Context‐dependent effects of cold stress on behavioral, physiological, and life‐history traits of the red flour beetle

Abstract

Animals are exposed in nature to a variety of stressors. While stress is generally harmful, mild stress can also be beneficial and contribute to reproduction and survival. We studied the effect of five cold shock events versus a single cold shock and a control group, representing three levels of stress (harsh, mild, and no stress), on behavioral, physiological, and life-history traits of the red flour beetle (Tribolium castaneum, Herbst 1797). Beetles exposed to harsh cold stress were less active than a control group: they moved less and failed more frequently to detect a food patch. Their probability to mate was also lower. Beetle pairs exposed to harsh cold stress frequently failed to reproduce at all, and if reproducing, females laid fewer eggs, which were, as larvae in mid-development, smaller than those in the control group. However, harsh cold stress led to improved female starvation tolerance, probably due to enhanced lipid accumulation. Harsh cold shock also improved tolerance to an additional cold shock compared to the control. Finally, a single cold shock event negatively affected fewer measured response variables than the harsh cold stress, but also enhanced neither starvation tolerance nor tolerance to an additional cold shock. The consequences of a harsher cold stress are thus not solely detrimental but might even enhance survival under stressful conditions. Under benign conditions, nevertheless, harsh stress impedes beetle performance. The harsh stress probably shifted the balance point of the survival-reproduction trade-off, a shift that did not take place following exposure to mild stress.

Effect of continuous and alternating episodes of starvation on behavior and reproduction in the red flour beetle

“…We cautiously interpret this to mean that females cope better with stress than males. Similarly, another stress type, repeated cold shock, led to accumulation of more fat, and improved female starvation tolerance and tolerance of a successive cold shock, while no such effects were evident for males (Scharf et al, 2018). Starvation effects differ between the sexes in other arthropods too.…”

“…This set-up was photographed after 10 min. The number of beetles in each quarter of the arena and the number of beetles settled in the flour were recorded (similar to Scharf et al, 2018). This response variable represents exploration.…”

“…By dividing the total larval mass by number of offspring, we obtained the mean larval mass. We preferred to count larvae rather than eggs, as this is a better proxy of reproductive success than eggs (due to frequent cannibalism and egg infertility in this species; similar to Scharf et al, 2018). We analyzed the number and mass of larvae using a linear regression and one-way ANOVA for the continuous and alternating starvation experiments, respectively.…”

“…The same level of stress can even lead to a mixture of positive and negative responses. For example, stress impairs reproduction but elevates tolerance to different stress types (Sejerkilde, Sørensen & Loeschcke, 2003;Le Bourg, 2007;Scharf et al, 2018).…”

“…We also tested for intersexual differences in such responses to starvation. T. castaneum females are larger, resist starvation longer, and respond differently to thermal stress than do males (Scharf, Galkin & Halle, 2015;Scharf et al, 2016Scharf et al, , 2018. Inter-sexual differences are common in other insects too.…”