Detectability and content as opposing signal characteristics in fruits.

Although often associated with consumers, fruit colours have rarely been assessed as signals. Here, we investigate the signal principles of 'detectability' and 'content' in bird-dispersed fruits. We determined detectability as the contrast between fruit and background and signal 'content' by correlating fruit colours and compounds. Red and black, the most common fruit colours globally, contrast more against background than other colours but do not indicate compounds. In other colours, 60% of the variation in long- to shortwave light correlated with protein, tannin and carbohydrate content. Because macronutrients stimulated fruit removal, while phenols, but not tannins, deterred it, signalling these macronutrients probably increases seed dispersal. Phenolic content was not signalled because it would reduce plants' fitness. Signalling tannins might be directed towards fruit pests rather than dispersers. In conclusion, plants may employ differential signalling strategies matching conspicuous signals in red and black fruits while other colours signal fruit quality. The latter implies that nutrient quality and fruit defence are communicated visually.