Mistletoebirds and Xylose: Australian Frugivores Differ in Their Handling of Dietary Sugars

Abstract

Carbohydrate-rich mistletoe fruits are consumed by a wide range of avian species. Small birds absorb a large portion of watersoluble nutrients, such as glucose, via the paracellular pathway. d-xylose, a pentose monosaccharide, is abundant in some nectars and mistletoe fruits consumed by birds, and it has been suggested that it is most likely absorbed via the paracellular pathway in birds. We measured apparent assimilation efficiency ( ) and * AE bioavailability (f) for d-xylose and d-and l-glucose in three frugivorous Australian bird species. Mistletoebirds, silvereyes, and singing honeyeaters showed significantly lower for d- * AE xylose than for d-glucose. Across two diet sugar concentrations, silvereyes and singing honeyeaters significantly increased f of both l-glucose (a metabolically inert isomer of d-glucose commonly used to quantify paracellular uptake) and d-xylose on the more concentrated diet, probably because of increased gut processing time. By contrast, mistletoebirds (mistletoe fruit specialists) did not vary f of either sugar with diet concentration. Mistletoebirds also showed higher f for d-xylose than l-glucose and eliminated d-xylose more slowly than silvereyes and singing honeyeaters, demonstrating differences in the handling of dietary xylose between these species. Our results suggest that d-xylose may be absorbed by both mediated and nonmediated mechanisms in mistletoebirds.

Digestive Adaptations of Aerial Lifestyles

“…We have since extended our measurements to include more birds, several bat species, other small nonflying mammals, and a lizard (12,14,27,28,67,75,76,78,87,88,107) (Price E, Karasov W, unpublished observations). In FIGURE 6A, we have plotted the 7 bats, 19 birds, 1 lizard, and 18 nonflying mammals for which we could find measurements of the fractional absorption of the following nutrient-sized probes: Diet was not a significant factor in either set.…”

“…When more than one probe was measured for a species, we chose L-arabinose or the probe closest in size to arabinose. Fractional absorption may vary depending on diet sugar concentration (75,78); when more than one measurement was available for a probe in a species, we averaged the measurements. B: absorption of L-arabinose by various species.…”

Effects of nicotine on the digestive performance of nectar-feeding birds reflect their relative tolerance to this alkaloid

“…The proportion of glucose absorbed through this pathway increases with more concentrated diets (Levey and Martínez del Rio 1998). This enhancement of paracellular uptake on energetically more profitable diets that require lower ingestion rates has been demonstrated in silvereyes Z. lateralis and in three species of specialist nectarivorous birds, including white-bellied sunbirds (McWhorter et al 2006;Napier et al 2014;Napier et al 2008). Generalist nectarivores visit flowers with lower nectar concentrations than do specialist nectarivores (Johnson and Nicolson 2008), and the absorption of any secondary metabolites via the paracellular pathway should be less than from the nectars consumed by specialist birds.…”