feeding behavior - movement

Hippo have a primary requirement to satisfy the aquatic conditions which provide their "daily living space" and, only after that, is the nature of their preferred pastures important.

A number of factors combine in creating the ideal daily living space (Figure 7). Where available, hippo select open stretches of permanent water with submerged sandbanks or gently shelving beaches where they can rest during the day with their back and heads just out of the water and their young can suckle without swimming. They prefer slack and relatively shallow water but like to have sufficient deep water nearby in which they can totally submerge. The daily movements between water and the grazing range result in trampling and erosion to river banks. When an exit point from a river becomes unusable due to erosion, hippo will begin another nearby and, over a number of years, this process can reshape rivers.

The significance of the relationship between daily living space and grazing range is that simple comparisons of hippo densities or attempts to assess carrying capacity are invalid without an understanding of the constraints affecting the daily living space of each population.

In the Caprivi, the influence of human activities on the daily living space of hippo is a highly significant factor (Figure 8). Hippos, humans and their livestock are competing for the same resources along river margins. Mendelsohn & Roberts (1997) refer to overgrazing by cattle in most of the Eastern Caprivi vegetation types. The final outcome of this competition will depend largely on the value (both economic and intrinsic) which the people living in northern Namibia place on hippos and the extent to which they are able to realise that value.

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Movement

With sufficient grazing available, hippo tend to remain close to rivers. However, drought, arid conditions or competition with humans may cause hippo to seek resources some distance from their daily living space. In the Serengeti hippo tend to remain within 1.5km of the Mara River (Olivier & Laurie 1974); in Kruger National Park hippo are found up to about 5km from the Letaba River (Pienaar et al 1966); up to 7km in the Queen Elizabeth National Park (now Ruwenzori National Park) (Field 1970); and up to 10km in Murchison Falls (Ian Parker, pers. comm.). The largest recorded distances are up to 30km (Fuente 1970, in Smithers 1983). Grazing pressure tends to decrease with distance from water (Lock 1972).

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Feeding behaviour

The food of hippos is almost entirely grass and sedges and Scotcher (et al 1978) recorded some 61 monocotyledonous species (Appendix 3). Small amounts of dicotyledonous plants are ingested but this is more likely to be by accident than intent and is a result of their feeding style. Hippo feed by clamping plants between their lips and using a sweep of the head to detach them from the ground. This tends to preclude selective grazing in diverse habitats.

The hippopotamus selects the plant community that best satisfies its feeding requirements - pure grassland, woodland or variations thereof, whilst thickets and dense forests are not preferred. Hippo might be termed an 'area selective' grazer based on the extent to which plant communities occurring within the same grazing environs are used. Although Bere (1959) described hippos as selective grazers in Queen Elizabeth National Park, Uganda, this selectivity was achieved more by feeding in monocultural patches rather than by individual plant selection.

Hippo food preferences change seasonally with an emphasis on those plant communities which offer a continuous dispersion of grasses. The factors determining palatability or acceptability of plants to a grazing animal are complex and it seems that the different stages of growth of herbage play a key rôle in the hippo diet. Hippo may be largely unselective as far as species are concerned but they show a preference for areas which offer freshly sprouting, highly nutritious forage which is relatively high in protein and low in fibre and they avoid dry stemmy material which is low in protein and high in fibre.

Numerous authors refer to the phenomenon of hippo 'lawns' - patches where the continuous grazing of hippos results in a short, productive green grass cover. A number of the floodplain vegetation types in the Eastern Caprivi, i.e. the Bukalo-Liambesi floodplains, dry Mamili grassland, Liambesi-Linyanti grassland, Okavango-Kwando grassland, Zambesi floodplain grassland, and the Gunkwe mulapos are characterised by extensive Cynodon dactylon 'lawns' (Mendelsohn & Roberts 1997).

Olivier & Laurie (1974) put forward the idea of a cycle in the relationship between hippos and their grazing pastures (Figure 9). Areas which are intensely grazed by hippos tend to exclude fire; as a result of this woody regeneration happens; grass becomes protected against hippo grazing by the woody regrowth; the hippo grazing pressure in such areas is reduced; the grass becomes long and flammable; fires occur; the woody regrowth is set back before the plants can develop into large fire-resistant trees; the grass becomes available to hippos; grazing intensity increases and the cycle repeats itself.

Thornton (1971) monitored the effects of the removal of the entire hippo population from the Mweya Peninsula in Queen Elizabeth National Park over a period of 4 years. The most significant effects were changes in the physiognomic structure of the pasture; changes in grass species composition with large increases in Sporobolus pyramidalis and Chloris gayana linked with a decline in Chrysochloa orientalis (a low-growing grass species) and the build-up of large amounts of plant litter which had the beneficial effect of protecting the eroded hard-pan soils against rainfall run-off. After four years the original grass climax species (Themeda triandra and Heteropogon contortus) had still not recovered to their former levels but were increasing.