Originally, two species of sifaka were recognized in Madagascar: P. verreauxi occupying the dry southern and western forests, and P. diadema occupying the eastern rainforests (Tattersall, 1982). Simons (1988) described a third species, P. tattersalli from Daraina in the far north, which appears to be most closely related to P. verreauxi (possibly the sister taxon of the subspecies P. v. coquereli; Pastorini et al., 2001; Rumpler et al., 2004).
Within P. diadema, four subspecies have been traditionally recognized: perri-eri, candidus, diadema, and edwardsi. These four "types" of diademed sifaka are allopatric, distributed along a north-south gradient, and easily distinguished by virtue of their distinct and colorful pelages. P. d. perrieri, the northernmost taxon, has the smallest distribution, being restricted to the Analamera Special Reserve and small forest fragments to the west. P. d. candidus has a slightly larger range, from the Marojejy massif in the north to the Antainambalana river in the south. P. d. diadema extends from the Antainambalana river in the north to the Onive river in the south; populations in the southwestern part of this range (between the Mangoro and Onive rivers) are morphologically different, and may be taxonom-ically distinct from P. d. diadema (CBSG, 2002; Glander and Irwin, unpublished data). Finally, P. d. edwardsi is found from the Onive river in the north to the Manampatrana river in the south.
A fifth "type," P. d. holomelas, had been recognized historically based on collection information, but has been subsumed (Tattersall, 1986) into P. d. edwardsi, as these two forms appear to have been sympatric. However, the extirpation of populations from areas thought to be inhabited by this variant means that we might well have lost a fifth taxon in historic times.
The taxonomic level at which these "types" of Propithecus diadema should be recognized has been subject to debate; all are allopatric in the wild, and therefore reproductive isolation cannot be demonstrated. Karyotypic differences exist (Rumper et al., 2004; Mayor et al., 2004) with P. d. edwardsi having a karyotype of 2n = 44 and all other types having 2n = 42. Mayor et al. (2004) propose that sequence differences in mitochondrial DNA warrant the elevation of these types to species, following the phylogenetic species concept. Following these authors, I will treat these four taxa as full species, within the "diadema group," which is still considered to be monophyletic on both morphological and molecular grounds.
All told, the distribution of eastern sifakas is extremely broad (Figure 1, Table 1), spanning from 12.75 to 22.75 degrees south, with an altitudinal range between sea level and 1650 m. Most remaining eastern forest is occupied by sifakas, except the extreme southeast (south of the Manampatrana river), the Masoala peninsula in the northeast, and the transitional "Sambirano" forest in the northwest (Mittermeier et al., 1994; Irwin et al., 2005). This broad range harbors marked climatic variation. Average temperature decreases from north to south, while seasonal variation increases; superimposed upon this is a decrease in temperature with increasing elevation (Donque, 1972). Rainfall is high throughout most of the east, owing to the steep ascent of the trade winds striking the eastern escarpment, and annual rainfall is typically 1500-4000 mm/year. However, the northern tip of the island, near Antsiranana and Vohemar (including the range of P. perrieri and P. tattersalli), has no escarpment; rainfall is much lower (1000-1500 mm/year) and this region's forests are consequently much drier.
Sifaka species have presumably evolved ecological and behavioral adaptations to these varying environments, but these have thus far been underexplored and are a promising direction of future research. For now, it is interesting to note this group's wide range in body mass, which correlates with climatic variables (Lehman et al., 2005); this is suggestive of ecological differences.
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