No abstract

Summary 1.Fruits are among the most antioxidant-rich foods in nature and thus can be important dietary sources for combating oxidative stress in animals and humans. 2. Because fruits are pigmented by important plant antioxidants such as anthocyanins and carotenoids, frugivores may be able to gauge the antioxidant value of a fruit by its colour. However, although the importance of dietary antioxidants and oxidative stress is increasingly being acknowledged in evolutionary ecology, it is unknown whether animals can use visual signals to detect the presence of antioxidants in their food. 3. We studied the colour and pigment content of 60 bird-dispersed fruits and used an avian eye model to assess the ability of birds to visually discriminate carotenoid and anthocyanin contents of fruits. We then tested whether the frugivorous European blackcap ( Sylvia atricapilla ) preferentially consumes food containing anthocyanins. 4. Fruit colour variation was explained by variation in anthocyanin contents, and birds were capable of discriminating anthocyanin concentrations in fruits based on colour because fruits rich in anthocyanins are black or UV reflecting. However, we found that birds could not use colouration to determine the carotenoid contents of fruits. Ripe fruits contained more anthocyanins than unripe fruits, while there was no difference between ripe and unripe fruits in carotenoid contents. Anthocyanin contents correlated with caloric value in fruits, while there was no such correlation between carotenoids and total energy contents. 5. In food choice experiments, blackcaps selected food containing anthocyanins over food without anthocyanins. 6. In sum, our results indicate that birds actively selected for anthocyanins in their food and that they may use fruit colour as a foraging signal of anthocyanin antioxidant rewards.

Summary 1.The intake of antioxidants confers health benefits to consumers by reducing oxidative stress and improving immune functions. Therefore, several life-history traits such as growth, immunity, senescence and the expression of sexually selected traits might be affected by dietary antioxidants. 2. Flavonoids are strong antioxidants in vitro , and are among the commonest found in fruits, which are a primary source of antioxidants for many animals. It is therefore likely that flavonoids play a beneficial role as dietary antioxidants, but their potential has been ignored in evolutionary ecology. 3. We investigated the ecological importance of flavonoids, using wild-caught blackcaps ( Sylvia atricapilla ), a frugivorous European songbird. 4. We verified the assumption that flavonoids can be absorbed and circulated by blackcaps. 5. In a food selection experiment, we showed that, when given a choice between food with and without fruit flavonoids, blackcaps actively select food with flavonoids. 6. We found a positive effect of flavonoids on humoral immune response. Birds supplemented for 4 weeks with a modest quantity of flavonoids were more likely to mount a humoral immune response after an immune-challenge, compared with control birds. 7. To conclude, our study demonstrates that birds select food with flavonoids and that these antioxidants increase consumer humoral immune response. Since these effects occurred at a moderate dose that is obtainable also by many omnivorous consumers, we suggest that flavonoids may play a beneficial role in ecological immunology for several wild species.