The View from the Trees: Nocturnal Bull Ants, Myrmecia midas, Use the Surrounding Panorama While Descending from Trees

Abstract

Solitary foraging ants commonly use visual cues from their environment for navigation. Foragers are known to store visual scenes from the surrounding panorama for later guidance to known resources and to return successfully back to the nest. Several ant species travel not only on the ground, but also climb trees to locate resources. The navigational information that guides animals back home during their descent, while their body is perpendicular to the ground, is largely unknown. Here, we investigate in a nocturnal ant, Myrmecia midas, whether foragers travelling down a tree use visual information to return home. These ants establish nests at the base of a tree on which they forage and in addition, they also forage on nearby trees. We collected foragers and placed them on the trunk of the nest tree or a foraging tree in multiple compass directions. Regardless of the displacement location, upon release ants immediately moved to the side of the trunk facing the nest during their descent. When ants were released on non-foraging trees near the nest, displaced foragers again travelled around the tree to the side facing the nest. All the displaced foragers reached the correct side of the tree well before reaching the ground. However, when the terrestrial cues around the tree were blocked, foragers were unable to orient correctly, suggesting that the surrounding panorama is critical to successful orientation on the tree. Through analysis of panoramic pictures, we show that views acquired at the base of the foraging tree nest can provide reliable nest-ward orientation up to 1.75 m above the ground. We discuss, how animals descending from trees compare their current scene to a memorised scene and report on the similarities in visually guided behaviour while navigating on the ground and descending from trees.

Keywords: ants; foraging; landmarks; navigation; nocturnal; scanning.

FIGURE 1

Circular distributions of individual Myrmecia midas foragers’ positions on the tree face during displacement experiments on their foraging tree. Figures show the raw data of forager positions at two heights after displacements to one of four sides of the tree at a 2 m height at Nest 1. The nest direction for each figure is at 0°, labelled by a black triangle. The arrow denotes the direction and length of the mean vector. Foragers were collected at the base of their foraging tree, held overnight and then released vertically on the tree face at one of four sites (0, 90, 180, and 270°). (A) The position of individual foragers released at the 0° location at 1 m in height. (B) Forager positions of individuals released at the 90° location at 1 m in height. (C) Forager position of individuals released at the 180° location at 1 m in height. (D) Forager positions of individuals released at the 270° location at 1 m in height. (E) The position of individual foragers released at the 0° location as they reach the ground. (F) Forager positions of individuals released at the 90° location as they reach the ground. (G) Forager position of individuals released at the 180° location as they reach the ground. (H) Forager positions of individuals released at the 270° location as they reach the ground.

FIGURE 2

Circular distributions of individual M. midas foragers’ positions on the tree face during displacement experiments on their foraging tree. Figures show the raw data of forager positions at two heights after displacements to one of four sides of the tree at a 2 m height at Nest 2. The nest direction for each figure is at 0°, labelled by a black triangle. The arrow denotes the direction and length of the mean vector. Foragers were collected at the base of their foraging tree, held overnight and then released vertically on the tree face at one of four sites (0, 90, 180, and 270°). (A) The position of individual foragers released at the 0° location at 1 m in height. (B) Forager positions at Nest 2 of individuals released at the 90° location at 1 m in height. (C) Forager position at Nest 1 of individuals released at the 180° location at 1 m in height. (D) Forager positions at Nest 1 of individuals released at the 270° location at 1 m in height. (E) The position of individual foragers released at the 0° location as they reach the ground. (F) Forager positions of individuals released at the 90° location as they reach the ground. (G) The position of individual foragers released at the 180° location as they reach the ground. (H) Forager positions of individuals released at the 270° location as they reach the ground.

