The effect of parental investment on immunocompetence and sexual immune dimorphism

Sexes of a species show different characteristics beyond the differences in their sex- ual organs; this is known as sexual dimorphism and applies to immunocompetence as well. Immunocompetence is the ability of an individual to mount an immune response when exposed to pathogens. Females are shown to have increased longevity that comes with higher immunocompetence as compared to males and this may also lead to an increased probability of autoimmune disease in females. However, for some species such as pipefishes and seahorses belonging to the Syngnathid family, studies show that the males have a higher immunocompetence. Experimental evidences suggest that this could be due to the fact that these males undergo pregnancy i.e. the males have brood pouches where the eggs are fertilized; the fathers provide oxygen and nutrition to their offspring until they give birth to the juveniles. Therefore, an increase in immunocompetence may also be related to the amount of parental investment. In this study, using state dependent life-history theory, we show that for most species systems it is optimal to invest more in immunocompetence when the time spent in parental investment is longer. Our findings also show that an increase in parental investment brings about an earlier immunosenescence i.e. the gradual deterioration of the immune system that occurs with aging. We observe that an increase in investment towards immunocompetence is more pronounced in short-lived species with long brooding periods whereas species with a longer lifespan allocate more reserves towards offspring production. Our model also accounts for intraspecies scenarios: if a sex spends a longer fraction of its reproductive season in pregnancy or brooding (as compared to the other sex), then we find that this sex would invest more towards immunocompetence.