Background: The disposable soma theory of ageing assumes that organisms optimally trade-off limited resources between reproduction and longevity to maximize fitness. Early reproduction should especially trade-off against late reproduction and longevity because of reduced investment into somatic protection, including immunity. Moreover, as optimal reproductive strategies of males and females differ, sexually dimorphic patterns of senescence may evolve. In particular, as males gain fitness through mating success, sexual competition should be a major factor accelerating male senescence. In a single experiment, we examined these possibilities by establishing artificial populations of the mealworm beetle, Tenebrio molitor, in which we manipulated the sex-ratio to generate variable levels of investment into reproductive effort and sexual competition in males and females.

Results: As predicted, variation in sex-ratio affected male and female reproductive efforts, with contrasted sex-specific trade-offs between lifetime reproduction, survival and immunity. High effort of reproduction accelerated mortality in females, without affecting immunity, but high early reproductive success was observed only in balanced sex-ratio condition. Male reproduction was costly on longevity and immunity, mainly because of their investment into copulations rather than in sexual competition.

Conclusions: Our results suggest that T. molitor males, like females, maximize fitness through enhanced longevity, partly explaining their comparable longevity. 

For the three data files provided, data were collected from a single experiment, which tested for the cost of reproductive effort at the population level on survival (BMC_EVOB-D-19-00289_Survival), reproduction (BMC_EVOB-D-19-00289_Fertility) and immunity (BMC_EVOB-D-19-00289_Immunity) of females and males of the mealworm beetle, Tenebrio molitor, in artificial populations in which we manipulated the sex-ratio to generate variable levels of investment into reproductive effort and sexual competition. Replicated populations (Replicat) of 100 adult beetles of the same age were maintained at different sex-ratio condition (75%_male, 50%_male and 25%male) for their whole lifespan and surveyed for age at death (in weeks).

During the experiment, age specific reproductive capacity of 4 females and 4 males haphazardly picked in each population was estimated weekly. To this purpose, each female was allowed to lay eggs for 3 days in a Petri dish, and was then returned to their initial population box. Two weeks later, the number of larvae was counted in each Petri dish to quantify female fertility as the number of viable larvae produced per female (Nb larvae). At the same time, each male was provided with a standardized virgin female for 24 hours and was then returned in its initial population. The female was then allowed to lay eggs in the Petri dish for three additional days to estimate male fertility, as the number of viable larvae produced by the female (Nb larvae).

Furthermore, 4 females and 4 males were also haphazardly picked in each population at the age of 2, 4, 6, and 12 weeks to estimate their body condition, as the residuals of the regression between their body mass and body size, and their immunity. Immunity was estimated from a single sample of hemolymph allowing measures of hemocyte concentration (Hemocyte), both the activity of naturally activated phenoloxidase enzymes only (PO) and the activity of the proenzymes in addition to that of naturally activated phenoloxidase enzymes (Total-PO), and antibacterial peptide activity (presence absence : ZI (yes/no) and magnitude: ZI size).