(c) Symmetric cues

To determine if a symmetric pattern of skylight, such as that presented by the main band of the Milky Way across the zenith, is sufficient for accurate orientation, the ability of nocturnal dung beetles to orient to two lights 180° apart in azimuth was tested. Under the ‘equal intensity’ condition (figure 2), two green (λ_max = 525 nm) LED arrays (LZ1-10G100: LedEngin Inc., San Jose, USA), acting as light spots, were displayed simultaneously, either at the north and south or east and west positions at an elevation of 45°. These light spots were displayed at a distance of 2 m from the centre of a 1 m diameter arena. To simulate nocturnal conditions, these LEDs were dimmed using four layers of 1.2ND filter (Lee Filters 299: Andover, UK) and one layer of 1/8 white diffusion filter (Lee Filters 252) to a photon flux of 1.25 × 10⁷ photons cm⁻² s⁻¹ (see electronic supplementary material, Intensity Measurements). For this experiment, the stimulus was not moved between trials, and orientation error was therefore calculated from the difference in headings between two trials with identical stimuli. These data were compared with orientation errors recorded with no light spots illuminated (no lights) and with an ‘intensity difference’ condition, in which one of these light spots was dimmed using a further layer of 1.2ND filter and both light spots were closer (650 mm as opposed to 2 m) to the arena centre, resulting in intensities of 1.99 × 10⁸ photons cm⁻² s⁻¹ and 1.45 × 10⁷ photons cm⁻² s⁻¹, respectively (a Michelson contrast of 86.5%). All trials were carried out at Lund University, between 7 and 13 May 2015, with the exception of the ‘intensity difference’ experiment, which was carried out at the ‘Stonehenge’ field site in South Africa on 2 February 2015. Trials at Lund University were carried out between 13.00 and 18.00 for beetles that had habituated to a 12.00–00.00 dark period, while trials in South Africa were carried out between 20.00 and 00.00 during the typical activity period for S. satyrus [18].

The arrangement used to create symmetric cues. The ‘equal intensity’ symmetric stimulus consisted of two light spots 180° apart. In the ‘intensity difference’ symmetric stimulus one of the two light spots was dimmed by one order of magnitude. In the ‘no lights’ condition no light spots were illuminated, but all other aspects of the arrangement were identical to the other conditions. (Online version in colour.)