Distribution, Diversity, And Latitude in African Primates

Abstract

According to Rapoport’s rule, the latitudinal ranges of species become progressively smaller toward the equator (Stevens 1989). Despite a number of studies that describe this pattern for a variety of taxa, the existence of this phenomenon in tropical organisms has recently been questioned (Rohde and Heap 1996). With respect to terrestrial taxa, several studies have included land masses and species whose ranges extend into the tropics (e.g., Nearctic mammals, Pagel et al. 1991; Palearctic mammals, Letcher and Harvey 1994; Australian mammals, Smith et al. 1994), but few have focused on a taxon that is predominantly tropical. In a notable exception, Ruggerio (1994) found support for Rapoport’s rule in Neotropical mammals, but this analysis was plagued by problems of data nonindependence (in both statistical and phylogenetic dimensions). The presence or absence of Rapoport’s rule in tropical organisms is accorded particular importance because Stevens (1989) has argued that the same process that underlies this pattern might also be responsible for the progressive increase in species richness toward the equator. In light of these facts, the purpose of this short note is to examine latitudinal gradients in the geographical range and diversity of a well-known group of terrestrial animals that exists in and around equatorial regions: the African primates. African primates were chosen for this study because they are predominantly tropical but also exist outside the tropics from the Mediterranean to the Cape (Wolfheim 1983); their geographic distribution is well-known, obviating the possible confounding effects of sampling bias among poorly known taxa for tests of Rapoport’s rule (Colwell and Hurtt 1994); and their phylogeny is wellknown (Purvis 1995). In total, 64 primate species were recognized following Oates’s (1996) classification (we exclude Homo sapiens). In keeping with previous studies of the Rapoport phenomenon, the ranges of these species were determined on the basis of the most recent data available. These geographical distributions are the net result of historical processes that may relate to both natural climate change (e.g., red colobus; Oates and Davies 1994) and human activities