Follower ants in a tandem pair are not always naïve

Abstract

In addition to foraging individually several species of ants guide nestmates to a goal by tandem running. We found that the Australian ant, Camponotus consobrinus, forages both individually and by tandem running to head to the same goal, nest-specific native Australian trees on which they forage. While paths of solitary foragers and initial paths of tandem followers showed no differences in heading directions or straightness, tandem followers moved at about half the speed of solitary runs. When leaders were experimentally removed, follower ants initially engaged in a systematic search around the point of interruption, following which they either (a) headed directly towards and successfully reached the foraging trees, or (b) continued searching or (c) returned to the nest. The high incidence of followers that successfully navigated towards the foraging trees on their own provides strong evidence that many tandem followers are in fact experienced foragers. Detailed analysis of the searching behaviour revealed that even seemingly lost followers displayed a directional bias towards the foraging trees in their search path. Our results show that in a foraging context follower ants in a tandem pair are not always naïve.

Figure 1. Tandem running in sugar ants.

(a) Pair of workers of Camponotus consobrinus engaged in tandem running with the leader on the right. (b) Study location showing nest site and three foraging trees (green circles). Foraging corridor is shown in grey shaded area. Arrow indicates typical foraging direction. (c) Panoramic view available to the ants from the nest, showing the three tree locations (green arrows). Photo credit: Ajay Narendra.

Figure 2. Outbound activity patterns of Camponotus consobrinus foragers from a single nest on eight days.

Recording started two hours before sunset, and continued until about one hour after sunset. Solitary foragers (bars) and tandem runs (red dots and line) are shown normalised to their respective maxima. The grey shade indicates the period after sunset.

Figure 3. Paths of Camponotus consobrinus foragers.

(a) Natural, undisturbed paths of solitary foragers (grey lines) and two tandem runs (black lines). Nest (filled blue circle), three foraging trees (green circles), interruption points where leaders where removed in later conditions (open blue circles). After leader removal, paths of followers (b) that successfully reached the foraging trees, (c) that did not reach either the trees or the nest, (d) returned to the nest. (e) In red, path of a follower after interruption (open arrow), that displaces a follower of another tandem pair (closed arrow) and heads to the tree; in black, a tandem pair leaves the nest and returns to the nest entrance. Colours represent different ants.

Figure 4. Histograms of heading directions of follower ants during interrupted tandem runs, separated into three segments.

Segment 1: tandem run until interruption; segment 2: the search phase after interruption; segment 3: the final part after the end of search, if present. Each subplot has been normalised to the respective maximum. 0° = bearing from nest to foraging tree.

Figure 5. Path measures of solitary foragers and followers from interrupted runs.

(a) Path straightness and (b) walking speed of solitary foragers (left) and followers from interrupted tandem runs (right), separated into three path segments. Boxes show median, upper and lower quartile, whiskers extend to upper and lower deciles. Average values are shown as black dots. The small star denotes significant differences between groups. S: successful followers, L: lost followers.

Figure 6. Searching behaviour of tandem followers after interruption of the tandem run.

(a) Example search paths, with the first two minutes of search shown in red. (b) Distance from interruption point over time, derived for the example path shown in (a). The first two minutes of search are shown in red. (c) Average position along the axis of the preceding tandem run, over the first two minutes of search for all tandem followers. Positive values are in the direction of the tandem run, negative values are in the direction back to the nest. Grey shading shows the standard deviation, red line shows the best linear fit.