Brood care in the dung beetle Onthophagus vacca (Coleoptera: Scarabaeidae): the effect of soil moisture on time budget, nest structure, and reproductive success

Under laboratory conditions brood care behaviour, nest structure and weight of dung supply in brood chambers of the dung beetle Onthophagus vcca proved to depend on water content of the soil beneath the dung. The substrate in a bucket beneath the dung pat was dry sand (4% water content) or moist sand (8% water content). Emigrating beetles were trapped and counted at 12 h intervals. In a total of 109 replicates one pair was released on an artificial 1000 g dung pat. From 95 replicates in which brood chambers were built the following results were derived: 1) Breeding females and resident males which helped the female stayed longer in dung pats on dry sand than in those on moist sand. 2) Nest architecture was influenced by substrate moisture: length of main tunnels did not differ between nests in dry and moist sand, but total length of side tunnels was shorter in dry sand. 3) Numbers of brood chambers were equal in both substrate types, weight of the dung supplies was larger in dry sand. 4) Offspring size was not only influenced by dung provision in the brood chambers. Beetles emerging from chambers in dry sand were smaller than those emerging from moist sand even if the amount of dung supply was equal.