Neural coding underlying the cue preference for celestial orientation

Diurnal and nocturnal African dung beetles use celestial cues, such as the sun, the moon, and the polarization pattern, to roll dung balls along straight paths across the savanna. Although nocturnal beetles move in the same manner through the same environment as their diurnal relatives, they do so when light conditions are at least 1 million-fold dimmer. Here, we show, for the first time to our knowledge, that the celestial cue preference differs between nocturnal and diurnal beetles in a manner that reflects their contrasting visual ecologies. We also demonstrate how these cue preferences are reflected in the activity of compass neurons in the brain. At night, polarized skylight is the dominant orientation cue for nocturnal beetles. However, if we coerce them to roll during the day, they instead use a celestial body (the sun) as their primary orientation cue. Diurnal beetles, however, persist in using a celestial body for their compass, day or night. Compass neurons in the central complex of diurnal beetles are tuned only to the sun, whereas the same neurons in the nocturnal species switch exclusively to polarized light at lunar light intensities. Thus, these neurons encode the preferences for particular celestial cues and alter their weighting according to ambient light conditions. This flexible encoding of celestial cue preferences relative to the prevailing visual scenery provides a simple, yet effective, mechanism for enabling visual orientation at any light intensity.

How dim is dim? Precision of the celestial compass in moonlight and sunlight

Prominent in the sky, but not visible to humans, is a pattern of polarized skylight formed around both the Sun and the Moon. Dung beetles are, at present, the only animal group known to use the much dimmer polarization pattern formed around the Moon as a compass cue for maintaining travel direction. However, the Moon is not visible every night and the intensity of the celestial polarization pattern gradually declines as the Moon wanes. Therefore, for nocturnal orientation on all moonlit nights, the absolute sensitivity of the dung beetle's polarization detector may limit the precision of this behaviour. To test this, we studied the straight-line foraging behaviour of the nocturnal ball-rolling dung beetle Scarabaeus satyrus to establish when the Moon is too dim—and the polarization pattern too weak—to provide a reliable cue for orientation. Our results show that celestial orientation is as accurate during crescent Moon as it is during full Moon. Moreover, this orientation accuracy is equal to that measured for diurnal species that orient under the 100 million times brighter polarization pattern formed around the Sun. This indicates that, in nocturnal species, the sensitivity of the optical polarization compass can be greatly increased without any loss of precision.

Kinematic-Dimensional Analysis of Parallel Robot Used for Orientation

In the paper a very simple parallel robot intended for some celestial orientation applications (satellite dishes, sun trackers, cameras, telescopes etc.) is proposed. After describing the mechanism and its kinematics, the workspace and the presence of the singularities are studied. The dimensional optimization to maximize the workspace is described, taking into account the constraints due to telescopic leg lengths and transmission angles.