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(Meyers, 1993), 3 groups a A subset of time devoted to "Other / Unknown." b Value for one group only; second group consumed soil "only rarely."

(Meyers, 1993), 3 groups a A subset of time devoted to "Other / Unknown." b Value for one group only; second group consumed soil "only rarely."

Among primate groups on other continents, eastern forest sifakas' diet is most similar to the asian colobines (e.g., Davies, 1991; Meyers, 1993; Koenig and Borries, 2001). These colobines also have morphological adaptations for folivory (in this case, foregut fermentation), a diverse diet, and seasonal variation quite similar to that of eastern sifakas.

While all Propithecus consume large amounts of foliage, it has been suggested that P. verreauxi in western and southern forests tends to be a frugivore-folivore, while eastern Propithecus tends to be a granivore-folivore (Richard, 2003). However, recent evidence does not completely bear out this generalization. While eastern sifakas at some sites (Ranomafana: Hemingway, 1995; Mantadia: Powzyk, 1997) fit this pattern, P. diadema at Tsinjoarivo more closely fit the frugivore-folivore model; they often consume fruit pulp and discard seeds (Irwin, 2006). The reason for this discrepancy is unclear, but it is possible that floristic changes related to Tsinjoarivo's high altitude create a fruit guild more like that of drier forests.

The sifakas' relatively catholic diet is in stark contrast to most other lemur groups, which tend to specialize on specific plant parts. Most Eulemur taxa in eastern rainforests concentrate heavily on reproductive parts (flowers and fruits), and consume very little foliage (Overdorff, 1993). Most other groups (Avahi laniger, Indri indri, and Lepilemur spp.) are more dedicated to folivory (Ganzhorn et al., 1985; Ganzhorn, 1988; Harcourt, 1991; Powzyk, 1997). In the southern part of their range, rainforest sifakas are the most folivorous of their diurnal lemur communities; in the north they are sympatric with Indri, a similar-sized indriid more fully devoted to folivory (Powzyk and Mowry, 2003). In all regions, they are considerably less folivorous than the sympatric nocturnal genera Avahi and Lepilemur.

There are conflicting reports concerning which plant parts are preferred. P. tat-tersalli at Daraina, P. edwardsi at Ranomafana, and P. diadema at Mantadia track immature leaf availability (consumption of this resource is positively correlated with its availability; Meyers and Wright, 1993; Powzyk, 1997). In contrast, Irwin (2006) found highly significant positive correlations between fruit availability and consumption in P. diadema at Tsinjoarivo, suggesting that fruit is the preferred resource. Finally, newer data from Ranomafana (Wright et al., 2005) also suggest that P. edwardsi at Ranomafana track fruit availability. Further research is necessary, particularly to control for such confounding factors as chemical variation among plant species and the preferred maturity level (i.e., ripeness) of selected foods.

Finally, eastern sifakas differ from other sympatric lemurs in their treatment of fruits and seeds (Overdorff and Strait, 1998). Eulemur species mainly derive nutrients from pulp, either dropping whole seeds at the feeding tree or ingesting and defecating them whole. Sifakas, in contrast, either consume pulp and drop seeds (Irwin, 2006) or, more commonly, masticate the seeds they consume (some smaller seeds [e.g., Ficus sp.] may be consumed whole). Eulemur feces often contain multiple whole seeds, while sifaka feces are usually homogeneous with no discernible plant parts. As a result, unlike Eulemur species (Overdorff, 1993; Dew and Wright, 1998), sifakas provide limited or no seed dispersal.

Seasonality

All populations for which long-term data are available show extreme seasonal variation in diet composition (Meyers, 1993; Hemingway, 1995; Powzyk, 1997; Irwin, 2006). Generally, sifakas consume high levels of fruit and/or seeds in the rainy season (December-April) when these are most abundant; during this time fruits and seeds can account for 70-90% of feeding time. Diet in the remaining months is more variable but fruit and seeds generally constitute less than 10% of feeding time. P. edwardsi at Ranomafana consume more leaves at this time, but still maintain a modest intake of fruit and seeds (including seeds from fallen, rotting fruit; Hemingway, 1995). P. diadema at Mantadia consume high levels of leaves, as well as flowers and fern fronds, during this time (Powzyk, 1997).

P. diadema at Tsinjoarivo follow a different strategy (Irwin, 2006). They consume high levels of young leaves at the beginning (May-June) and end (October-November) of the dry season, but rely on flowers during the height of the dry season (July-September), spending up to 50% of feeding time on this resource. Their diet at this time is heavily monotonous, with the buds, flowers, and leaves of a hemiparasitic mistletoe (Bakerella clavata) accounting for 45-70% of feeding time. This is an extreme and unusual level of devotion to a single species.

Taxonomic Composition of Diet

The taxonomic composition of diet appears to be relatively flexible, varying widely between study sites (Table 3). Myrtaceae is the dominant plant family for P. edwardsi at Ranomafana and P. diadema at Mantadia but Loranthaceae

Table 3. Preferred food resources for eastern sifakas, ranked by feeding time

(a) "diadema"

group

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