Large mammals in Ruaha National Park, Tanzania, dig for water when water stops flowing and water bacterial load increases

As water is essential for life, animals have adaptations that increase their ability to survive during periods of water shortage. Accessing water by digging is one behavioural adaptation to water shortage used by some African mammals. Digging might also provide access to higher quality water below ground when surface water quality is poor. We investigated the digging of waterholes by wildlife in the Ruaha National Park (NP), in central Tanzania, during three dry seasons (June to November from 2011 to 2013). We monitored surface water availability and water quality at 10 sites along the Great Ruaha River (GRR) and eight non-GRR sites. We used camera-traps and direct observations to determine when and where digging to access water occurred. Elephant (Loxodonta africana), plains zebra (Equus quagga), warthog (Phacochoerus africanus) and yellow baboon (Papio cynocephalus) dug waterholes and a further four species drunk from these holes. Waterholes were dug later in the dry season along the GRR (October) than at other sites (July). The likelihood of digging and drinking from waterholes was lower along the GRR than at non-GRR sites and did not depend on the absence of surface water but increased when surface water stopped flowing. Digging of waterholes was also significantly more likely when the bacterial load in available surface water increased but was independent of salinity levels. Escherichia coli load, indicative of faecal contamination, significantly increased with total aerobic bacterial load. Our results suggest that digging is an adaptation to avoid the ingestion of poor quality surface water highly contaminated with faeces, and thereby possibly also potentially pathogenic microbes, in addition to providing access to water when surface water is absent. Our findings also highlight (1) the essential role of the GRR as the key water source for wildlife in the Ruaha NP during the dry season, and (2) that maintenance of water flow throughout the dry season is essential to prevent deterioration of water quality in the GRR.