Rates of fermentative digestion in the howler monkey, Alouatta palliata (primates: ceboidea).

1. Caecal material of wild howler monkeys was analyzed by gas chromatography for evidence of fermentation activity and rates of production and absorption of volatile fatty acids. 2. Results showed a high rate of production of acetic acid and lesser production of propionic, butyric and isobutyric acids. The VFA content of the blood was increase in passage through the caecal vascular system. 3. We estimate that howler monkeys may obtain as much as 31% of their required daily energy from fermentation end products. 4. Energy rich fatty acids may be of particular importance to howlers when they are living on diets high in leaves, which have high cell wall contents and low contents of nonstructural carbohydrates.