Dominant herbivores, like leafcutter ants, have a strong impact on the ecosystems they inhabit. Understanding which factors regulate their foraging rates is crucial for understanding ecosystem dynamics. In desert habitats, environmental factors, such as temperature, play a major role in regulating ants' behavior. We studied the role of ground temperature in regulating daily and seasonal activity patterns of two coexisting leafcutters ant species, Acromyrmex lobicornis (EMERY, 1888) and A. striatus (ROGER, 1863), in the Monte desert of Argentina. We measured the variations in activity levels and soil temperature every two hours throughout the day in colonies of both species every season for two consecutive years. Temperature was a good predictor of both the timing of colony activation (the onset and end of their daily foraging activities) and foraging intensity (the number of workers devoted to foraging tasks). However, temperature affected each species differently: Acromyrmex lobicornis foraged at lower temperatures (10 - 35 °C) than A. striatus (27 - 45 °C). Our results suggest that these two species have different thermal tolerance ranges that result in temporally separated foraging activities. We suggest that interference competition may have driven this temperature and temporal specialization in these two sympatric species, given their similar sizes and diets. Field observations of activity vs. temperature in allopatry, and behavioral tests in controlled conditions should provide further evidence to test this hypothesis. Fil: Nobúa Behrmann, Beatriz Elisa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Lopez de Casenave, Javier Nestor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Milesi, Fernando Adrian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina Fil: Farji Brener, Alejandro Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina