Food Choice and Digestive Strategies of Two Sympatric Primate Species

Abstract

Critical determinants of dietary choice in animals may be internal rather than external and therefore not readily detected by field observation. Digestive strategies of 2 sympatric primate species, howler monkeys (Alouatta palliata) and spider monkeys (Ateles geoffroyi), both primary consumers, were investigated by carrying out feeding trials to determine food passage rates and by examining selected aspects of gut morphology. On the average, a given marker took 20.4 .+-. 3.5 h to first appear in the feces of howler monkeys as contrasted with a mean of 4.4 .+-. 1.5 h for spider monkeys. Gut morphology showed that howlers had colons approximately double the size of those of spider monkeys. Howler monkeys are highly folivorous while spider monkeys are primarily frugivorous. Yet leaves are generally low in nonstructural carbohydrates while fruits are low in protein. Howlers, with their capacious hindguts and slow food passage rates, are able to ferment refractory plant parts more efficiently than spider monkeys and in this way can maximize energy returns from leaves. Spider monkeys, with smaller hindguts than howlers, are able to process greater quantities of food per unit time. In this way they can specialize on fruits, which are generally too low in protein to support howlers. Once a particular digestive strategy has evolved, with its attendant morphological, physiological and behavioral adaptations, diet switching, at least over the short run, does not appear possible.