Passage through bird guts causes interspecific differences in seed germination characteristics

1. Seed germination characteristics are often modified after seeds are inge sted by frugivores. Factors that are intrinsic either to the plant or to th e frugivore's digestive tract are responsible for the great variation obser ved in germination response. 2. Our objectives were to determine whether and how the seed germination pa tterns of five common western Mediterranean plant species are affected by s eed passage through the guts of their major dispersers, and to elucidate th e mechanism by which such patterns are changed. 3. We used captive birds (Turdus merula and Sylvia melanocephala) to obtain ingested seeds and compared their germination rate (speed) and germinabili ty (final percent germination) with those of controls (uningested, pulp-rem oved seeds), controlling for seed age, size and source. Germination was mon itored for 2 years in an experimental garden. We evaluated the possible cha nges in seed traits after ingestion by measuring weight and coat thickness, and by observing seed coat sculpture. 4. Rate of seed germination, but not germinability, changed in all species after gut treatment. The greatest effect was in Osyris, in which germinatio n was much enhanced. A great acceleration of germination, which is likely t o translate into a seedling size advantage, was also found in Asparagus. In the other three species tested, germination was slower for ingested than f or control seeds. 5. For Rubus and Rubia seeds, we found a different germination response dep ending upon the frugivore species tested. A different degree of seed coat s carification caused by differences in gut retention time, chemical and/or m echanical abrasion probably account for such responses. 6. In three of the species (Osyris, Rubia and Phillyrea), seed weight decre ased after gut treatment. Such weight loss was not caused by any change in coat thickness, but may have been because of the scarification and conseque nt alteration of the seed coat structure. 7. The five Mediterranean species studied germinate when rains are most lik ely to fall (mostly autumn and spring). The different speed of germination promoted by gut treatment within frugivores may increase the probability th at seeds can recruit successfully at a given time and in a given place. 8. This study suggests that frugivores contribute to the heterogeneity in g ermination characteristics not only within plant populations but also withi n plant communities, each frugivore species having a particular effect on t he seeds of each plant consumed.