All members of the Indriidae (Avahi laniger, Propithecus spp., Indri indri) have specializations to accommodate a folivorous diet. Early dissections showed them to be anatomical folivores with hypertrophied salivary glands, voluminous stomachs, sacculated ceca, and looped colons, which facilitates efficient digestion of leaf parts (Hill, 1953). Anatomical folivores can be classified as either foregut (e.g., colobine monkeys) or midgut fermenters (previously known as hindgut fermenters) (Chivers, 1994). All indriids are the latter, exhibiting increased surface area in the midgut where nutrients are made available through fermentation of fiber by symbiotic gut flora (protozoan and bacterial) (Bauchop, 1978; Hladik, 1978; Parra, 1978). During fermentation, volatile fatty acids are released and then passed into the bloodstream where they are taken up by the animal as a form of assimilable energy. Compared to foregut specialists, midgut fermenters are able to consume more fibrous forage but need larger volumes to pass through their digestive tract to ensure sufficient extraction of nutrients (Janis, 1976). Within the indriid clade, Indri shows the highest degree of specializations for foliage digestion over any other confamilial (Hill, 1953).
The teeth of Indri indri are also indicative of a folivorous lemurid. All indri-ids have a reduced dentition, with just 30 teeth rather than the lemur-typical 36 teeth (Swindler, 1976). The molars of Indri have high crowns and long shearing blades (Kay and Hylander, 1978) to slice up fibrous plant matter and fruit seeds. In addition, Indri's molars are squared off with a bilophodont morphology that only the Indriidae and Cercopithecoidea exhibit (Maier, 1977). The large crushing basins of these molars are highly effective at breaking down plant matter to even finer grades, the first step toward efficient digestion (Kay and Hylander, 1978).
Indriids have a toothcomb comprised of a single set of incisors and canines (four teeth total) rather than the two sets of incisors and one set of canines (six teeth total) that are typically found in other lemurids. Indri indri uses its tooth-comb for both grooming and feeding purposes. At Mantadia, Indri utilized its toothcomb to pry out the large seeds from the leathery exocarp of Beilschmiedia fruits (Lauraceae) (Powzyk, 1997). This fruit resembles a small avocado with an oversized cotyledon without the fleshy fruit. Indri would first pluck a fruit with its front teeth and then, while clenching it in one hand, slice through the skin with its cheek teeth. Once the fruit was opened, the toothcomb was used to scoop out the seed by moving the lower jaw in an upward motion to release the hard seed from the outer skin. Toothcomb grooves were clearly observed on the fallen debris (Powzyk, 1997). Since Indri fully masticates all food items, it should be considered a seed predator rather than a seed disperser (Powzyk, 1997).
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