EVOLUTIONARY IMPLICATIONS OF FRUIT-PROCESSING LIMITATIONS IN CEDAR WAXWINGS

Waxwings fed two types of artificial fruits with equal seed loads but with different seed sizes consumed significantly more of the larger-seeded fruits. Because large seeds were defecated more quickly than small seeds, the increase in fruit consumption indicated that waxwings are process rate limited. Our experiments demonstrate that, for small seeds, differential processing of pulp and seeds can be much more complex than previously believed. Waxwings separated pulp from seeds and defecated seeds well in advance of pulp, thereby allowing both an increase in consumption and, presumably, a greater assimilation of nutrients in the pulp. We propose that rapid seed processing has influenced pulp composition and seed-packaging traits. In particular, the link between rapid passage of small seeds and pulp through the intestine may help explain why fleshy fruits are high in simple sugars and free amino acids. Likewise, the size and number of seeds in a fruit may in part be determined by a trade-off between the number of seeds ingested and how far they are dispersed. This evolutionary trade-off is likely influenced by the digestive strategies of volant frugivores to cope with gut processing limitations.