| dc.description | In this paper, I discuss recent evidence concerning the adaptive significance of secondary metabolites in ripe fleshy fruits, and implications for seed dispersal by birds and other vertebrates. Specifically, I revisit a number of adaptive hypotheses originally presented and discussed by Cipollini & Levey in 1997, including the Attraction/Association, Seed Germination Effects, Attraction/Repulsion, Protein Assimilation, Gut Retention Time, Directed and General Toxicity, and Defense Tradeoffs hypotheses. I also present and discuss a new adaptive hypothesis, the Direct Nutritional Benefits hypothesis, posed to reflect recent discoveries concerning the positive dietary effects of some secondary metabolites. From this review, I conclude that focused studies are much needed to provide direct tests of these hypotheses. Evidence addressing many of the hypotheses is either observational or indirect, and gleaned from studies not directly designed to address these hypotheses. Despite this, most hypotheses find at least some level of support - even when the same metabolite is being considered (e.g., anthocyanins and carotenoids having functions as pigments as well as nutritional antioxidants). I conclude with a discussion of the nature of multiple molecular targets of plant secondary chemicals. In doing so, I reinforce the notion that synergistic interactions and multifunctionality of secondary metabolites may provide economical evolutionary solutions for plants facing the disparate and temporally variable selective pressures that impinge upon fruits and seeds. As such, it may not be surprising that specific secondary metabolites serve more than one adaptive function in ripe fleshy fruits; finding support for several non-mutually exclusive hypotheses is likely. Comparative studies designed to address these hypotheses should be undertaken with a careful consideration of the potential underlying effects of phylogeny and physiological constraints on such patterns | |
