The use of dosed and herbage n-alkanes as markers for the determination of intake, digestibility, mean retention time and diet selection in galapagos tortoises (geochelone nigra)

Abstract

Eleven captive Galapagos tortoises (Geochelone nigra) were used as study subjects to estimate digesta kinetics, diet composition, intake and apparent digestibility (aD) using n-alkanes. The results were compared to observed intakes and digestibility estimated through total faecal collection. Acid-insoluble ash (AIA) and acid-detergent lignin (ADL) were compared to the alkane method for the estimation of aD. Mean retention times (MRT) were estimated in four adult tortoises (ca 40-60 years old) and four juvenile tortoises (4-5 years old) fed a pulse dose of Co-EDTA, Cr-mordanted fibre (particle size <2 mm) and n-alkane hexatriacontane (C 36). Average MRT for the liquid phase marker Co was nine days in both adult and juvenile tortoises. For the particle phase markers Cr and C36, MRT was 12 days in adult tortoises and eight and nine days, respectively, in juveniles. Digestibility, diet intake and diet composition were estimated in nine Galapagos tortoises fed for 32 days on a standardized diet containing the synthetic n-alkanes octacosane (C 28), dotriacontane (C32) and C36. In four juvenile tortoises kept individually, total faecal collection was performed and the faecal recovery rates of n-alkanes were estimated for pentacosane (C25), heptacosane (C27), C28, nonacosane (C29), hentriacontane (C31), C32, tritriacontane (C33) and C36. Intakes calculated with the alkane-pair C31 and C32 overestimated intake by a factor 1.5. After correction for the relative recoveries of alkanes there was no significant difference between estimated and observed intakes. Observed aD of organic matter (OM) was 67.5%. Estimated aD of OM with the internal marker C 36 alkane, ADL and AIA were 48.5%, 38.9% and 18.3%, respectively. Estimates of diet composition using alkanes in individual animals accurately reflected directly-observed composition. Observed selection of a certain feedstuff was recognized with the alkane method. This is the first report of the use of n-alkanes as digestive markers in reptiles and it confirms that n-alkanes may be used for determining diet intake and the passage through the gut of the particulate digesta phase, and for estimating diet composition. The possibility of estimating different aspects of digestive strategies with the same marker type is a major asset of the alkane technique.