Sex-specific patterns of senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

Abstract

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.

Keywords: Ageing; Cost of reproduction; Disposable soma theory; Immunity; Immuno-senescence; Tenebrio molitor.

Fig. 1

Age-specific survival according to sex-ratio condition. a females; b males

Fig. 2

Age-specific fertility of females (a) and males (b) according to sex-ratio condition. a the tested females were those coming from the experimental tanks. b the tested females were virgin females mated with males coming from the experimental tanks. Dots are the means (for variation around the means see Additional file 1: Figure S2) and lines are the predictions of the models

Fig. 3

Estimated mean reproductive effort. Reproductive effort (RE - mean number of viable offspring produced per individual and per week of survival in the population) of females (left panel) and males (right panel) according to sex-ratio condition. Lines are means, dots are values of single replicates. Values surrounded by different letters were significantly different after Tukey-Kramer HSD post-hoc test (α = 0.05)

Fig. 4

Immune parameters in females and males according to individual age and/or sex-ratio condition. a females PO activity; b proportion of females exhibiting antibacterial activity; c male PO activity; d proportion of males exhibiting antibacterial activity; e male intensity of antibacterial activity as the size (in mm) of zones of inhibition; f male haemocyte count. Values are means among replicates ± s. e. m. Number in the bars are sample size