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

The terminal investment hypothesis predicts that as an organism’s
prospects for survival decrease, through age or when exposed to a
pathogenic infection, it will invest more in reproduction, which should
trade-off against somatic maintenance (including immunity) and therefore
future survival. Attempts to test this hypothesis have produced mixed
results, which, in addition, mainly rely on the assessment of changes in
reproductive effort and often overlooking its impact on somatic defences
and survival. Alternatively, animals may restrain current reproduction to
sustain somatic protection, increasing the chance of surviving for
additional reproductive opportunities. We tested both of these hypotheses
in females of the yellow mealworm beetle, Tenebrio molitor, an iteroparous
insect with reproductive tactics similar to that of long-lived organisms.
To achieve this, we mimicked pathogenic bacterial infections early or late
in the life of 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, body mass and immunity.
Inconsistent with a terminal investment, females given either an early or
late-life immune challenge did not exhibit reduced 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. Our
results clearly demonstrate that age-specific reproduction of T. molitor
females under pathogenic threat is inconsistent with a terminal
investment. In contrast, our results instead 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.