Distinct decision-making properties underlying the species specificity of group formation of flies

Heat-avoidance behavior associates with thermal sensitivity rather than tolerance in aphid assemblages

How to predict animals' heat-avoidance behaviors is critical since behavior stands the first line for animals dealing with frequent heat events under ongoing climate warming. However, the discrepancy between the scarcity of research on heat-avoidance behaviors and the commonness of eco-physiological data for thermal tolerance and for thermal sensitivity such as the temperature-dependent survival time makes it difficult to link physiological thermal traits to heat-avoidance behavior. Aphids usually suck plant sap on a fixed site on the host plants at moderate temperatures, but they will leave and seek cooler feeding sites under stressful temperatures. Here we take the cereal aphid assemblages comprising different species with various development stages as a model system. We tested the hypotheses that heat tolerance (critical thermal maximum, CT_max) or heat sensitivity (temperature-dependent declining rate of survival time, similarly hereinafter) would associate with the temperature at which aphid activate heat-avoidance behavior. Specifically, we hypothesized the aphids with less heat tolerance or greater heat sensitivity would take a lower heat risk by leaving the host plant earlier. By mimicking the linear increase in ambient temperature during the daytime, we measured the CT_max and the heat-avoidance temperature (HAT, at which aphids leave the host plant to find cooler places) to understand their heat tolerance and heat-avoidance behavior. Then, we tested the survival time of aphids at different temperatures and calculated the slope of survival time declining with temperature to assess their heat sensitivity (HS). Finally, we examined the relationships between CT_max and HAT and between HS and HAT to understand if the heat-avoidance behavior associates with heat tolerance or with heat sensitivity. The results showed that HS and HAT had a strong correlation, with more heat sensitive individuals displayed lower HAT. By contrast, CT_max and HAT had a weak correlation. Our results thus provide evidence that heat sensitivity is a more reliable indicator than thermal tolerance linking with the heat-avoidance behavior in the aphid assemblages. Most existing studies use the indexes related to thermal tolerance to predict warming impacts. Our findings highlight the urgency to incorporate thermal sensitivity when predicting animal responses to climate change.

Distinct decision-making properties underlying the species specificity of group formation of flies

Many animal species form groups. Group characteristics differ between species, suggesting that the decision-making of individuals for grouping varies across species. However, the actual decision-making properties that lead to interspecific differences in group characteristics remain unclear. Here, we compared the group formation processes of two Drosophilinae fly species, Colocasiomyia alocasiae and Drosophila melanogaster, which form dense and sparse groups, respectively. A high-throughput tracking system revealed that C. alocasiae flies formed groups faster than D. melanogaster flies, and the probability of C. alocasiae remaining in groups was far higher than that of D. melanogaster. C. alocasiae flies joined groups even when the group size was small, whereas D. melanogaster flies joined groups only when the group size was sufficiently large. C. alocasiae flies attenuated their walking speed when the inter-individual distance between flies became small, whereas such behavioural properties were not clearly observed in D. melanogaster. Furthermore, depriving C. alocasiae flies of visual input affected grouping behaviours, resulting in a severe reduction in group formation. These findings show that C. alocasiae decision-making regarding grouping, which greatly depends on vision, is significantly different from D. melanogaster, leading to species-specific group formation properties.