A glycoalkaloid in ripe fruit deters consumption by Cedar Waxwings

Abstract

Many ripe fruits contain toxic chemicals that presumably protect them against attacks by frugivores that do not disperse seeds. These secondary metabolites may be generally toxic, representing an evolutionary compromise between defense from pathogens and attraction of seed dispersers (the General Toxicity hypothesis). Alternatively, they may be nontoxic to seed dispersers and simultaneously toxic to vertebrate frugivores that do not disperse seeds (the Directed Toxicity hypothesis). To tease apart these hypotheses, we tested whether consumption of artificial fruit agar by captive Cedar Waxwings (Bombycilia cedrorum) was reduced by the presence of α-solamargine, a glycoalkaloid common in solanaceous fruits. We recorded consumption of three artificial fruit types differing in α-solamargine concentration and a control fruit that lacked α-solamargine. Waxwings were strongly and equally deterred by all concentrations of α-solamargine. These concentrations, 0.1, 0.2, and 0.3% wet mass, are commonly found in some Solanum fruits. In a second trial, we addressed another hypothesis, the Nutrient-Toxin Titration hypothesis, which predicts that the deterrent effects of α-solamargine can be overridden by highly nutritious fruit pulp. We offered waxwings three types of artificial fruits that varied in nutrient concentration but not in α-solamargine concentration. Nutrient content had no effect on consumption when α-solamargine was present. In summary, our results are inconsistent with both the Directed Toxicity and the Nutrient-Toxin Titration hypotheses.