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Nocturnal activity of common warthog in Rimoi Game Reserve, central Kenya

Yvonne A. de Jong and Thomas M. Butynski

Common warthog Phacochoerus africanus are the smallest of the diurnal bare-skinned large mammals (adult males ≈ 80 kg; adult females ≈ 56 kg). Partly due to their sparse cover of hair and low body fat, they are intolerant of cold and hot air temperatures. They avoid the warmer periods of the day by resting in the shade, mud-wallowing (Figure 1), or retreating to a burrow (Cumming, 2013; Butynski & De Jong, 2018). Although typically diurnal, the odd record exists of common warthog being active at night.

Figure 1. Young adult male common warthog Phacochoerus africanus mud-wallowing during the heat of the day (16:40 h) in Tsavo East National Park, southeastern Kenya. Wallowing is probably important for thermo-regulation and for removal of ectoparasites. Photograph by the authors.

On 21 October 2021, we spend one night at Croc Camp, Rimoi Game Reserve (1,065 m asl; 66 km²), southwestern Kenya. This camp is located on the left bank of the Kerio River and holds about a 1.5 hectare open area of dense short green grass. When we arrived at about l6:00 h there were 12 relatively tame common warthog grazing on the campground. To our surprise, at least six individuals continued grazing until at least 22:40 h under a near (98%) full moon. Large predators were neither heard nor seen in Rimoi and we were told by the rangers that lion are absent and leopard are very rare. The 12 warthog encountered at Croc Camp were grazing alongside cattle and within 20 m of people (three campers, one ranger, three herders). This suggests that predation by humans was also not a concern for warthog at this site. This small Reserve is surrounded by a high human population density and has a long and tense history of encroachment by people and their livestock (Chebet, 2019). We encountered many cattle within Rimoi (Figure 2), including at Croc Camp.

Figure 2. Common warthog Phacochoerus africanus were observed grazing near cattle and plains zebra Equus quagga in Rimoi Game Reserve, southwestern Kenya. Photograph by the authors.

Although only rarely observed, this was not the first time we recorded nocturnal activity by common warthog. In 2012 we captured a sounder of common warthog on a camera trap located 20 km south of Sibiloi National Park, northwestern Kenya (Figure 3; De Jong & Butynski 2012). Also, during about 70,000 nocturnal-hours of camera-trapping on Lolldaiga Hills Ranch, central Kenya (where it is dark during about 19:00–06:00 h), we obtained two photographs of adult male warthog—one just after dark (19:28 h) and one near daybreak (05:30 h).

Figure 3. Sounder of two adult and six large juvenile common warthog Phacochoerus africanus captured on a camera trap on a moonlit night south of Sibiloi National Park, northwestern Kenya. Camera trap image by the authors.

Nocturnal activity by warthog is probably constrained by low temperatures, poor visibility, and predators. Our observations in Rimoi and south of Sibiloi were made when early night temperatures were warm (>18°C), when there was considerable moonlight (98%), and where large predators were either absent or at low abundance. However, in stark contrast with Rimoi, where food and water were ample and day-time temperatures moderate, food appeared to be scarce, water absent, and day-time temperatures high at the site south of Sibiloi. 

Rimoi appears a suitable site at which to conduct research on nocturnal activity of common warthog. The results could be compared with those from a similar study in a site where it is hot during much of the day, food and water are often scarce, and the large predator fauna is intact (e.g., Tsavo East NP, Tsavo West NP). Such a comparative study should help determine the environmental conditions that promote and constrain nocturnal activity by common warthog.

Although there are no records (that we are aware of) for nocturnal activity by the relatively poorly known desert warthog Phacochoerus aethiopicus (De Jong & Butynski, 2014, 2018), we suspect that this species is also occasionally active at night.

We are interested in records of nocturnal activity by either species of warthog. If you have encountered warthog at night, please send your record(s) to Yvonne de Jong at: yvonne@wildsolutions.nl

Adult female common warthog Phacochoerus africanus in Rimoi Game Reserve, southwestern Kenya. Photograph by the authors.

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