Feeding ecology and diet

Primate species exhibit a wide range of diets, although most of them include at least some fruits in their food intake. If there is a typical dietary category for primates generally, it is surely fruit consumption, as this is found from the smallest to the largest species. Although most primates eat at least some fruits, primates can be classified into three main dietary categories representing at least 50% of food intake: (1) insectivores, feeding mainly on arthropods (e.g., tarsiers); (2) frugivores, feeding mainly on fruits (e.g., most forest-living monkeys); (3) folivores, feeding mainly on leaves (e.g., leaf-monkeys). There is a general trend among primates for the diet to shift progressively from insectivory through frugivory to folivory as body size increases. This is understandable because small-bodied mammals have relatively high-energy requirements per unit body weight and must eat foods with a rich, easily available energy content. Large-bodied mammals have relatively low energy requirements per unit body weight and can consume foods that have a poor energy content and require extensive digestion. As a general rule, insectivorous primates do not exceed 1.5 lb (700 g) in body weight, while folivorous primates tend to be quite large-bodied species. Sportive lemurs (Lepilemur) and avahis (Avahi), which weigh between 1.4 lb (650 g) and 2.2 lb (1 kg), are both exceptions to this rule, but they can cope with their relatively low-energy food intake because they have unusually low metabolic rates. In fact, a fourth dietary category known as gummivory must be recognized for primates whose food intake includes more than 50% of plant exudates (gums). Gums resemble fruits in that they are a major source of carbohydrates, but they resemble leaves in that the carbohydrates are polymerized and require extensive digestion. Many primate species include at least some plant exudates in their diets, but there are just a small number of gum-feeding specialists, such as the fork-

Crowned lemur (Eulemur coronatus) females feeding on bark. (Photo by Harald Schutz. Reproduced by permission.)

crowned lemur, the needle-clawed bushbaby and some marmoset species.

Most primates forage primarily in trees or bushes for insects, fruits, leaves and/or gums. Regardless of the diet, the visual sense plays a major part in searching for food. Nocturnal primates generally have only a very restricted capacity for distinguishing colors and must rely on other dietary cues, but diurnal primates usually have some form of color vision. Fully developed trichromatic color vision of the kind found in humans occurs in Old World monkeys and apes and a few New World monkeys. Most New World monkeys and all diurnal lemurs have fundamentally dichromatic vision, although in certain New World monkeys there is an unusual polymorphism of the gene coding for a retinal pigment on the X-chromosome, such that some females have a form of trichromatic vision. Prosimian primates generally collect their food primarily with the mouth, but in higher primates the hands play an increased role. As a rule, food items are consumed directly, but in some cases there is some pretreatment of food items. For instance, some capuchin monkeys break nuts by pounding them on branches or tree trunks, while certain chimpanzee populations show nut-cracking involving the use of some kind of hammer and anvil. Chimpanzees have also been reported to use twigs or stems as tools to extract termites from their mounds.

Most primates lack any obvious special foraging adaptations, but there are a few conspicuous exceptions. The tooth-comb in the lower jaw of strepsirrhine primates is, for instance, commonly used in gathering food as well as for grooming. Some lemurs, bushbabies and lorises use the tooth-comb to harvest gum, and many species use it to scoop out the pulp of large fruits. However, the tooth-comb is quite fragile, so it is typically used simply to scrape up plant exu-dates that seep out following insect damage to tree trunks and branches. In marmosets, by contrast, the lower incisors are elongated to match the canines and all of these stout teeth are used together as a dental tool to gouge holes in tree-trunks to promote the flow of gum. This dental adaptation distinguishes the marmosets from the closely related Goeldi's monkey and tamarins. Undoubtedly the most striking foraging adaptation in primates is found in the aye-aye (Daubentonia) of Madagascar, which has rodent-like incisors in both upper and lower jaws and a very thin middle finger in each hand. The gnawing incisors are used to open up channels occupied by wood-boring larvae in tree trunks, and the thin finger is used as a probe to extract the prey. Experiments have confirmed that the aye-aye can locate larvae in a tree trunk by tapping with the probe-like finger and listening to the echoes. It should also be mentioned that the leaf-monkeys (Colobi-nae) are unique among primates in that they have a complex stomach to permit efficient digestion of leaves.

A red mouse lemur (Microcebus rufus) feeds in the trees in Madagascar. (Photo by Harald Schütz. Reproduced by permission.)
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