Inferences from Primitive Mammals

A review of the patterns of social organization in those groups of placental mammals that are thought to be primitive and/or related to primates also failed to support a harem system as the ancestral pattern for either primates or placental mammals. Very little is known about patterns of social organization in insectivores, but the majority of species exhibit a nonsocial lifestyle (Müller and Thalmann, 2000). Only few species live in social networks. These include the lesser moonrats (Hylomyssuillus), American least shrews (Cryptotis parva), desert shrews (Notiosorex crawfordi), southern water shrews (Neomys anomalus), some white-toothed shrew species (Crocidura sp.), desmans (Desmana moschata), American shrew moles (Neurotrichusgibbsii), eastern moles (Scalopus aquaticus) and star-nosed moles (Condylura cristata) (e.g., Cantoni and Vogel, 1989; Harvey, 1976; Krushinka and Rychlik, 1993; Nowak, 1999). The dominant pattern, however, is promiscuity with the ranges of several males overlapping the ranges of several females and an overall lack of social contacts (Figure 1). Males generally compete for estrous females. Only two species are known to be monogamous: the greater white-toothed shrew (Crocidura russula) and the Pyrenean desman (Galemyspyre-naicus) (Cantoni and Vogel, 1989; Favre et al., 1997; Stone, 1987). To date no species of insectivore has been reported to live in a harem system (Müller and Thalmann, 2000).

By contrast, elephant shrews and tree shrews exhibit monogamy in that the home ranges of an adult male and an adult female coincide (FitzGibbon, 1997; Gould, 1978; Kawamichi and Kawamichi, 1979, 1982; Nowak, 1999; Rathbun, 1979; Sauer, 1973) (Figure 1). In elephant shrews the adult pair has no social contacts (FitzGibbon, 1997; Rathbun, 1979; Sauer, 1973) and their social organization can therefore be referred to as spatial monogamy. In wild tree shrews, the situation remains somewhat unclear. Kawamichi and Kawamichi (1979) report that in common tree shrews (Tupaia glis), 18% of encounters were social encounters and that one-third of these social encounters occurred between an adult male and an adult female. The majority of these intersexual encounters occur between partners (Kawamichi and Kawamichi, 1979). It is not known whether partners share their sleeping sites in the wild, but in captivity, the adult pair shares a nest (Martin, 1968). Pen-tailed tree shrews (Ptilocercus lowii) also occur in pairs, but Madras tree shrews (Anathana ellioti) are mostly seen alone and do not share their nests (Chorazyna and Kurup, 1975; Emmons, 2000; Gould, 1978; Nowak, 1999).

Nothing is known regarding the social organization of flying lemurs, except that individuals forage solitarily and may share sleeping sites (Nowak, 1999). This would indicate that their social organization is of the dispersed type (Figure 1).

Insectivores, tree shrews, and elephant shrews all seem to be relatively close to the ancestral placental stock. It remains unclear, however, which of these orders is most representative of the ancestral placental stock. There are still two possible interpretations regarding the ancestral pattern of social organization in placental mammals: promiscuity and spatial monogamy.

Didelphids and dasyurids generally have a nonsocial lifestyle and the males compete over estrous females (e.g., Bradley, 1997; Buchman and Guiler, 1977; Collins, 1973; Cuttle, 1982; Fleming, 1972; Fox and Whitford, 1982; Jarman and Kruuk, 1996; Lazenby-Cohen and Cockburn, 1988; Morton, 1978; Nowak, 1999; Righetti, 1996; Ryser, 1992; Serena and Soderquist, 1989; Soderquist, 1995; Sunquist et al., 1987; Woolley, 1991). The available data strongly suggest promiscuity to be the dominant pattern of social organization among those marsupials that are believed to approach the ancestral marsupial condition most closely (Müller and Thalmann, 2000) (Figure 1). The available data also suggest promiscuity to be the pattern of social organization present in the monotremes (e.g., Augee et al., 1975; Brattstrom, 1973; Gardner and Serena, 1995; Griffiths, 1978; Gust and Handasyde, 1995; Nowak, 1999; Serena, 1994). It is therefore most likely that promiscuity, being the dominant pattern of social organization among "primitive" mammals, represents the ancestral condition for mammals in general (Müller and Thalmann, 2000) (Figure 1).

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