CP = crude protein, BP = bound protein, AP = available protein, NDF = neutral detergent fiber, ADF = acid detergent fiber, Ls = sulfuric acid lignin, WSC = water-soluble carbohydrates. Mean values are % dry matter. il = immature leaves.

CP = crude protein, BP = bound protein, AP = available protein, NDF = neutral detergent fiber, ADF = acid detergent fiber, Ls = sulfuric acid lignin, WSC = water-soluble carbohydrates. Mean values are % dry matter. il = immature leaves.

These additional food samples were eaten by either Indri or Propithecus diadema and included important seasonal food items such as tree bark, ferns, and fruits. Plant samples were assayed for total phenolics, hydrolyzable tannins, condensed tannins, and alkaloids. Ecological theorists have long proposed a defensive role for such plant secondary metabolites (Karban and Baldwin, 1997; Rosenthal and Berenbaum, 1992) and studies have suggested that these compounds act as deterrents in primate (as well as other mammalian) food choices (Glander, 1978; Hladik and Simmen, 1996; Hume, 1999; Lawler et al., 1998; McKey et al., 1981; Oates et al., 1977, 1980; Simmen et al., 1999). Tannins and alkaloids have been the most widely studied classes of secondary compounds in relation to folivory (Rosenthal and Berenbaum, 1991). Plant tannins can form indigestible complexes with digestive enzymes and ingested proteins and thus limit nitrogen assimilation by folivores (Hagerman and Butler, 1991); they may also have an antibiotic effect on gut microflora (Waterman and Mole, 1994). Nitrogen-based alkaloids can be potentially toxic by crossing cell membrane barriers and disrupting metabolic activity (Harborne, 1982) and include compounds such as strychnine and cyanide.

Nevertheless, no significant correlations between the preference of a particular food item by Indri indri and the item's corresponding level of putative antifeedants (total phenolics, hydrolyzable and condensed tannins, or fiber) were found. However, Hematodendron glabrum fruit, the only fruit eaten by Indri that was assayed, was quite low in phenolics and lacked alkaloids, and none of the other Indri food samples tested contained alkaloids, even though these items represented nearly half of Indri's total annual diet. In contrast, sympatric Propithecus diadema routinely ate two alkaloid-containing species of fruit (not within the top ten preferred food items) and consumed soils twice as often during the study as did Indri. Soil consumption by Indri occurred on 22% of the sample days on which they were followed for full-day data collection (Powzyk, 1997). Geophagy can detoxify deleterious compounds (Diamond, 1999; Gilardi et al., 1999; Hladik, 1977a,b; Krishnamani and Mahaney, 2000; Oates, 1978). Results that show Indri's limited intake of digestion-inhibiting carbon-based compounds (e.g., tannins) are either inconclusive or nonexistent, but they do appear to be avoiding potentially toxic alkaloids in their dietary choices. The entire secondary compound data set can be found in Powzyk and Mowry (2003).

Plant Species Consumed

Indri indri consumed 76 different plant species within the Mantadia research site, with an average of 11.19±2.52 on a daily basis (Powzyk, 1997). At Betampona, Indri consumed a total of 42 plant species (Britt et al., 2002). This difference in plant species choice may be a reflection of forest type (plant species diversity) and/or a reflection of disturbance levels. Mantadia has not been selectively logged while Betampona has experienced both logging and hunting (Welch, personal communication) Lauraceae (35.5%), Clusiaceae (29.3%), and Myristicaceae (16.3%) were the most preferred plant families at Betampona and accounted for 78% of all identified feeding records. At Mantadia, the most preferred plant family was Lauraceae (26 plant species consumed, 34.2% of feeding time), followed by Euphorbiaceae, with four "Uapaca" species eaten (22.3%), including their immature leaves, galls, and seasonal flowers. The third most preferred plant family at Mantadia was the Clusiaceae with 13 species representing 16.8% of total feeding time (Powzyk, 1997). Thus, the top three plant families accounted for 73.3% of total feeding time observed. Interestingly, the fourth most popular plant family at Mantadia was Myristicaceae with a single species, Haematodendron glabrum, known locally as "Rara." This tree species represents a vital food resource of immature leaves and fruit for Mantadia Indri. Britt et al. (2002:235) found this plant species to be the "most preferred single food source" at Betampona and labeled it a "keystone species" for Indri. Betampona Indri consumed every aspect of this tree, including its mature foliage, immature foliage, fruit, bark, seeds, flowers, and petioles (Britt et al., 2002). Knowing which plant species are most preferred by Indri will have direct applications to its conservation (Britt et al., 2002). Pollock (1977) reported that Indri fed upon 62 plant species, yet this number was derived from several forest locations in and around the Special Reserve Analamazaotra.

