Fruit color and size provide reliable signals of macronutritional rewards for seed dispersers

Fruit color and size provide vital cues for determining seed dispersers' preferences; however, any consistent underlying link between these traits and fruit macronutritional rewards remains to be established by large-scale analyses. We collated a global database comprising 2626 observations of 1626 fruiting plant species from 460 global sites to test whether fruit color and size traits do indeed consistently signal macronutritional rewards to frugivores. We found that consistent fruit color and size signaled macronutrient reward specifically relating to bird-, mammal-, and mixed-dispersed dispersal syndromes. Lipids were the predominant macronutrient in black, blue, red, orange, and yellow fruits with birds (27.24%), whereas sugar predominated in mammal-dispersed fruits (72.55%). Fruits dispersed by mammals (25.55 mm) were consistently larger than those dispersed by birds (10.17 mm) regardless of fruit color, and total macronutrient mass per fruit increased with fruit size. These signal-mutualist relationships are shaped by associated disperser traits, demonstrating co-evolutionary mechanisms driving adaptive differentiation of fruit morphological and macronutrient traits. By integrating the disperser syndrome hypothesis and signal theory with nutritional geometry theory and trait-matching theory, our study expounds the role of seed disperser selective preferences in the evolution of fruit traits, thus advancing the seed disperser mutualism paradigm.