Constraints on the foraging patterns of sympatric leafcutter ants

Background/Question/Methods

Leafcutter ants are the dominant native herbivores in neotropical ecosystems. Understanding what regulates their herbivory is a crucial component of the ecology of these systems. Herbivory rates are directly controlled by the number of hours foragers are active during each day and the number of foragers a colony deploys at any given moment. Both abiotic (e.g., temperature, moisture) and biotic (e.g., interactions with other species) factors regulate the foraging pattern of a colony and have important implications for the dynamics of the ecosystem. It is therefore important to quantify their effects on leafcutter foraging. We studied the daily and seasonal activity patterns of two sympatric species of leafcutter ants (Acromyrmex lobicornis and A. striatus) in the central Monte desert (Argentina), and assessed the role of temperature in regulating those patterns. We measured foraging activity and soil temperature every two hours during a whole day in 10 colonies per species once per season for two years. We modeled the relationship between temperature and foraging activity and their variations among seasons using a logistic linear mixed model.

Results/Conclusions

Daily foraging patterns varied seasonally, with higher activity levels during spring and summer. Both species were less active during autumn, and only A. lobicornis was active in the winter, albeit scarcely. The two species had minimal overlap in daily activity patterns during the spring and summer, when A. lobicornis was primarily nocturnal and A. striatus was strictly diurnal. However, in autumn both species were active during the day. Soil temperature was a good predictor of foraging patterns for both species, affecting both the timing and intensity of foraging activity. Acromyrmex lobicornis was active under cooler temperatures than A. striatus, which explained the daily and seasonal differences in their foraging patterns. These results suggest that different physiological tolerances may allow coexistence of these two leafcutter species that otherwise share similar food requirements and life histories. However, the lack of overlap found in their foraging timing and preferred temperature ranges also suggests that interference competition in their sympatric range may be an important factor contributing to the differences in foraging patterns.