Preferred Plant Types and Forest Height When Feeding

Indri indri fed 5-15 m above ground level for the majority of feeding records at Betampona with only 0.7% above 25 m (Britt et al., 2002), while at Mantadia, feeding heights varied from 1 to 32 m with a preferred height of 12.7 m (Powzyk, 1997). Clearly, Indri chooses most of its food from trees: 98.3% of foods consumed at Mantadia came from trees, followed by 0.8% from lianas, and 0.1% from parasitic plants. Indri was not observed to feed on any ground herbs, hemiepi-phytic plants, or ferns, all of which are eaten by Propithecus diadema at Mantadia. P. diadema spent more time on or closer to the ground than Indri. P. diadema often came down to the ground to feed on fallen fruit in the leaf litter, as well as browsing on low-growing herbs, ferns, and lianas. One particular parasitic plant genus, Bakerella, comprised 17% of P. diadema's flower eating feed time while Indri spent just 0.11% of its feed time on this plant genus (Powzyk, 1997). Following a cyclone in 1994, numerous canopy gaps formed in Mantadia from the loss of large mature trees. Lianas were often the first colonizers in these new sun-lit patches, and P. diadema spent long periods of time feasting on the new foliage from these quick-growing vines. Indri did not appear to benefit from this sudden resource availability, but rather incurred a cost since many of the fallen trees had been their food trees (Powzyk, personal observation).

Feeding Behavior and Activity Patterns

Ecological comparisons between sympatric Indri indri and Propithecus diadema at Mantadia provided insight into the effects of diet on behavior. The more foliv-orous Indri's feeding bouts were fewer in number but longer in duration than those of P. diadema, whose diet was more varied in both the number and types of foods eaten. Indri's average number of daily feeding bouts changed seasonally and ranged from 14.5 to 27.4, with fewer feeding bouts occurring during the dry months (Figure 2). P. diadema usually had twice as many daily feeding bouts as Indri in any given month. The availability of food types also corresponded to the duration of Indri's feeding bouts: feeding bouts were shortest on ubiquitous young leaves with increasingly longer bouts occurring on more ephemeral flowers and fruit (see Figure 2). Indri's longest feeding bouts were observed in an adult female who fed continuously on a Cryptocarya (Lauraceae) fruit; one bout lasted 2 hours and 37 minutes, while another bout lasted for 1 hour and 27 minutes. This individual carefully chewed and softened the tough exocarp, causing the husk to tear open, whereby the seeds were squeezed out while the remaining fruit was dropped (Powzyk, 1997).

Indri indri also had significantly shorter daily path lengths (yearly means for dpl: Indri =740 m, Propithecus diadema =1629 m) and shorter active periods than Propithecus diadema (Powzyk, 1997). The Indri is highly diurnal; on one Indri sample day, the focal pair awoke at 9:12 and settled into a sleep site at 13:18, while P. diadema typically spent an additional 2-3.7 hours active per day than Indri (Powzyk, 1997). Indri established/defended their territories using long call vocalizations and appeared to avoid all intergroup contact with no observed intergroup fighting within the Mantadia site. Within the smaller Analamazaotra Special Reserve, intense intergroup fighting has been observed, although this may be an outcome of a "hyperconcentration" of Indri owing to a lack of viable corridors for animal transfer (Petter and Peyrieras, 1974). Not surprisingly, Analamazaotra Indri had smaller territories than Betampona or Mantadia Indri (Glessner and Britt, 2005; Pollock, 1975a,b, 1977; Powzyk, 1997) (Table 1). In contrast to Indri, P. diadema maintained their territorial defense with more energetically costly behaviors such as extensive territorial patrols, scent marking, and intergroup fighting. P. diadema also exhibited significantly more play behavior than Indri both in the trees and when P. diadema ventured down to the ground to wrestle (Powzyk, 1997). Finally, Indri's daily defecation rate was half that of P. diadema's (2 versus 4.7), while Indri spent significantly more time resting during the 12-hour daylight period than did P. diadema (Powzyk, 1997). These are important behavioral adaptations since increased resting facilitates efficient fermentation of plant fiber. Such "inactivity" allows blood to be relegated to its digestive tract; when a primate is traveling, its sympathetic nervous system sends blood from its gut out to the limb muscles and heart (Smith, 1977). Based on Indri's digestive anatomy, dietary choices, and behavioral patterns, we classified it as an animal who makes efficient use of its cecocolic fermenting ability while conserving energy (Powzyk and Mowry, 2003).

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