Organisms often respond in ways that appear to benefit others rather than themselves. This phenomenon is consistent with the views of Darwin (1859) and Dawkins (1999) that individuals may exploit the responses of others. This phenomenon, “social parasitism”, has been extensively investigated in social insects, particularly, ants. Other empirical studies have demonstrated social parasitism in fish, birds, and mammals. This paper reviews several possible examples of mammalian social parasitism, with an emphasis upon intraspecific social parasitism (ISP) in Neotropical primates. Social parasitism is discussed as a life history feature of long-lived, social organisms such as many primates, including humans. A simple mathematical model, applied to social parasitism, is presented linking parasite transmission to a parasite’s influence on its host. Phenotypic manipulation is assessed as a mechanism of social parasitism, and possible examples from the literature on Neotropical primates are provided. Social parasitism is discussed in relation to the evolution of higher grades of sociality (eusociality, cooperative breeding), manipulation success (infectivity), and the evolution of virulence (e.g., aggression, punishment). It is proposed that an understanding of variations in virulence and infectivity by social parasites is likely to reveal important evolutionary dynamics for an integrated view of social evolution.
A rare species contains a small absolute number of individuals, and theoretical and empirical ecologists have provided quantitative approaches to the study of differential species abundance (e.g., Preston, 1962 a, b; Gaston, 1994). The studies show that rare species are more “extinction prone” because they are more vulnerable to demographic, genetic, environmental, or catastrophic perturbations (e.g., mortality, inbreeding, habitat fragmentation, or drought, respectively). These perturbations cause “imbalances” of varying intensities, durations, frequencies, and rates which may cause population numbers to fluctuate below thresholds required for recovery. Some authors have defined and analyzed three domains of rarity: a “within-habitat” domain (alpha-rarity) (e.g., population density); a “between-habitat” domain (beta-rarity) (e.g., the number of different habitats occupied by a local population); and, a “geographic” domain (gamma-rarity) (e.g., the areal range of a species). It has been argued that species may be “extinction prone” because they occur in one or more domain of rarity and that causes of extinction may be multidimensional. These factors were studied in the Primate Order employing a subset of 97 species extracted from Wolfheim (1983). Trophic patterns for each dimension of rarity and for their combinations were also studied and found to vary from domain to domain. The broad habitat specificity of the Order implies that most species are distributed across a mosaic of edaphic and phytogeographic areas, responding with differential “norms of reaction” to stimulus patterns as they occur within and between populations. The conservation implications of the observed patterns are discussed, and a “signature” may be identified whereby Primates, broad habitat specialists, appear to display an association between endemism and low differentiation into subspecies and races. Endemism is favored where one finds poor vagility, poor survivorship, or poor colonization, traits thought to characterize large mammals, such as Primates. The traits that predispose Primates to extinction-vulnerability are components of the dimensions of rarity and may predict those strategies most likely to maximize the preservation of primate species diversity. It is concluded that Primates will be conserved where they coexist with other fauna and flora of greater ecological significance in “hotspots” of biological activity in large reserves that are close together.