Age-specific fecundity under parasitic threat in an insect: terminal investment versus reproductive restraint

International audienceThe terminal investment hypothesis predicts that animals increase their reproductive effort as they age, and their chances for survival and reproduction decline. An infection predicts further diminished residual reproductive value, and could therefore trigger increased breeding effort at the expense of somatic defences and survival. Attempts to test this hypothesis have produced mixed results, which also often relied on potential changes in the breeding effort only. Alternatively, animals may restrain their current reproduction to sustain immunity and so later reproduction. 2. We tested these possibilities in females of the yellow mealworm beetle, Tenebrio molitor, an iteroparous insect with reproductive tactics similar to that of long-lived organisms. 3. For this purpose we mimicked pathogenic bacterial infections early or late in life in breeding females by injecting them with a suspension of inactivated Bacillus cereus, a known natural pathogen of T. molitor, and measured female age-specific fecundity, survival and immunity. 4. Inconsistent with a terminal investment, early or late-life immune challenges of females did not reduce their survival or enhance their reproductive output. Female fecundity declined with age and was reduced by the early but not the late immune challenge. Both early and late-life fecundity correlated positively with life expectancy. Finally, young and old females exhibited similar antibacterial immune responses, suggesting that they both restrained reproduction to sustain immunity. 5. Our results clearly demonstrate that age-specific reproduction of T. molitor females under parasitic threat is discrepant with a terminal investment. By contrast, our results rather suggest that females used a reproductive restraint strategy to sustain immunity and therefore subsequent reproductive opportunities. However, as infections were mimicked only, the fitness benefit of this reproductive restraint could not be shown