Measurement of Elemental Assimilation by Animals from Radioisotope Retention Patterns

Measurements and nutrient assimilation by animals from dietary inputs often are based upon chemical analyses of food and feces. Therefore, those budgets of the difference between nutrient values of food consumed and feces produced provide an estimate of the digestive assimilation of nutrients–equivalent to metabolizable energy in energy balance calculations (Harris 1966)–if it can be assumed that intestional secretion of nutrients is not a significant source of error. Although this experimental approach is reasonable for those animals where urinary and digestive products are distinct, e.g., mammals (Petrusewicz 1967), it cannot be applied to those organisms where metabolic and digestive products are intimately mixed in feces. In the latter case, assimilation measurements are biased by the organism's turnover of materials through normal excretory and secretory processes. Fecal analyses on insects and other invertebrates describe only net assimilation–equivalent to apparent digestible energy (Harris 1966)–and do not reflect true digestive efficiency as often implied in the literature.