Origin And Evolution Of Primate Social Organization

As it is impossible to investigate the social life of fossil prosimians (but see Krishtalka et al., 1990; Plavcan et al., 2002), a reconstruction of the ancestral condition of primate social organization has to be based on what is known of the biology of extant species. It is widely accepted that the cheirogaleids and the lorisiforms are likely to represent the first primates most closely (CharlesDominique and Martin, 1970; Martin, 1972). The social organization patterns of these primates will therefore give the best indication of the ancestral primate condition. In addition, the whole lemur group is of major interest because lemurs exhibit a wide range of size and activity types (e.g., Fleagle, 1999; Sussman, 1999; Tattersall, 1982, 1987). The high diversity of lemur adaptations may provide insights into the evolution of primate social organization, since the evolutionary shift from nocturnal to diurnal activity rhythms is believed to have involved the change from solitary foraging to foraging in cohesive groups (Martin, 1981; van Schaik and van Hooff, 1983). In addition, information on other mammals should be included in any analysis that aims to trace evolutionary patterns among primates (Martin, 1981), because "early 'advances' in primate evolution can only be defined in relation to the hypothetical ancestral condition for placental mammals [...]", and "[...] in order to reach a clear formulation of the evolutionary origins of any single order of placental mammals, it is also necessary to take into account the evolution of monotremes and the marsupials [...]" (Martin, 1990: 141).

Müller and Thalmann (2000) reviewed the social organization of the strep-sirrhine primates, as well as that of several orders of mammals in order to investigate the ancestral pattern of social organization in primates and mammals in general. The ordinal relationships between primates and other mammals have yet to be resolved (see Müller and Thalmann, 2000 for references). For that reason, Müller and Thalmann (2000) used those orders that have either primitive morphological characteristic and/or are believed to be related to the primates in their comparative analysis of the evolution of social organization in primates and other mammals. The orders of placental mammals that were investigated include the insectivores (Insectivora), elephant shrews (Macroscelidea), tree shrews (Scandentia), and flying lemurs (Dermoptera). They further investigated those marsupials that are believed to approach the ancestral marsupial condition most closely, such as the didelphids (Didelphimorphia) and dasyurids (Dasyuromorphia) (Springer et al., 1997) as well as the monotremes (Monotremata). For every group, the most frequent pattern of social organization was inferred to be the ancestral condition of the group (Müller and Thalmann, 2000).

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