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In otherwise nutrient-poor savannas, fertile vegetation patches are particularly attractive to ungulates because of the higher-quality food they provide. We investigated forage plants and diet of the common warthog (Phacochoerus africanus) on an abandoned cattle ranch in coastal Tanzania. The forage grasses of highest nutritional quality occurred in former paddock enclosures (bomas) where cattle had been herded at night. In the dry season, grass samples from bomas contained approximately 4 times as much nitrogen and phosphorus as those of the surrounding vegetation. δ^sup 15^N values of soil and plants also were highest in bomas and decreased significantly with distance, and high δ^sup 15^N values in feces suggest that warthogs preferentially fed in the vicinity of the former bomas. δ^sup 13^C values of warthog feces indicate that warthogs ingested on average 83% (77-98%) C^sub 4^ grasses, with this proportion varying regionally but not seasonally. We conclude that, for medium-sized selective grazers such as warthogs, bomas represent attractive feeding grounds. We also hypothesize that by promoting nutrient turnover in these patchily distributed areas, grazing animals help to maintain them as sources of high-quality forage.
Key words: African savanna, isotopic analyses, livestock, medium-sized grazer, nitrogen, phosphorus
Understanding the nutritional status of plants and animals is essential for wildlife population and habitat management (van der Waal et al. 2003). Compared to dicotyledons and C^sub 3^ grasses, the dominant C^sub 4^ grasses of tropical savannas are nutrient-poor, and also vary in nutritional quality both seasonally and between sites (Owen-Smith 1982). Thus, to meet their nutritional requirements, small ungulates must be very selective in their diet (Murray 1993; Wilmshurst et al. 1999). For this reason, some East African grazers undertake long-distance migrations (Dörgeloh et al. 1998), following flushes of new grass growth triggered by rainfall (Durant et al. 1988; Mduma et al. 1999).
The common warthog (Phacochoerus africanus) is a medium-sized, nonmigratory ungulate. It is a hindgut fermenter with significant microbial fermentation in parts of the digestive tract (Boomker and Booyse 2003). Warthogs are predominantly grazers and depend on high-quality food. Their wide distribution in African savanna ecosystems can partly be explained by their dietary flexibility (Estes 1991; Kingdon 1997; Rodgers 1984).
The nutrient content of grass depends on rainfall and on the availability of macro- and micronutrients in the...