that yield the highest benefit-to-cost ratio of harvesting them and discuss its implications for the evolution of clutch size in bird-dispersed species.