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

With a little help from my friends: Individual and collaborative performance during trail clearing in leaf‐cutting ants

“…To determine the effect of obstacle size , we placed square obstacles of (a) 10 cm 2 (3.1 × 3.1 cm); (b) 20 cm 2 (4.5 × 4.5 cm); (c) 40 cm 2 (6.3 × 6.3 cm); and (d) 60 cm 2 (7.7 × 7.7 cm), in high ant flux (197 ± 9.1 ants per min, mean ± SE) and in cleared trails over the forest floor (Figure S1). The smallest obstacle measured 10 cm 2 because smaller obstacles (e.g., 4 cm 2 ) barely reduced the ant flux (~20%), and they were hardly ever removed from the trail (Alma et al, ); while our largest obstacles were 60 cm 2 to evaluate situations where both strategies could be negatively affected by obstacle size. The percentage of trail physically blocked by our obstacles (1–4) was 37 ± 1.70, 52 ± 2.36, 75 ± 2.95 and 90 ± 2.12% (mean ± SE), respectively.…”

“…We considered that ants abandoned the obstacles when they left the obstacle in the trail and walked toward the nest or resources. The lag between the introduction of the obstacle and the start of removals did not differ between individual and collaborative removals (mean ± SD : 339 ± 260 and 254 ± 241 s, respectively, Wilcoxon rank‐sum test: 4 cm 2 : W = 13, p = .11, n = 9; 10 cm 2 : W = 5, p = .38, n = 9; and 20 cm 2 : W = 10, p = .26, n = 13; Alma et al, ); thus, we did not considered that variable here.…”

“…We defined collaborative removals when two or more ants worked as a group, team or sequentially (Anderson & Franks, ; Anderson et al, ). We decided to use only 2–3 workers because (a) the smallest obstacles of 10 and 20 cm 2 are removed by 2 ± 0.45 or 3 ± 0.47 ants (mean ± SE), respectively (Alma et al, ); thus, expecting a higher number of ants that remove them is unfeasible; and (b) comparing removals carried out by groups of ants of very different size may confound results, since group size affects performance (Biro & Sasaki, ). We removed the excess of individuals in collaborative removals with forceps (mainly in the 40 and 60 cm 2 obstacle treatments), without delaying the movement of the other ants working with the obstacle; and we neither observed that the ants removing obstacles lose their grip.…”

“…When comparing the removal time of natural clearing (without manipulating the number of clearing ants) to removal time of our experiment (controlling the number of clearing ants to only 2–3), ants took the same time ( t test, t = 0.38, p = .70, df = 38, mean ± SE of natural and experimental removals 513 ± 85 and 440 ± 181 s, respectively). Although this methodological procedure did not reflect the natural behavior of ants, since the number of ants increases with obstacle size (Alma et al, ), it allowed us to decrease variability in performance within collaborative removals at the potential cost of sub‐estimating its performance due to smaller group size.…”

“…Removal of these obstacles can be carried out by one or several workers. In collaborative removals, ants behave cooperatively or cut the obstacle in pieces before removing it (Alma, Farji‐Brener, & Elizalde, ; Bochynek et al, ; Howard, ). Despite the existence of previous studies analyzing the clearing behavior of leaf‐cutting ants (Alma et al, ; Bochynek et al, , ; Bruce et al, ; Caldato et al, ; Cevallos Dupuis & Harrison, ; Howard, ), as far as we know, none has analyzed which contexts are more advantageous for individual or collaborative removals.…”