Effects of age on immune function in free-living animals: Ontogeny, senescence, and their implications

Abstract

Little is known about the development of immune function in wild animals. We investigated the ontogeny of immune defense in a free-living bird, the tree swallow. We assessed total and differential leukocyte counts, natural antibodies, complement activity, in vivo skin swelling response, and in vitro lymphocyte proliferation and compared the levels of development between nestlings and young adults. We also assessed whether body condition explained variation in these immune components. We found some support for the prediction that innate defenses, which do not need to generate a broad repertoire of specific receptors, would reach adult levels earlier than adaptive defenses. In contrast, we found limited support for the prediction that adaptive defenses, which are thought to be more costly to develop, would be more related to body condition than innate defenses. We discuss our findings in the context of other studies on the ontogeny of immune function. Introduction Ecoimmunology is a rapidly growing field of evolutionary ecology that studies immune defense strategies and their relationship to the life history and ecology of organisms [1-4]. A major aim of ecoimmunology is to understand the causes and elucidate the consequences underlying the broad variation in immune defense strategies observed within and among species. One key insight from such studies has been that immune function is costly in terms of energy and/or limiting nutrients (e.g., protein, carotenoids) and therefore traded-off with other costly functions such as growth and reproduction. An important aspect of ecoimmunology that has been relatively understudied is the age-specific variation in immune function [3].