Light orientation in the ball-rolling dung beetle, Gymnopleurus sturmi (MacLeay, 1821), in Morocco (Coleoptera: Scarabaeinae: Gymnopleurini)

The ball-rolling dung beetle, Gymnopleurus sturmi, is widely distributed in the Mediterranean Sea Basin. Individuals aggregate to consume dung and only make dung balls for reproduction. The mating pairs formed on dung piles quickly make and roll their ball away from potential competition, mainly from conspecific individuals. We studied the behaviour of G. sturmi in the field, and more particularly the direction of ball rolling after leaving the dung pile. The possible rolling directions were demarcated as 16 numbered sectors oriented according to the four cardinal directions. The experimental results showed that ball rolling was almost always done in a straight line from the food source. The rolling direction was chosen before rolling commenced, during an orientation dance that was similar to that described for Scarabaeini species. In the morning, G. sturmi predominantly rolled their dung balls east-south-east, while in the early afternoon the main rolling direction was south-south-west, shifting to north-north-east at the end of the activity period. The position of the sun seemed to be the major cue for guiding the ball rolling direction, with a rapid readjustment to the initial direction in cases of deviation due to obstacles. We discuss the hypothesis that the opposite rolling directions taken by beetles between the beginning and the end of the afternoon may be due to the switching from direct solar cueing to polarised light cueing near dusk. The results are also discussed in the context of competition.