Relationship between early body condition, energetic reserves and fitness in an iteroparous insect

Body condition can be defined as the amount of energetic reserves present within an individual after structural size had been accounted for (i.e. relative amounts of energetic reserves), and estimated by Body Condition indices (BCIs). Several methods have been proposed to calculate BCIs. However, they have traditionally been validated in vertebrate studies and evidence of their power to predict fitness in invertebrates is scarce. Ideally, the use of a particular BCI in an animal population should be validated based on its ability to accurately reflect the relative amount of reserves available to the animal as well as its relationship to fitness. We aimed at increasing the variance in female body condition of Tenebrio molitor beetles by subjecting them to restricted or optimal food conditions at both the larval and/or adult stage. We then explored the predictive power of several BCIs on both the absolute and relative amount of lipids and sugars present in the insect9s body, and their link with adult fitness. Using an iteroparous income breeder allowed us to assess the relative effects of larval vs. adult access to nutritional resources on fecundity along several reproductive events. Simple measurements of condition dependent traits (i.e. mass and volume) correlated well with absolute, but poorly with relative, measures of body reserves. Conversely, we found that BCIs that corrected for the interdependence between plastic phenotypic traits and structural size strongly correlated with relative amounts of body components. We found that even though the adult feeding treatment had a stronger effect, body condition at emergence, but not larval feeding treatment, also affected fecundity. Moreover, while the effect of the adult feeding treatment varied along time (i.e. egg laying rank), the effect of body condition at emergence remained constant. These results show that by carefully using simple morphometric measures and BCIs, it is possible to distinguish the effect of structural size and body condition on fitness traits in invertebrates, and to show that an iteroparous income breeder can partially rely on its early energetic state for its later fecundity.