Colobines

Food and Digestion

Colobine monkeys differ from cercopithecines in having large, multi-chambered stomachs, an [Page 531]anatomical feature closely related to their behavior and ecology. Anterior sacculations of the fore-stomach are fermentation chambers. They contain bacteria capable of digesting the celluloses and hemicelluloses of dietary fiber. In the forest habitat that most colobines favor, leaves are the most obvious and abundant high-fiber food, and the colobines are sometimes referred to collectively as the “leaf-eating monkeys.” However, the digestive system of colobines does not confine them to a completely folivorous (leaf-eating) diet. Less than a tenth of studied species have diets with more than 30% leaves, and when feeding on leaves, they tend to select the youngest ones. Fruits and seeds/nuts are other major constituents of their diets, making up more than a third of the annual diets of several species. Seed storage tissues, whether carbohydrate or lipid, are richer sources of energy than are young leaves. However, seed storage tissues are also substantially tougher than young leaves. Colobines have molar teeth whose shapes and sizes appear to be well adapted to chewing both leaves and tough seeds.

A second possible function of colobines' fermentation chambers is that they may lower the gut's concentrations of various toxic compounds that are produced by plants as a defense against destructive plant eaters. For example, detoxification may account for the ability of purple-faced langurs to eat the toxic fruits ofStrychnos trees, which contain toxic alkaloids such as strychnine. Conversely, Strychnos fruits are avoided by toque macaques, which are cercopithecine monkeys that also have access to these fruits but do not have highly developed fermentation chambers in their guts.

The fermentation chambers of colobines enable them to process relatively indigestible forest foliage when other foods are scarce, thus enabling them to attain unusually high biomass. In many forest communities, the biomass of colobine monkeys is greater than that of all other primates combined.

Social Groups

Colobine monkeys of many species live in relatively small social groups, consisting of a single adult male, several adult females, and their offspring. In other colobines, social groups range from small, monogamous associations to groups of over 200 with many adult males. Compared with cercopithecine primates, aggressive and other interactions occur infrequently among females in forest-dwelling colobines. This difference in behavior has been attributed to leaves as a key food resource for many colobines. The abundance and dispersion of leaves within large tree crowns allows several animals to feed together without competition. In contrast, foods of cercopithecine monkeys tend to be more clumped: to occur in smaller, more dispersed patches. In cercopithecines, cheek pouches, which colobines do not have, may be an adaptation to the relatively clumped distribution of fruits and other foods that they commonly eat, enabling them to take quantities of foods into their mouths faster than they can chew and swallow them, thereby reducing competition.

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