Polarized Light Orientation in Ball-Rolling Dung Beetles
Many dung beetles, unlike most insect navigators, do not need to locate a stationary nest at the end of their foraging journey. This makes the nature of their orientation task fundamentally different, and in the case of straight-line orientation, the beet
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Polarized Light Orientation in Ball-Rolling Dung Beetles Marie Dacke
Abstract Many dung beetles, unlike most insect navigators, do not need to locate a stationary nest at the end of their foraging journey. This makes the nature of their orientation task fundamentally different, and in the case of straight-line orientation, the beetles appear to rely single-handedly on celestial compass cues to move along straight paths. With a focus on the sky, diurnal dung beetles rely on the sun and the skylight cues that span the entire sky. These cues include the linear polarization pattern of skylight. As day turns to night, crepuscular and nocturnal dung beetles start to fly at around sunset. At this time, the full sky is polarized in one single direction, and the ball-rolling beetles can be observed to turn sharply when rolling under a linear polarizer placed with its E-vector oriented 90 to that of skylight. When the moon has risen, the beetles continue to orient along straight paths well after sunset, guided by the celestial polarization pattern created by the scattered moonlight. The intensity of this relatively dim polarization pattern will gradually decline as the moon wanes. Remarkably, even the extremely dim celestial polarization pattern formed around a crescent moon is sufficient to guide the nocturnal beetles along straight paths. Moreover, straight-line orientation on these dark nights is performed with the same precision and speed as in dung beetles orienting under the much brighter polarization pattern of the sun or full moon. So strong is their desire to roll their balls of dung that nocturnal beetles can be made to roll at day and diurnal beetles can be compelled to roll in the middle of the night. This incredible flexibility opens up the possibility to perform a new set of experiments directed towards an understanding of how celestial compasses have been adapted to the visual environment in which the insect is normally active.
M. Dacke (*) Department of Biology, Lund Vision Group, Lund University, 221 00 Lund, Sweden e-mail: [email protected] G. Horva´th (ed.), Polarized Light and Polarization Vision in Animal Sciences, Springer Series in Vision Research 2, DOI 10.1007/978-3-642-54718-8_2, © Springer-Verlag Berlin Heidelberg 2014
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Introduction
A freshly made dung pile on the African savannah attracts visitors from near and far. Ball-rolling dung beetles fly in from kilometres away and gather at this food source to feed and mate. Once the foraging beetles have climbed onto the dung pile, they begin to sculpt the dung into a transportable package of food. Using their flat heads and legs, they quickly form even the sloppiest excrement into a ball and start to push it away. The beetles exit the pile in different directions (Fig. 2.1a) (Dacke et al. 2003a, 2013a, b, 2014) in search of a suitable (but not predetermined) place to bury and consume their smelly meal. Newly arriving beetles are hot from flying (and therefore faster and stronger than colder beetles) and will try to steal
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