Secondary metabolites of fleshy vertebrate-dispersed fruits: adaptive hypotheses and implications for seed dispersal.

We discuss seven hypotheses to explain the adaptive significance of secondary metabolites in ripe fleshy fruits and their implications for seed dispersal. These hypotheses are the attraction/association, seed germination inhibition, attraction/repulsion, protein assimilation, gut retention time, directed toxicity, and defense trade-off hypotheses. We examine evidence that supports or refutes these hypotheses and suggest further tests of each. In addition, we summarize recent work with Solanum fruit pulp glycoalkaloids that bears directly on three of these hypotheses (directed toxicity, gut retention time, and defense trade-off). We conclude that evidence addressing many of these hypotheses is either observational or indirect, but most hypotheses find at least some level of support. Because many of the hypotheses are not mutually exclusive, we also conclude that synergistic interactions and multifunctionality in secondary metabolites may provide economical evolutionary solutions for plants facing disparate and temporally variable selective pressures that impinge on fruits and seeds.