The galloping dung beetle: A new gait in insects and its consequences for navigation
Lund University, Department of Biology, Sweden
University of the Witwatersrand, South Africa
University of Pretoria, South Africa
Like most animals, insects walk in a variety of gaits depending on their speed and on the substrate. In all gaits described so far, each pair of legs shows a 180° phase difference (i.e. the legs alternate), while the phase difference between adjacent pairs of legs is altered to create different gaits. Synchronous (in-phase) stepping of a leg pair is only observed in exceptional circumstances, e. g. when crossing obstacles or at the start of a walk. However, in the deserts of south-western Africa, some species of flightless dung beetles Pachysoma (=Scarabaeus) walk in a unique "galloping" gait. Like a bounding hare, the beetles propel their body forward by synchronously stepping with both middle legs and then both front legs (see video). In this study, we describe this unique gait and explore three possible reasons for its evolution.
We observed Pachysoma at three field sites in South Africa and Namibia, and used high-speed video to describe their gait. Only three of seven observed species, all part of the same evolutionary lineage, employ the galloping gait. All three collect plant detritus which they drag home on their hind legs. We tested three hypotheses about why this gait has evolved:
1. Substrate: Galloping might be more energy efficient on the soft sand of the beetles' natural habitat. If this is the case, we might expect beetles to change gait on different substrates or slopes. We found that, compared to flat ground, beetles are less likely to gallop on a 30° upward slope (Wilcoxon signed-rank test, n=29, p=0.005), but there is no difference when walking down a slope (p=0.68) or on a slippery acetate sheet (p=0.51).
2. Vision: Pachysoma use a skylight compass for path-integration. Galloping might aid visual navigation, if it helps keep the visual image constant on rough terrain. Comparing the tripod-walking P. striatum (Figure 1a) and the sympatric, galloping P. endroedyi (Figure 1d), we found that the standard deviation of head orientation was approximately two times larger in tripod-walkers (preliminary data, Bartlett-Box test, n=6, p<0.001). We also compared eye morphology and size across twelve species using scanning electron micrographs. In all three galloping species, the ventral eyes are drastically reduced in size (Figure 1b,c).
3. Odometry: Preliminary data suggests that Pachysoma use stride integration to measure distance while homing. Each step of the gallop consists of a fast forward motion while the middle legs step, followed by a period of near standstill while the front legs step. We argue that this sharper profile, combined with the reduced lateral motion, might provide a more reliable input for a stride integration odometer on rough, slippery terrain.
Conclusion: We describe a gallop/bound-like gait in South African dung beetles. To our knowledge, this is the first time such a gait has been described in insects. Possible reasons for the evolution of this gait include energy efficiency, gaze stabilisation and odometry. Future research into this novel gait will provide an interesting new model organism for the study of biomechanics and insect navigation.
Tenth International Congress of Neuroethology, College Park. Maryland USA, USA, 5 Aug - 10 Aug, 2012.
Poster (but consider for participant symposium and student poster award)
Orientation and Navigation
(2012). The galloping dung beetle: A new gait in insects and its consequences for navigation
Front. Behav. Neurosci.
Tenth International Congress of Neuroethology.
29 Apr 2012;
07 Jul 2012.