Thermal tolerance regulates foraging behaviour of ants

Abstract

Abstract Theory suggests that performance increases with temperature up to an optimization point before rapidly decreasing as an animal approaches its upper thermal limit. Here, we use the red harvester ant, Pogonomyrmex barbatus, to test predictions about how daily temperature fluctuations and thermal tolerance combine to influence one metric of performance—foraging. Over 2 years, we tracked 322 foraging trips from 15 colonies in a mixed grass prairie of southwestern Oklahoma. During each trip, we measured surface temperature, distance, time, worker mass, seed mass, and foraging tempo (i.e., running speed). To assess P. barbatus heat tolerance, we measured CTmax and knock‐down resistance of field‐collected workers in the lab. Trip time, but not distance, decreased with increasing temperature, resulting in an increased foraging tempo as ants neared their CTmax of 50°C. Knock‐down resistance trials confirmed that 50°C is an upper thermal limit, as individuals showed decreasing survival from 100% at 45°C to 0% at 50°C. Worker size and collected seed size were unrelated to temperature. Our results highlight how daily temperature fluctuations drive activity, not only by limiting foraging but also by increasing foraging rates near the thermal limit. If temperatures continue to increase, the foraging ability of this and similar species may be restricted to an ever‐narrowing window with effects potentially extending to the surrounding community.