Response of silvereyes (Zosterops lateralis) to dietary tannins: the paradox of secondary metabolites in ripe fruit

Abstract

There are many secondary metabolites in ripe fruit that are toxic to vertebrate consumers. The most prevalent explanation for their presence in ripe fruit is to protect the fruit against consumers that do not disperse viable seeds. It has been hypothesised that seed dispersers are not deterred by, or can tolerate, the consumption of secondary metabolites in ripe fruit. We tested whether silvereyes (Zosterops lateralis), which are known seed dispersers, were deterred by quebracho (condensed tannins) presented in two different food types. In the first experiment, silvereyes were given artificial fruit containing either 0% or 5% quebracho and their fruit consumption was measured. A second experiment recorded consumption of a cereal-based, long-term maintenance diet containing either 0% or 5% quebracho. Silvereyes did not exhibit a significant preference for artificial fruit that did not contain quebracho: 39.9% of the fruit consumed did contain quebracho. However, silvereyes strongly avoided the cereal diet containing quebracho, consuming, on average, only 0.36 g (0.06 g) of cereal per 5 h compared with 17.3 g (0.23 g) of cereal that did not contain quebracho. We suggest that because the artificial fruit were swallowed whole by silvereyes, the quebracho may not have been detected as easily in the time available as it would have been in the aqueous cereal diet. Consumption of fruit containing secondary metabolites by wild silvereyes is likely to depend on factors such as food availability, nutrient content of the fruit and the degree of diet mixing.