Differential thermal sensitivity may explain the temporal distribution of foraging activity among different-sized workers in a polymorphic ant species

One of the most stressful factors for insects is increasing temperature because of the risk of potentially fatal dehydration linked to their small size. We used respirometry to study the effect of both temperature and body mass on water loss and metabolic rate in individual workers of the polymorphic ant species Messor barbarus. As expected, we found that large ants exposed to increasing temperatures have a lower rate of water loss than small ants and that their mass-specific metabolic rate increases more slowly. However, counterintuitively, the measure of worker sensitivity to changes in temperature, as assessed by the instantaneous Q10 value (i.e., the rate of change across 10°C temperature intervals), shows that large ants are more sensitive than small ants to changes in temperature in terms of both water loss and metabolic rate. Such differential thermal sensitivity allows to make testable predictions on the temporal distribution of foraging activity among workers of different sizes in polymorphic ant species, as well as how these species may alter their colony demographics in response to rising temperatures.