Digestive Efficiencies of Wild Howler Monkeys

Abstract

(1) Two sets of feeding trials were carried out on temporarily caged wild howler monkeys (Alouatta palliata) to determine digestive efficiencies with respect to cell-wall constituents and protein in natural items of diet. One trial diet consisted primarily (81%) of ripe fruit; the other consisted primarily (87%) of young leaves. (2) Samples of all foods used in the trials, as well as urine and feces, were collected and preserved. These were analyzed for total cell-wall content, cell-wall components (cellulose, hemicelluloses, lignin, cutin), and crude protein. In vitro digestibility of crude protein in trial foods was examined, using pepsin in a solution of HCl. (3) Adult howler monkeys averaged a digestive efficiency of 24% with respect to total cell-wall material on the fruit diet and 42% on the leaf diet. All animals were more efficient at degrading cellulose than hemicelluloses. In both trials fecal nitrogen content was high, and apparent digestibility of protein was low. True digestibility estimates, however, indicated that an average of 77% of dietary protein was removed in transit on the fruit diet and 89% on the leaf diet. The results of the in vitro digestibility trials indicated that tannins or their precursors are present in many natural foods of howlers. (4) It is concluded that considerable fermentation of plant structural carbohydrates takes place in the digestive tract of howler monkeys. Fermentation end products may be of particular importance to howlers at times of the year when foods rich in ready energy are in short supply.