Effects of heat stress on the foraging activity of incipient ant colonies

Global warming affects organismal performance across biological levels, from molecular processes to complex behaviours. Ectotherms are particularly vulnerable to thermal stress, as their body temperature critically depends on ambient environmental conditions. Warming may affect their foraging behaviour and/or their activity patterns. Here, we experimentally investigated the effects of acute and chronic thermal stresses on foraging by an ectotherm species. Social insects are important ecosystem engineers. Colony survival and growth depend on successful foraging, and colonies are more susceptible to warming when they are young and have fewer workers. We investigate how heat intensity and exposure duration influence foraging dynamics and survivorship in young colonies of the Lasius niger ant. Using laboratory assays, we studied the effects of acute heat stress by exposing foragers to short (10 min) and intermediate (1 h) heat pulses of 25°C (control), 30°C, 35°C and 40°C. Furthermore, we investigated the effects of chronic heat stress by rearing colonies at 25°C:20°C (day:night, control conditions) or 35°C:30°C (heat stress) for 2 weeks. After heat exposure, we recorded the latency to initiate foraging, the number of foraging visits and individual and colony survival between treatments. We hypothesized that ants could cope with relatively low and relatively short heat shocks, but not with high or long heat exposures. We found that prolonged and high heat exposure indeed increased the mortality of foragers and colonies, increased the delay before foraging, and decreased the number of foraging trips. Reduced foraging capacity in young colonies is likely to have strong fitness consequences. The establishment of colonies is dependent on the rapid acquisition of resources to sustain exponential growth. Any slowdown in resource intake can limit colony development and prolong the duration of the incipient (newly founded) and vulnerable stage, ultimately reducing the likelihood of successful establishment.