Age, but not an immune challenge, triggers terminal investment in the Pacific field cricket, Teleogryllus oceanicus

The terminal investment hypothesis proposes that, when individuals are faced with a threat to survival, they will increase investment in current reproduction. The level of the threat necessary to elicit terminal investment (the dynamic terminal investment threshold) may vary based on other factors that also influence future reproduction. Here, we tested whether there is an interactive effect of age and an immune challenge on the dynamic terminal investment threshold in the Pacific field cricket, Teleogryllus oceanicus. We measured the courtship call, mating attractiveness, ejaculate size, and offspring production of T. oceanicus males. We found only limited support for the dynamic terminal investment threshold: there was no consistent evidence of a positive interaction between male age and immune challenge intensity. However, we found evidence for age-related terminal investment: older males produced a larger spermatophore than younger males. Older males also had a slower calling rate compared to younger males, suggesting a potential trade-off between these two pre- and post-copulatory traits. As some, but not all, reproductive traits responded plastically to cues for terminal investment, our research highlights the importance of considering a broad range of pre-and post-copulatory traits when exploring the potential for terminal investment to occur.

Terminal investment theory suggests that when time is running out, or if you are really sick, you should go all-in on reproduction. In the Pacific field cricket, we show that, while older males produce heavier ejaculates than their younger counterparts, their immune status has little effect on their reproductive investment. Thus, we demonstrate that Pacific field crickets do show terminal investment, but in only one of a suite of pre- and post-copulatory reproductive traits.