Do Seeds Hinder Digestive Processing of Fruit Pulp? Implications for Plant/Frugivore Mutualisms

Abstract

The seeds of bird-dispersed fruits could impose significant costs on frugi- vores if seed bulk reduces the effective capacity of the gut and, hence, reduces the rate of nutrient intake. This has led to the notion that avian frugivores pass food through their di- gestive tracts rapidly to minimize the effects of seeds on nutrient acquisition. Consequently, avian frugivores are thought to utilize fruit sugars inefficiently, because this permits higher intake and sugar-assimilation rates from a supposedly energy-dilute food. I evaluated the influence of seed bulk on intake and absorption of sugars from chokecherry fruits (Prunus virginiana) by Cedar Waxwings (Bombycilla cedrorum), American Robins (Turdus migratorius ), Wood Thrushes (Hylocichla mustelina), Gray-cheeked Thrushes (Catharus minimus), and Her- mit Thrushes (Catharus guttatus). I compared Cedar Waxwings with thrushes to determine if they differed in how seed bulk affects digestion of fruit. Cedar Waxwings have higher mass-specific intake rates of sugary fruits than do thrushes; because waxwings defecate all seeds, whereas thrushes regurgitate many seeds, the difference in intake of sugary fruits between waxwings and thrushes may result from ingestion/fruit-processing limitations as- sociated with mode of seed processing. For all species, seeds did not reduce rates of intake and sugar absorption from chokecherry fruit pulp. All birds assimilated sugars from choke- cherry fruits efficiently, and digestive processing of seeds did not reduce sugar absorption. Rates of fruit processing were closely tied to rates of sugar absorption, rather than to the physical composition of the diet. My results suggest that current models of digestive func- tion in avian frugivores are seriously flawed. Because differences in intake rates of sugary fruits between waxwings and thrushes were independent of the presence of seeds, these differences were not due to different modes of seed processing. Received 13 January 1997,