1. Several fundamental aspects of communication between plants and their mutualists are poorly understood. It remains unclear whether plant signals are accurate and reliable, that is, whether they recruit mutualists by informing them about the nutritional rewards provided by flowers and fleshy fruits.
2. We evaluated fruit characteristics of 105 vertebrate-dispersed species from an Atlantic rainforest community to examine the relationship between visual fruit stimuli and fruit quality. We combined morphological, biochemical and spectrophotometric methods and employed comparative analyses to assess the evolutionary association among morphological, biochemical and visual characters.
3. We detected a significant phylogenetic signal in most morphological and nutritional traits but not in colour traits, which suggests that fruit colours are evolutionarily labile. We also found that two properties of fruit coloration (brightness and chroma) explain 24–29% of variation in protein and sugar contents, respectively. High sugar content in fruit is associated with dark colour and low chroma (colour saturation). However, fruit brightness is not strongly correlated to protein content and its signalling role remains unknown.
4. We suggest that the negative relationship between chroma and sugar content in fruit can be explained by the fact that sugars up-regulate the synthesis of anthocyanins, the pigments primarily imparting achromatic coloration in fruits. Biochemical pathways could consequently explain the covariance between visual fruit traits and nutritional fruit traits. In this scenario, information could arise as by-product rather than as an adaptation to signal to seed dispersers.
5. Synthesis. We show that fruit visual traits are evolutionary labile and that fruit chroma and brightness can reliably indicate sugar and protein content, respectively. We propose that shared biochemistry is a likely mechanism inducing the covariance between colour (the stimulus) and nutritional quality (as the unseen quality). We hypothesize that by-product information represents a widespread and so far neglected mechanism leading to reliable visual traits in mutualistic and possibly also antagonistic plant–animal interactions.