Seasonal variation in the dung of african grazing mammals, and its consequences for coprophagous insects

The dung of African grazing mammals varies in nutrient and moisture content according to the condition of the pasture on which the animals feed. This study investigated the effect of variation in quality of herbivore dung on the survival and reproduction of coprophagous insects. Seasonal variation was recorded in physical and chemical characteristics of zebra, wildebeest and impala dung. Dung was collected from free-ranging animals grazing in natural habitat in Mkuzi Game Reserve, a hot summer-rainfall region of South Africa. Interspecific differences in dung were related to the feeding ecology, digestive physiology and size of each species. Seasonal changes in water and nitrogen content of dung were related to patterns of rainfall and hence pasture growth. Dung moisture was significantly correlated with the amount of rain that fell in the preceding 2 weeks for wildebeest, in the preceding 4 weeks for impala and in the period 2-6 weeks before collection for zebra dung. Seasonal variability in wildebeest dung affected the reproductive rate of the dung beetle Euoniticellus intermedius (Reiche). Egg production at 25^⚬C ranged from 0.1 per female per week in winter dung to 12.1 in summer dung, and was significantly correlated with dung moisture. Euoniticellus intermedius and the African buffalo fly Haematobia thirouxi potans (Bezzi) could not breed in fresh wildebeest dung of 62% water content. However when the water content was raised to 68% and above, breeding by Euoniticellus intermedius increased; and at moisture contents of 73% and above, buffalo fly size increased and survival improved. Higher water content was correlated with an increase in availability of dung fluid, the component of dung used by these insects.