Immunocompetence and Animal Population Regulation

Abstract

Although a plethora of hypotheses exists to explain the natural regulation of animal populations, nearly all have one important attribute in common; numerical fluctuations in animal populations occur to a large extent as the result of individuals responding to environmental factors via altered mortality, particularly among the neonatal age classes. Protein availability has been implicated as the most influential environmental factor driving these demographic changes, especially among herbivore populations. I believe the evidence supports involvement of host immunocompetence as the primary mechanism by which environmental selection forces influence fitness (survival) of individuals within herbivore populations, mostly in response to changing protein nutrition. Because immune responsiveness is largely genetically based, and often associated with certain reproductive attributes, 1 propose a model of population regulation whereby environment can influence the frequency of immunocompetent and possibly fecund genotypes (within the major histocompatibility complex) within animal populations through selection. The proposed model represents a synthesis of several recent studies that have implicated involvement of host immunity in population declines and mate selection. The evidence in support of the proposed immunocompetence selection hypothesis of population regulation is reviewed and future research directions for testing this hypothesis are discussed.