Exploring the males' terminal investment strategy: Impact of the dose of stress and the time lapse between stress and mating

In the terminal investment strategy, organisms allocate resources to maximize current reproduction when their future reproductive opportunities are uncertain due to a high risk of mortality. However, whether the role of time lapse between stress and reproductive investment (courtship vs. courtship and mating) triggers the terminal investment strategy remains unknown. We examined this hypothesis in Tenebrio molitor using oxidative stress as the stressor. Given that this is the first time that oxidative stress is used to trigger terminal investment, we first conducted mate choice trials where males exposed to 5, 20, 40 or 80 mM of the pro-oxidant, paraquat (PQ) engaged in courtship behaviour towards the females. Subsequently, we analysed the male mortality against each of these doses. Males exposed to a 5 mM treatment exhibited higher survival rates compared with the control group, whereas males exposed to 80 mM experienced a shorter lifespan. After courtship, no significant differences in mortality were observed between the control group and those treated with 20 or 40 mM of PQ. Notably, only males exposed to 40 mM of PQ were preferred over the control group. In a third experiment, we quantify the survival costs when males courted and mated at varying time intervals after oxidative stress (40 mM). When males were exposed to 40 mM of PQ and mated 3 h later, their mortality rates significantly decreased compared with males exposed to oxidative stress, courted and mated, but 24 h later. We suggest that the interaction of the dose of oxidative stress and the time lapse between courtship and mating may explain opposed results to support either the terminal investment or honest signalling.