Natural pans are one of the most interesting features of the African bush because so much happens in and around them. Natural pans are usually filled by rain during the wet season and depending on their size, level of utilization by animals, subsequent refilling and rates of evaporation, may empty within a week or two or may last months. Natural pans form most easily in soils with a high clay content. These soils are found mostly in basalt areas that give rise to clay soils and in low lying sodic areas along drainage lines. Natural pans are often areas of intense activity – providing drinking water for local wildlife, breeding habitat for frogs and water birds and mud for animals, which enjoy wallowing such as warthog, bush pig, buffalo and rhino.
Natural pans are often surrounded by the foamy nests of grey tree frogs (Chiromantis xerampelina) that construct their nests in trees or on objects overhanging the water. When the eggs hatch the tadpoles fall directly into the water.
Seasonal pans also play a significant role in the spread of wildlife diseases. Anthrax, which is endemic to the Kruger National Park and surrounding areas, has a close association with natural pans. Highly resistant anthrax spores gravitate to and are washed down into low-lying catchments areas where natural pools form. When these pans are full the concentration of spores is low and although animals are ingesting spores there are too few to precipitate the disease. During dry months of the year as the volume of water decreases in natural pans the relative concentration of anthrax spores increases to a point where a threshold is exceeded and animals ingest a sufficient quantity of spores to contract the disease. When the rain months return the water volume increases once again and the dilution factor decreases the anthrax spore concentration to a level where it is once again safe for the animals to drink the water. A similar cycle occurs with a disease spread by rodents in which a bacterium effects the myocardium (heart muscle) of elephants – especially bulls which drink more frequently from small natural pans than breeding herds – and can cause death – a condition referred to as endomyocarditis.