Keep it rolling : Straight-line orientation in South African ball-rolling dung beetles

Abstract: Representing a substantial range and variety in morphological and ecological niche, found on all continents of the globe (except for the Antarctic), the ball-rolling dung beetles provide an excellent model in which to study the heading direction network and the factors by which it is influenced. As soon as a ball-rolling dung beetle has located a fresh dung pile to feed on, it immediately starts shaping a piece of dung into a ball, rolling it away from the dung pat in as straight of a trajectory as the terrain allows. This straight-line orientation behaviour is thought to be a strategy to escape the fierce competition of dung at the pile. By investigating how size (Paper I), ecological niche, phylogeny (Paper II and Paper III) and visual conditions (Paper IV) influence this relatively straightforward orientation behaviour, I explore the orientation challenges faced, and the solutions presented. In the first paper (Paper I), I investigated the effect of directional error on straight-line orientation in two differently sized beetles, and concluded that the directional error that unavoidably accumulates over a given distance as the beetle travels, is inversely proportional to the step size of the animal. Next (Paper II), I investigated straight-line orientation in a diurnal woodland-living ball-rolling species. In this study I demonstrated that the woodland-living species, present in habitats of densely packed trees and tall grass, relies predominantly on directional information from the celestial pattern of polarised light. This stands in contrast to all previous observations on diurnal ball-rolling beetles, where the sun has been demonstrated as the predominant source of directional information in their heading direction networks.In the third paper (Paper III) I continued to explore the relative weighting of directional information in three species of ball-rolling South African dung beetles from three different tribes living within the same savanna biome, but in different habitat types. In this study I found that species within a tribe share the same orientation strategy, but that this strategy differs across tribes. In my final paper (Paper IV), I further explored the weight relationship of directional information from the sun (simulated by a green LED) and the celestial polarisation pattern (simulated by an overhead band of polarisation) in the heading direction network of the beetle. I concluded that the directional information conveying the highest certainty at a given moment in time is afforded the greatest weight in the heading direction network of the animal.With my work, I hope to provide an insight to the dynamic nature of the biological compass and its ability to change and adapt to different visual environments.

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