FIGURE 3

Circular distributions of individual M. midas foragers’ positions on the tree face during displacement experiments with cue conflicts. Figures show the raw data of forager positions at two heights after displacements to one of two sides of the tree at a 1.5 m height at Nest 3. The nest direction for each figure is at 0°, labelled by a black triangle. The foragers’ accumulated vector was at 270°, labelled by a white triangle. The arrow denotes the direction and length of the mean vector. Foragers were collected at the base of their foraging tree, held overnight and then released vertically on the tree face of the nest tree at one of two sites (0 and 180°). (A) The position of individual foragers released at the 0° location at 1 m in height. (B) Forager position at Nest 1 of individuals released at the 180° location at 1 m in height. (C) The position of individual foragers released at the 0° location as they reach the ground. (D) The position of individual foragers released at the 180° location as they reach the ground.

FIGURE 4

Circular distributions of individual M. midas nest tree foragers’ positions on the tree face during the landmark blocking experiments on the nest tree. Figures show the raw data of forager positions at three heights after displacements to one of two sides of the tree at a 1.5 m height at Nest 3. The nest direction for each figure is at 0°. The arrow denotes the direction and length of the mean vector. Foragers were collected at the base of the nest tree, held overnight and then released vertically on the tree face of the nest tree opposite the nest entrance (180°) with the surrounding landmark panorama either unblocked or blocked. (A) The position on the tree face of individual foragers released at the 180° location as they begin their descent at 1.4 m in height with the surrounding landmarks unblocked. (B) The position on the tree face of individual foragers released at the 180° location as they begin their descent at 1.4 m in height with the surrounding landmarks blocked. (C) The position on the tree face of individual foragers released at the 180° location at 1 m in height with the surrounding landmarks unblocked. (D) The position on the tree face of individual foragers released at the 180° location at 1 m in height with the surrounding landmarks blocked. (E) The position on the tree face of individual foragers released at the 180° location as the forager reaches the ground with the surrounding landmarks unblocked. (F) The position on the tree face of individual foragers released at the 180° location as the forager reaches the ground with the surrounding landmarks blocked.

FIGURE 5

Individual M. midas nest tree foragers’ paths descending the tree face in the landmark blocking experiment. Circular positions on the tree face have been unwrapped to show individuals’ paths from the 180° off-route, 1.5 m high displacement site (open square) to the ground. The plots are cylindrical, with +180° and –180° being the same position on the side of the tree opposite the nest. The open circle at ground level (0 cm) denotes the nest entrance direction. (A) Forager paths in the unblocked condition with the surrounding landmarks visible. (B) Forager paths in the blocked condition with all surrounding landmarks below 2 m blocked using a plastic screen. The grey area in the background signifies the blocking screen surrounding the tree from 0 to 2 m.

FIGURE 6

Quantifying the change in the panorama at different elevations on the foraging tree at the three nests. (A) Panoramic images at the base of the foraging tree (blue), 1 m in height (green), and 1.75 m in height (red). Images were downscaled to 1 pixel per 1° to resemble the ant’s visual acuity, filtered through only the blue colour channel and oriented with the nest centred. (B) The rotIDF compares the root mean square pixel difference between the panorama at the base of the foraging tree with itself (blue), the 1 m (green), and the 1.75 m (red) panoramas. The nest direction in all comparisons is centred at 0°.

FIGURE 7

Quantifying panorama changes at the four displacement directions and at two elevations on the foraging tree at the three nests. (A) Panoramic images at the base of the foraging tree (blue), 1 m in height at 0° (red), 90° (green), 180° (black), 270° (orange) and 1.75 m in height at 0° (red), 90° (green), 180° (black), 270° (orange). Nest orientation is at the centre of each image and images were downscaled to 1 pixel per 1° to resemble the ant’s visual acuity, filtered through only the blue colour channel and oriented with the nest centred. (B) The rotIDF compares the root mean square pixel difference between the panorama at the base of the foraging tree with itself, and the foraging tree at both 1 and 1.75 m at each direction. The nest direction in all comparisons is centred at 0°.

FIGURE 8

The four described vertical scanning behaviours. All images were taken as foragers were descending their foraging tree after displacement. (A) The horizontal scan. (B) The downward pitch scan. (C) The head roll scan. (D) The push up or upward pitch scan.