Testing for age-dependent effects of dietary restriction on the strength of condition dependence in ejaculate traits in the guppy (Poecilia reticulata)

Ejaculates can be costly to produce and depend on an individual's condition, defined as the pool of resources allocated to fitness. A method for assessing condition dependence is to manipulate resource availability and test for a reduction in trait expression. Here, we assess the effects of dietary restriction on two determinants of reproductive fitness in the guppy Poecilia reticulata—sperm production and sperm motility. Importantly, we administered dietary restriction at distinct developmental stages to test: (1) whether dietary restriction, when applied exclusively to juveniles, compromised the ejaculates of newly mature males; (2) whether any observed effects of dietary restriction seen in (1) were reversible when fish returned to an unrestricted diet; and (3) whether dietary restriction applied exclusively to adults influenced ejaculates. We found detrimental effects of resource limitation on both traits, and these were consistent across the three developmental stages tested. Furthermore, dietary restriction reduced male body weight, but this was partially reversed when diet-stressed juveniles (i.e. group 2) returned to unrestricted diets. This latter result suggests that diet-stressed males may sacrifice growth in order to maintain their investment in ejaculates. Together these findings underscore the importance of resource acquisition in determining the expression of ejaculate traits.