Minding the gap: learning and visual scanning behaviour in nocturnal bull ants

Abstract

Insects possess small brains but exhibit sophisticated behaviour, specifically their ability to learn to navigate within complex environments. To understand how they learn to navigate in a cluttered environment, we focused on learning and visual scanning behaviour in the Australian nocturnal bull ant, Myrmecia midas, which are exceptional visual navigators. We tested how individual ants learn to detour via a gap and how they cope with substantial spatial changes over trips. Homing M. midas ants encountered a barrier on their foraging route and had to find a 50 cm gap between symmetrical large black screens, at 1 m distance towards the nest direction from the centre of the releasing platform in both familiar (on-route) and semi-familiar (off-route) environments. Foragers were tested for up to 3 learning trips with the changed conditions in both environments. The results showed that on the familiar route, individual foragers learned the gap quickly compared with when they were tested in the semi-familiar environment. When the route was less familiar, and the panorama was changed, foragers were less successful at finding the gap and performed more scans on their way home. Scene familiarity thus played a significant role in visual scanning behaviour. In both on-route and off-route environments, panoramic changes significantly affected learning, initial orientation and scanning behaviour. Nevertheless, over a few trips, success at gap finding increased, visual scans were reduced, the paths became straighter, and individuals took less time to reach the goal.

Keywords: Gap learning; Scene familiarity; Success rate; Visual navigation.

Fig. 1.

Experimental set up for examining gap learning in Myrmecia midas ants in a semi-familiar environment. (A) The control condition where a goniometer was placed 7 m away from the nest, and 90 deg clockwise from the natural foraging route (experiment 2). (B) The gap position and symmetrical screens on either side on the return journey of displaced foragers. In both cases, a video camera was fixed just above the displacement centre to record initial heading of foragers on the goniometer.

Fig. 2.

Success rate of individual foragers during gap learning with changes of surrounding panorama. Data are for the familiar (experiment 1) and semi-familiar (experiment 2) environments.

Fig. 3.

Circular distributions of initial headings under the different conditions. Forager directions are shown at 50 cm from the release point on (A) the familiar route (experiment 1) and (B) the semi-familiar route (experiment 2) in both control and gap-learning conditions. The nest direction for each figure is at 0 deg. The arrows denote the length of the mean vector of each condition. Asterisks indicate significant differences of initial heading compared with the (third) control trip.

Fig. 4.

Path straightness of foragers on the familiar route. The box plots show path straightness for section A (1 m) and section B (6 m) separately in both control and gap-learning conditions for experiment 1. Asterisks (* and **) and arrows indicate significant differences of path straightness compared with the third trip of the control condition. Circles indicate exceptional path straightness outside of the box plot range. The box plots indicate medians (solid line), box margins (25th and 75th percentiles) and whiskers (5th and 95th percentiles) in this and all following figures. Asterisks represent significant differences (*section A and **section B).

Fig. 5.

Path straightness of foragers on the semi-familiar route. The box plots show path straightness in both control and gap-learning conditions for experiment 2. For the control condition, path straightness was measured from the release point to the nest (section A+B=7 m); in the gap-learning condition, path straightness is shown separately for section A (1 m) and section B (6 m). Asterisks (*first trip of control condition, **section A and ***section B) indicate significant differences of path straightness compared with the third trip of the control condition.

Fig. 6.

Duration of foragers’ trips on the familiar route. The box plots show duration for section A (1 m) and section B (6 m) separately in both control and gap-learning conditions for experiment 1. Asterisks (*section A and **section B) indicate significant differences of duration during homeward navigation compared with the third trip of the control condition.

Fig. 7.

Duration of foragers’ trips on the semi-familiar route. The box plots show duration in both control and gap-learning conditions for experiment 2. For the control condition, the duration was measured from the release point to the nest (section A+B=7 m). In the gap-learning condition, duration is shown separately for section A (1 m) and section B (6 m). Asterisks (*first trip of control condition, **section A and ***section B) indicate significant differences of duration during homeward navigation compared with the third trip of the control condition.

Fig. 8.

Foragers’ path and scanning positions (black dots) on the goniometer in different conditions on the familiar and semi-familiar route. (A–D) Proportion of scans in the ‘start zone’ (dotted circle, 15 cm radius) on the familiar route: (A) 3rd control trial 94%, (B) 1st gap-learning trip 63%, (C) 2nd gap-learning trip 78%, (D) 3rd gap-learning trip 84%. (E–J) Proportion of scans in the start zone on the semi-familiar route: (E) 1st control trip 74%, (F) 2nd control trip 82%, (G) 3rd control trip 91%, (H) 1st gap-learning trip 58%, (I) 2nd gap-learning trip 66%, (J) 3rd gap-learning trip 75%. Individual foragers were released on the centre of a wooden goniometer and the headings were recorded using a camera (see Materials and Methods). The solid circles indicate the goniometer border. The ‘route zone’ occupies all areas outside of the start zone on the goniometer. The black arrow indicates the nest direction. Each path colour represents an individual ant. In all of the trials of all conditions, foragers performed more scans in the start zone than in the route zone.

Fig. 9.

Example paths of two foragers, one in each panel, from the release point to the nest in each experiment. The circles show the positions of scans. If a forager did not find the gap within 4 min, we considered it ‘unsuccessful’ and released it in the middle of the gap and allowed it to return home. Over learning trips, the paths became straighter, scans became reduced, and foragers took less time to reach their nest. On the familiar route (A), the ant was successful in the three consecutive learning trips whereas on the unfamiliar route (B), the 1st and 2nd gap-learning trips were unsuccessful, but the 3rd gap-learning trip was successful.