Carrying oversized loads may create pheromone “blind spots” in leafcutter ants

Understanding animals’ sensorial abilities and cognitive processes while foraging helps explain why animals depart from theoretical optimal foraging. Here, we studied foraging decisions in leafcutter ants ( Atta colombica ), which form and follow pheromone foraging trails that workers smell by tapping the substrate with the antennae. Carrying oversized items transports more plant material in a single trip, but workers walk more slowly and can delay the nestmates walking behind them. We tested the hypothesis that balancing an oversized load limits the ability to tap the ground with the antennae, therefore reducing the ability to smell the foraging trail. As expected, we found that the number of antennae taps per step was (1) fewer in laden vs. unladen workers, (2) fewer as loads increased in area, but only for larger ants, and (3) unrelated to the load shape. Second, workers increased the antennae taps and speed after experimentally reducing standardized loads. Last, we evaluated the allometric relation between the antennae length and worker size, and found that it showed negative allometry. Hence, larger ants had proportionally shorter antennae, which could explain why larger workers are more impacted by oversized loads in the number of antennae taps. Overall, our results support that carrying an oversized load limits the ability of workers to tap the ground, analogous to the blind spots in a large truck. Our findings reveal a sensory limitation to foraging and reflect how foraging decisions in a social organism are adjusted to the individual capacities and the colony needs.