|Eger's Earth Boring Beetle - Geotrupes egeriei |
New research shows dung-rolling beetles perform an orientation dance atop their dung ball to maintain a straight-line rolling path . Live adult dung beetles photographed at Bartlett, Illinois.
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Dung beetles are those which feed partly or exclusively on feces, most often from herbivorous animals. Most of these species belong to the subfamilies Scarabaeinae and Aphodiinae of the family Scarabaeidae. There are dung-feeding beetles which belong to other families, such as the Geotrupidae (the earth-boring dung beetles, pictured here.)
The Scarabaeinae alone comprises more than 5,050 species. Many dung beetles, known as rollers, are noted for rolling dung into spherical balls, which are used as a food source or brooding chambers. Other dung beetles, known as tunnellers, bury the dung wherever they find it. A third group, the dwellers, neither roll nor burrow; they simply live in manure.
An interesting feature of dung beetle behaviour is that once they have formed a piece of dung into a ball, they roll it along a straight path away from the dung pile. This straight-line orientation ensures that the beetles depart along the most direct route, guaranteeing that they will not return to the intense competition (from other beetles) that occurs near the dung pile. Before rolling a new ball away from the dung pile, dung beetles perform a characteristic “dance,” in which they climb on top of the ball and rotate about their vertical axis. This dance behaviour can also be observed during the beetles' straight-line departure from the dung pile.
Experiments were performed on the farm “Stonehenge” in North-West Province, South Africa using adult beetles of the diurnal ball-rolling species Scarabaeus (Kheper) nigroaeneus (Coleoptera: Scarabaeidae) collected in the local area. Captured beetles were individually marked on their elytra with paint and placed in plastic bins filled with soil, where they could make and roll balls from the fresh cow dung provided.
Researchers calculated the percentage of beetles that danced under the following conditions: a) at the dung pile, just after making a ball and just before rolling, b) rolling into an obstacle (experiment 2), c) falling off a step (experiment 3), d) rolling off course in a curved tunnel (experiment 4, results from both the 1 m and 1.5 m diameter tunnels are represented with one bar as they were the same), e) 180° rotation with a view of the sky (experiment 5), f) 180° rotation with no view of the sky (experiment 5) and g) 180° reflection of the sun (experiment 6) . Results are show in figure 1. below.
Figure 1. Percentage of beetles that danced
If the function of the dance is to take a compass reading of the celestial cues or find a match between this snapshot and the current view of the sky, then why do beetles climb on top of their ball to scan the sky, instead of staying on the ground? Climbing on top of the ball would have several advantages. First, getting higher would maximise the beetles' chances of getting a good view of the sky. This would be especially useful in grassy areas where the view of the sky from the ground may be occluded by obstacles.
Another advantage of climbing on top of the ball would be that it would allow the beetles to perform the rotation on a single plane above the ground, where the ball would not obstruct the field of view. A third advantage of getting on top of the ball would be that it would allow the beetle to easily defend its ball against other beetles that might try to steal it while it is otherwise engaged in an orientation behaviour. Given that beetles rolling on rough ground perform their first dance when they are very close to the dung pile, this defensive approach would be necessary, due to the fierce competition for dung balls near the dung pile .
Unlike many other animal navigators, the task of the dung beetles is not to find their way back to a familiar location after a foraging trip. Instead, foraging dung beetles need to roll from a known location to an unknown destination in the most direct and efficient manner possible, which is a straight line. The researchers propose that the characteristic dance that dung beetles perform before rolling away from the dung pile, and after encountering a disturbance while rolling, is an orientation mechanism that allows beetles to set an initial roll bearing, and to regain this original bearing if they experience an unintentional disturbance .
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Order Coleoptera: Beetles are the dominant form of life on earth: one of every five living species is a beetle. Coleoptera is the largest order in the animal kingdom, containing a third of all insect species. There are about 400,000 known species worldwide, ~30,000 of which live in North America. Beetles live in nearly every habitat, and for every kind of food, there's probably a beetle species that eats it.
Beetles first appeared during the lower Permian period, about 270 mya
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