Trail using ants follow idiosyncratic routes in complex landscapes

Abstract

A large volume of research on individually navigating ants has shown how path integration and visually guided navigation form a major part of the ant navigation toolkit for many species and are sufficient mechanisms for successful navigation. One of the behavioural markers of the interaction of these mechanisms is that experienced foragers develop idiosyncratic routes that require that individual ants have personal and unique visual memories that they use to guide habitual routes between the nest and feeding sites. The majority of ants, however, inhabit complex cluttered environments and social pheromone trails are often part of the collective recruitment, organisation and navigation of these foragers. We do not know how individual navigation interacts with collective behaviour along shared trails in complex natural environments. We thus asked here if wood ants that forage through densely cluttered woodlands where they travel along shared trails repeatedly follow the same routes or if they choose a spread of paths within the shared trail. We recorded three long homing trajectories of 20 individual wood ants in their natural woodland habitat. We found that wood ants follow idiosyncratic routes when navigating along shared trails through highly complex visual landscapes. This shows that ants rely on individual memories for habitual route guidance even in cluttered environments when chemical trail information is available. We argue that visual cues are likely to be the dominant sensory modality for the idiosyncratic routes. These experiments shed new light on how ants, or insects in general, navigate through complex multimodal environments.

Keywords: Idiosyncratic routes; Navigation; Pheromone trails; Visual navigation; Wood ants.

Fig. 1

Experimental procedures. (A) Three consecutive homing routes of individual ants travelling from the tree to their nest (N) were filmed with a handheld camera. Solid lines: First, second and third homing routes of an individual ant. Dashed lines: When an ant arrived at the nest, we carried her back to the tree to record another homing route. (B) Combined animal tracking and environment reconstruction algorithm (CATER; Haalck et al., 2023) showing three homing routes of an individual ant. N, nest. F, foraging tree. (C) Area of a video frame showing the tracked ant (marked with the black circle) on the cluttered woodland ground, ground markers are also visible in the picture, with the distance between the two wooden markers placed on the ground being 20 cm. (D) For path analysis, we extracted the coordinates of the ants’ paths from the videos using the markers on the woodland ground (x axis: 0.2 m intervals; y axis: 1 m intervals). A single trajectory with high temporal resolution (shown in blue; extracted using CATER) is overlaid with the path plotted using the extracted coordinates (shown in black)

Fig. 2

Idiosyncratic routes from homing ants travelling from the tree (T) to their nest (N). Three consecutive homing routes were recorded for all 20 ants. Grey lines show the total 60 homing routes and black lines show the three routes for each individual in turn. Black filled circles show path coordinates every 1 m along the tree-to-nest distance. The ant from Fig. 1B is marked with an asterisk

Fig. 3

Paths from individual ants are more similar to each other than to routes from the other ants. (A) Mean inter-path differences for intra- and inter-ant comparisons. Intra-ant comparisons (n = 56) and inter-ant comparisons (n = 1,597) are shown as boxplots (median, 25th and 75th percentiles (edges of the boxes) and whiskers for extreme values not considered as outliers (o)). Statistically significant differences between the groups are indicated by asterisks (Mann Whitney test, p < 0.001). (B) Inter-path differences shown for every 1 m along the tree-to-nest distance. Red, intra-ant differences (n = 56). Blue, inter-ant differences (n = 1,597). Lines show the medians for every metre along the tree-to-nest distance, and lower and upper edge of the shaded areas show the 25th and 75th percentiles, respectively. Mann Whitney tests: tree-to-nest distance; 1 m; p > 0.05 (ns); tree-to-nest distance, 2 m, p < 0.01 (*); all other tree-to-nest distances, p < 0.001 (**)

Fig. 4

Ant’s eye views from route divergence points. (A) All paths from Fig. 2 are shown in grey (trimmed here at 2 m and 10 m). The asterisk indicates the nest direction. At 3 m along the tree-to nest direction, five ants consistently navigate to the left (shown in dark blue), whereas 13 ants consistently navigate to the right (shown in red). From the ants travelling to the right, one ant then navigates consistently to the left (shown in black) and 11 ants navigate consistently to the right (shown in light blue) when they reach 6 m. (B) Path sections between 6 m and 9 m from (A) are shown for two selected locations. At 6 m, two ant’s-eye-view pictures are shown for the highlighted locations (1) and (2). The asterisk indicates the nest direction. Coloured circles below the picture show the heading directions from 6 m to 9 m for the coloured paths in their respective colours. (C) Path sections between 3 m and 6 m from (A) are shown for two selected locations. At 3 m, two ant’s-eye-view pictures are shown for the highlighted locations (3) and (4). The asterisk indicates the nest direction. Coloured circles below the picture show the heading directions from 3 m to 6 m for the coloured paths in their respective colours