Refining approaches and diversifying directions in ecoimmunology

Ecoimmunologists have made many important discoveries about the immune systems of wild animals including (1) immune activity is usually costly, (2) counter-intuitive decrements in immune activity are often due to trade-offs with other physiological activities or behaviors, and (3) immune activity is a currency by which sexually selected traits are indices of individual quality. The use of single assays to characterize "immunocompetence," however, as was, and is, the common practice in ecoimmunology, ignores the inherent complexity of the immune system and may have led to inappropriate conclusions or even positive publication bias. Recently, some have suggested that ecoimmunologists measure disease resistance or the fitness consequences of immunological insults instead of the immune system itself. We propose that researchers continue to use the techniques that have already been fruitful in ecoimmunology, but better incorporate the underlying immunophysiology of such techniques into their study designs and interpretation. We review the benefits and some recent successes of such an approach. Then, we discuss several under-explored but potentially rewarding areas of ecoimmunology, including development of the immune system, immunosenescence, and immunoredistribution. All three areas are well studied in biomedicine and are likely to be relevant in ecological contexts. For instance, because of the inherent costliness of immune defense and reproduction, variation in rates of development and senescence of the immune system likely impacts the ways in which individuals of different species mature and/or breed. Likewise, differential capacity to redistribute immune resources in response to changes within the endocrine system may explain some of the inconsistencies regarding the immunocompetence handicap hypothesis of sexual selection.