Control of Gut Retention Time by Secondary Metabolites in Ripe Solanum Fruits

We tested whether compounds in ripe Solanum americanum fruits affect gut retention time of S. americanum seeds in Cedar Waxwings (Bombycilla cedrorum). Glycoalkaloids were of special interest because they commonly occur in ripe Solanum fruits and are associated with diarrhea in humans. Also, we determined the influence of gut retention time and the presence/absence of two glycoalkaloids, a-solasonine and a-solamargine, on germination of S. americanum seeds. In one trial, we measured seed retention times of 10 waxwings fed three types of artificial fruits, Control (containing no secondary metabolites), Low Concentration (containing low concentrations of a-solasonine and asolamargine, matching those in ripe S. americanum fruit), and Extract (containing an ethanol extract of ripe S. americanum fruits, with many unidentified secondary metabolites). Seeds in Low Concentration fruits were not defecated more quickly than those in Control fruits, but seeds in Extract fruits were. Thus, ripe S. americanum fruits contain a chemical or chemicals with a laxative effect. Also, seeds from Control fruits were deposited in more defecations and at lower densities in each defecation than those from Extract fruits. In a second trial, we compared retention times of seeds in control fruits and in fruits with high concentrations of glycoalkaloids, typical in ripe fruits of other Solanum species. These high concentrations had a significant constipative effect on seed passage. Percentage germination and mean germination time of defecated seeds were not influenced by mean retention time in waxwing guts. However, proportionately fewer seeds germinated from Low Concentration fruits than from Control fruits. These results suggest that plants have more control over seed processing by frugivores than generally acknowledged. Secondary metabolites in ripe fruits can increase or decrease retention time and thereby influence seed deposition patterns (e.g., number of defecations with seeds, number of seeds per defecation, and presumably, dispersal distance).