Effects of time spent in pregnancy or brooding on immunocompetence

Sexes of a species may show different characteristics beyond the differences in their sexual organs and such sexual dimorphism often occurs in the level of immune response when exposed to pathogens (immunocompetence). In general, females have increased longevity relative to males, which is associated with higher immunocompetence. However, males have higher immunocompetence in some species, such as pipefishes and seahorses. Experimental evidence suggests that this could be because males, rather than females, carry fertilized eggs to birth in these species. This observation suggests that an increase in immunocompetence may be related to the level of parental investment and not to a particular sex. We use state-dependent life-history theory to study optimal investment in offspring production relative to parent immunocompetence, varying the relative time that a parent spends in brooding or pregnancy within a breeding cycle. When offspring is dependent on a parent's survival for a large part of the breeding cycle, we predict higher investments in immunity and longer life expectancies. We hypothesize that (1) the amount of parental care has a direct effect on immunocompetence and (2) this effect should be seen while comparing individuals of the different sexes within a species or while comparing the major parental care providing sex between species. We explore this idea with a state-dependent life-history model to predict how an individual allocates its resources toward its own immunity while also investing in caring for its offspring in a way that it can maximize the fitness. The main result from our model is that an increase in investment toward immunocompetence is more pronounced in short-lived species with longer brooding periods (in comparison to long-lived species with shorter brooding or pregnancy periods). Our model also shows that in general, longer brooding times lead to more investment in immunity and hence higher life expectancy.image