How Do Frugivores Process Fruit? Gastrointestinal Transit and Glucose Absorption in Cedar Waxwings (Bombycilla cedrorum)

We used image-intensification radiology to examine gut function in six Cedar Waxwings (Bombycilla cedrorum) fed artificial fruits that contained barium-labeled pulp and seeds. Despite the absence of an anatomical crop, waxwings were able to store fruits orad to the gizzard in a distensible portion of the esophagus. By doing so, they were able to increase meal sizes above what their gizzards could contain at any one time. Thus, they could partially overcome the digestive bottleneck that results from a bulky diet. Gizzard contraction increased in frequency with gizzard fullness. Seeds and pulp were separated in the gizzard; most pulp passed into the intestine before the seeds. Once in the intestine, however, seeds moved especially quickly (g = 0.14 cm/s) into the rectum. Both pulp and seeds resided for a relatively long time in the rectum, where antiperistalsis was clearly visible. Unlike other birds that have been studied, the functions of antiperistalsis in waxwings probably do not include cecal filling, since ceca of waxwings are minute. We hypothesize that the function of antiperistalsis in waxwings (and other fruit-eating birds) is to mix digesta in the intraluminal zone adjacent to the rectal mucosa, thereby increasing nutrient absorption. This hypothesis is consistent with the observationt hat fruit-eating birds typically have low digestive efficienciesI.n particular, bsorption of nutrients in the rectum may be important, given that digesta therein may still contain significant amounts of nutrients. Indeed, active uptake of D-glucose, a primary constituent of fruit pulp, was as high in the rectum as elsewhere in the gut. We also found that waxwings typically defecated only the distal 50% of rectal contents. Presumably, digesta in the proximal portion was higher in nutrients and so was retained longer for more thorough absorption. This pattern contrasts to most other vertebrates in which the rectum empties completely and sugar uptake in the rectum is negligible. We suggest that functional adaptations to frugivory (i.e. how fruit is processed) are likely more important than structural adaptations.