What view information is most important in the homeward navigation of an Australian bull ant, Myrmecia midas?

Abstract

Many insects orient by comparing current panoramic views of their environment to memorised views. We tested the navigational abilities of night-active Myrmecia midas foragers while we blocked segments of their visual panorama. Foragers failed to orient homewards when the front view, lower elevations, entire terrestrial surround, or the full panorama was blocked. Initial scanning increased whenever the visual panorama was blocked but scanning only increased along the rest of the route when the front, back, higher, or lower elevations were blocked. Ants meandered more when the front, the back, or the higher elevations were obscured. When everything except the canopy was blocked, the ants were quick and direct, but moved in random directions, as if to escape. We conclude that a clear front view, or a clear lower panorama is necessary for initial homeward headings. Furthermore, the canopy is neither necessary nor sufficient for homeward initial heading, and the back and upper segments of views, while not necessary, do make finding home easier. Discrepancies between image analysis and ant behaviour when the upper and lower views were blocked suggests that ants are selective in what portions of the scene they attend to or learn.

Keywords: Celestial cue; Familiar environment; Heading direction; Homeward navigation; Terrestrial panorama.

Fig. 1

View regions blocked in different conditions (a–h), their rotational image difference functions (rotIDF) at each nest (i and j), and their rotIDF depth (k and l). In tests, foragers were released from the centre of a wooden platform, which was 50 cm in radius. The goniometer was divided into 24 sectors of 15° each. Ants were tested in eight different conditions: a Positive Control Condition: no view or cue blocking around the goniometer; b Front Condition: 50% of the view blocked in the front portion at 1 m radius from the goniometer centre; c. Back Condition: 50% of the view blocked at the back portion at 1 m radius from the goniometer centre; d Lower Condition: a 30 cm wall surrounding the goniometer (16.7° elevation); e. Upper Condition: a 90 cm black screen above a 30 cm gap; f Terrestrial Condition: a 120 cm wall around the goniometer; g Celestial Condition: a 50 cm screen blocking the upper portion and covering the canopy, leaving a 70 cm gap from the ground up; h Negative Control Condition: a 120 cm wall and roof top around the goniometer blocking the all of the visual cues. The bottom panel shows the rotation image difference function for each treatment, i at Nest A, and j at Nest B, each with an inset k and l which shows the rotIDF depth (depth = mean – minima)

Fig. 2

Circular histograms of initial headings of individual foragers of M. midas at 50 cm in the different conditions of their familiar routes. The histograms show orientation data in 15° bins, and the nest direction is indicated at 0°. The arrows in each histogram represent the length and the direction of the mean vector of foragers (see Table 2). The olive arrow indicates the combined mean vector of both nests whereas the black and sky colours denote Nest-A and Nest-B, respectively

Fig. 3

The paths of foragers in different experimental conditions on the goniometer. Ant path samples are selected randomly from all paths collected. The olive arrow indicates the nest direction, and the circle indicates the goniometer area (50 cm radius). In the Positive Control Condition (Fig. 3a), foragers were tested without any changes around the goniometer centre. The paths were randomly selected samples among all paths collected. The surrounding panoramic view and cues around the goniometer centre were changed in different ways (see details in Methods) in other conditions (Fig. 3b–h). Below each circle, the mean scans, durations, and Path Straightness (PS) with standard deviations are reported

Fig. 4

a The number of scans on the goniometer (50 cm radius) in different experimental conditions. A ‘*’ indicates significant differences in number of scans compared to the Positive Control Condition. The box plot indicates medians (solid black line), box margins (25th and 75th percentiles) and whiskers (5th and 95th percentiles). b The mean number of scans as a function of location on the goniometer in different view-blocking conditions. Same conventions for box plots

Fig. 5

The duration of time that foragers took to pass 50 cm from the releasing point. Same conventions as in Fig. 4a

Fig. 6

The Path Straightness of foragers in different experimental conditions. Maximum Path Straightness (a straight line) is 1. Same conventions as Fig. 4a