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When and how obstacle size and the number of foragers affect clearing a foraging trail in leaf-cutting ants

Abstract

Taking bad decisions to solve problems can negatively affect organism fitness, hence, the costs accrued by them should modulate decisions about when and how problems should be solved. We studied the problem of trail maintenance in leaf-cutting ants. We evaluated whether colonies have individuals exclusively dedicated to trail maintenance, and how obstacle size and ant forager flux influence the decision of removing obstacles, and the number of trail-clearing ants. We placed obstacles of different sizes, in low and high ant fluxes in different trails of Atta sexdens, and measured ant flux without and with obstacles, if ants removed obstacles, the number of trail-clearing ants and removal time. Obstacle cost was estimated as the proportion of ants that were blocked by the obstacle. We found that colonies lack exclusive trail-clearing ants, i.e., clearing ants also foraged and vice versa. The obstacle cost increased with obstacle size and ant flux. Removal probability increased with this cost. The number of trail-clearing ants increased with obstacle size but did not vary with ant flux, suggesting that the number of trail-clearing ants depends on intrinsic problem characteristics (i.e., higher resistance to traction in bigger obstacles), but does not depend on social context (i.e., interference with foragers). Regardless of the obstacle size, the removal time increased with the number of trail-clearing ants suggesting that interference among individuals is higher in larger working groups than in smaller ones. Our results suggest that individual capabilities as well as the coordination level among individuals influence the solving of a problem and the number of individuals involved in it. We discuss possible mechanisms behind results and propose a conceptual model about the costs and benefits of the removal task.

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Acknowledgements

We thank R. Josens, M. Lozada and J. Corley for helpful comments on an earlier version of this article. We thank J. Queiroz, Corina Barrera, Eder Cleyton Barbosa de França and Marina for help during fieldwork. This work was supported by the Fondo para la Investigación Científica y Tecnológica (FONCYT; PICT 2015-1319) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET; PIP 2014–2016, 11220130100665-CO).

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Correspondence to Andrea Marina Alma.

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Supplementary Tables and Figures (DOCX 5544 KB)

Supplementary Movie S1: Collective removal of an obstacle by cooperative transport. Leaf-cutting ant (Atta sexdens) cooperatively removing an obstacle of 10 cm2. The ant flux was 118 ants per min. At the end of the video, two workers cooperatively transport the obstacle away from the trail. During the collective removal, both individuals change the directions of pulling or pushing or the place where they picked up the obstacle (MP4 65544 KB)

Supplementary Movie S1: Collective removal of an obstacle by cooperative transport. Leaf-cutting ant (Atta sexdens) cooperatively removing an obstacle of 10 cm2. The ant flux was 118 ants per min. At the end of the video, two workers cooperatively transport the obstacle away from the trail. During the collective removal, both individuals change the directions of pulling or pushing or the place where they picked up the obstacle (MP4 65544 KB)

Supplementary Movie S2: Partitioned collective removal of an obstacle. Leaf-cutting ant (Atta sexdens) collectively removing an obstacle of 20 cm2. The ant flux was 70 ants per min. First the removal is made by one worker and then by another, until the first worker finishes the removal (MP4 344810 KB)

Supplementary Movie S2: Partitioned collective removal of an obstacle. Leaf-cutting ant (Atta sexdens) collectively removing an obstacle of 20 cm2. The ant flux was 70 ants per min. First the removal is made by one worker and then by another, until the first worker finishes the removal (MP4 344810 KB)

Supplementary Movie S3: Collective removal of an obstacle. Leaf-cutting ant (Atta sexdens) collectively removing an obstacle of 20 cm2. The ant flux was 108 ants per min. First the removal is made by one worker but then two workers cooperatively transport the obstacle away from the trail. During the collective removal, both individuals change the directions of pulling or pushing or the place where they picked up the obstacle (MP4 341926 KB)

Supplementary Movie S3: Collective removal of an obstacle. Leaf-cutting ant (Atta sexdens) collectively removing an obstacle of 20 cm2. The ant flux was 108 ants per min. First the removal is made by one worker but then two workers cooperatively transport the obstacle away from the trail. During the collective removal, both individuals change the directions of pulling or pushing or the place where they picked up the obstacle (MP4 341926 KB)

Supplementary Movie S4: Individual removal of an obstacle. Leaf-cutting ant (Atta sexdens) individually removing an obstacle of 10 cm2. The ant flux was 54 ants per min. One worker drags the obstacle and never picks it up with the mandible carrying it over its head. Foraging ants walk along the edge of the obstacles and, even some ants cross over it (MP4 39731 KB)

Supplementary Movie S4: Individual removal of an obstacle. Leaf-cutting ant (Atta sexdens) individually removing an obstacle of 10 cm2. The ant flux was 54 ants per min. One worker drags the obstacle and never picks it up with the mandible carrying it over its head. Foraging ants walk along the edge of the obstacles and, even some ants cross over it (MP4 39731 KB)

Supplementary Movie S5: Individual removal of an obstacle. Leaf-cutting ant (Atta sexdens) individually removing an obstacle of 30 cm2. The ant flux was 65 ants per min. One worker drags the obstacle and never picks it up with the mandible carrying it over its head (MP4 75832 KB)

Supplementary Movie S5: Individual removal of an obstacle. Leaf-cutting ant (Atta sexdens) individually removing an obstacle of 30 cm2. The ant flux was 65 ants per min. One worker drags the obstacle and never picks it up with the mandible carrying it over its head (MP4 75832 KB)

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Alma, A.M., Farji-Brener, A.G. & Elizalde, L. When and how obstacle size and the number of foragers affect clearing a foraging trail in leaf-cutting ants. Insect. Soc. 66, 305–316 (2019). https://doi.org/10.1007/s00040-018-00680-x

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Keywords

  • Atta sexdens
  • Behaviour
  • Decision making
  • Foraging
  • Problem
  • Social organism