Intraspecific constraints Organisms allocate energy between competing processes including self-maintenance, reproduction, or growth. Energy availability is limited, and the resources used for one of the competing metabolic tasks (i.e., growth, immune response) cannot be directed towards another. The cost of immunity is central to the understanding of ecoimmunology. Natural selection should favor the optimal immune response that maximizes total lifetime reproductive output. The costs of immunity to parasites occur at the individual and the evolutionary scale.
Innate versus acquired One axis on which these trade-offs occur is the trade-off between
innate and
acquired immunity.
McDade applies a framework that considers three ecological factors that shape life-history trade-offs. The framework suggests that environments with high
extrinsic mortality should favor innate immunity or short-term immunity while low extrinsic mortality should allow for a longer time horizon in order to invest in acquired or long-term immunity. In
Tsimane children, a 49% reduction in growth was observed in children with mild immune activation.
Body size Body size affects the extent to which an organism is exposed to parasites as well as limitations on how organisms can mount an immune response.
Reproduction Physiological and behavioral changes during reproduction are known to influence the
immune system. Trade-offs occur between bodily maintenance (which includes immune function) and reproduction, as metabolic energy expenditure is increased during
pregnancy and
lactation. The reproductive system is unique in that its function is to produce offspring while the immune system provides internal protection. Both systems are regulated by chemical signals in response to environmental stimuli and rely on interactions between both systems in order for each to function properly. however it is unclear if changes in the immune system are causing this as few studies include measures for both immunity and parasitism. A study of wild red deer on the
Isle of Rum, off the coast of
Scotland, found that reproducing females had lower antibody levels and higher parasite counts.
Testosterone The Immunocompetence Handicap Hypothesis and similar theories propose that
testosterone mediates a trade-off between longevity and reproductive effort in males, prioritizing investment in
secondary sexual characteristics such as
sexually dimorphic muscle mass. Energetically expensive secondary
sexual characteristics, such as skeletal muscle mass, have been shown to predict a relationship between
testosterone levels and reproductive effort. Human males experience muscle mass deterioration during times of immunological and nutritional stress. and
HIV. A different theoretical model has been proposed for testosterone variability as
phenotypic plasticity taking into account behavioral and environmental impacts as well as the role of immune activation on testosterone levels. Activation of the
hypothalamic-pituitary adrenal (HPA) axis is one of the main mechanisms by which the immune system interacts with stress. In animal studies, stressors such as
social disruption and restraint stress active HPA axis in mice and nasal shedding in bovids. In humans, stress is a predictor of recurrences of
herpes simplex virus outbreaks and
Epstein-Barr virus.
Interactions with parasites Host feeding behavior Parasite-altered feeding behaviors have been observed in several species. Most studies conclude that there is a fitness benefit of altering host feeding behavior to either the host or the parasite. The species
S.littoralis caterpillar when infected with
nucleopolyhedrovirus will self-select a protein-rich diet, which increases its probability of survival.
Parasite manipulation Selection is expected to favor parasite manipulation of the host when the host's behavior creates a suboptimal environment for the parasite's fitness. An application of coevolutionary theory would predict sophisticated manipulations of host behavior when host-specificity is high. This phenomenon has been observed in several species, with the most prominent examples including the ingestion of whole leaves by
primate species to reduce nematode infections and the ingestion of secondary plant metabolites by caterpillars and bumblebees. In social insects, behaviors that reduce colony-level parasite loads are termed "
social immunity". An example of this,
Apis mellifera incorporate plant resins in their nest building as this can reduce the chronic elevation of an immune response at the individual level. Immune costs are often seen when organisms are in stressful environments such as experiencing nutritional stress. In animal models, fruit flies that were selected for parasitoid resistance showed reduced larval competitiveness only when they were subject to food limitations.
Leptin has been proposed as a mediator of energetic trade-offs, as a potential provider of signal for current energy availability.
Microbiome Rapid changes in the
gut microbiome occurred during
human evolution Because the
microbiome is influenced by the host environment, researchers believe that it played a role in facilitating human adaptation to novel environments facilitated through periods of
climate change and migration. For instance,
commensal microbes influence the host's ability to survive pathogenic exposures through several mechanisms including inter-microbial competition and interaction with the
immune system.
Population genetics Population genetic characteristics such as population size,
mutation frequency, and
selective processes are important
host-parasite co-evolutionary dynamics and therefore influence the evolution of different aspects of the immune system. Pathogen-driven selection has been supported in
allele frequency studies including
MHC I and
blood group antigens.
Gene networks have also been correlated with specific pathogens including
helminths. == Critiques ==