Biochemical functions of geophagy in parrots: Detoxification of dietary toxins and cytoprotective effects

This study tests hypotheses on the biochemical functions of geophagy in par rots: mechanical enhancement of digestion, acid buffering capacity, mineral supplementation, adsorption of dietary toxins, and gastrointestinal cytopr otection. Parrots showed clear preferences for specific soil horizons. Comp arisons of preferred and nonpreferred soils from several sites suggest that soils have little ability to enhance grinding and no measurable ability to buffer gastric pH. Soils offered insignificant mineral supplementation sin ce most minerals occurred at similar levels in samples regardless of prefer ence, and the minerals were generally more plentiful in the birds' diets. S odium was available in moderate levels at some sites (>1000 ppm), but was w ell below sodium detection thresholds of parrots. X-ray diffraction, cation exchange capacity, and in vitro adsorptive trials showed that the preferre d soils are capable of exchanging substantial quantities of cations and are capable of adsorbing low-molecular-weight secondary compounds. In captive Amazona parrots, orally administered clay reduced the bioavailability of th e alkaloid quinidine by roughly 60%, demonstrating that in vivo adsorption of potentially toxic compounds may be a biologically important function of geophagy. Labeled clay remained in the lower gastrointestinal tract of capt ive parrots for >12 hr, which along with high adsorptive capacities, furthe r suggests a potential role in protecting the gastrointestinal lining from various biological and chemical insults. Detoxification and cytoprotection are the most likely functions of geophagy for parrots and herbivores with s imilar ecologies. Given the variety of chemically defended seeds consumed b y these herbivores, geophagy likely protects consumers from dietary toxins, allowing increased diet breadth and/or enhancing digestibility.