Social and Behavioral Barriers to Pathogen Transmission in Wild Animal Populations

Disease and pathogens have been studied as regulators of animal populations but not really as selective forces. I propose that pathogens can be major selective forces influencing social behaviors when these are successful at reducing disease transmission. The behaviors whose evolution could have been influenced by pathogen effects include group size, group isolation, mixed species flocking, migration, seasonal sociality, social avoidance, and dominance behaviors. Mate choice, mating system, and sexual selection are put in a new light when examined in terms of disease transmission. It is concluded that pathogen avoidance is a more powerful selective force than has heretofore been recognized. INTRODUCTION Pathogens (diseases, internal and external parasites) are a significant source of mortality, particularly in larger organisms not so limited by predation (Freeland, 1976; May, 1983). May (1988) argues that the significance of pathogens is generally underestimated in ecology. When pathogens are considered, they are generally viewed as simple regulators of population size. For overly large, and thus stressed populations it has often been shown that disease is an effective regulatory mechanism. Thus the most common question asked about disease is its contribution to the total mortality or population dynamics of a population (e.g., Anderson and May, 1979a; 1979b; Ball, 1985; May, 1983; Murray, 1987; Murray et ai., 1986). Freeland (1983) has proposed that coexistence of taxonomically related species may in fact be limited by pathogens, which may therefore control invasions and the overall structure of communities. While the role of pathogens in regulating animal populations and structuring communities is gaining increasing attention, the evolutionary consequences of pathogen-induced mortality have received inadequate attention. In particular, since pathogen transmission may be affected by the rate of contact between individuals (which is affected by group size, mating, and social behavior), it s eem reasonable to wonder whether social behaviors might also be affected by selective pressure to avoid pathogens. However, very little has been done on the possible social implications of disease (as noted by