Methods of studying the food habits of some wild ungulates in Uganda

Digestion studies in captive Hippopotamidae: a group of large ungulates with an unusually low metabolic rate

We performed intake and digestibility studies in four common (Hippopotamus amphibius) and four pygmy (Hexaprotodon liberiensis) hippos from two zoological institutions, using acid detergent lignin as an internal marker for the quantification of faecal output. In the case of one pygmy hippo, where total faecal collection was also possible, there was no distinct difference between the two methods of faecal output quantification. Two animals from each species were tested on a conventional zoo diet of hay and concentrates (diet HC) and on hay only (diet H). The other two animals received fresh grass at two different levels of intake (diets G1 and G2). Dry matter (DM) intake was higher on HC than on H or G diets, and averaged 37 +/- 11 for common and 35 +/- 14 g/kg(0.75) for pygmy hippos. There were no species differences in the average digestibility (aD) coefficients. Non-dietary faecal nitrogen averaged 65 +/- 4% of total faecal nitrogen, aD of crude protein (CP) averaged 67 +/- 9% and true protein digestibility 89 +/- 3%. Average digestibility of DM and crude fibre averaged 54 +/- 11% and 45 +/- 17%, respectively. In comparison with ruminants, hippos generally achieve lower aD for DM, organic matter and fibre parameters, but equal or higher aD CP coefficients. This is most likely due to the absence of significant fermentative activity in the hindgut and the corresponding low metabolic faecal nitrogen losses. Digestible energy intake was higher on HC than on H or G diets and averaged 0.30 +/- 0.11 MJ/kg(0.75) metabolic body mass. This value is extremely low for ungulates, supporting earlier suspicions that hippos have particularly low metabolic rates, and explains the proneness of this species to obesity in captivity when fed energy-dense pelleted feeds.

Intake, ingesta retention, particle size distribution and digestibility in the hippopotamidae

Although several aspects of the digestive physiology of the hippopotamidae-non-ruminating foregut fermenters-have been described, ingesta kinetics and passage characteristics of these species are not well understood. The most outstanding feature of the hippo digestive physiology reported so far is the very long mean ingesta retention times (MRTs) measured by Foose [Foose, T., 1982. Trophic strategies of ruminant versus nonruminant ungulates. PhD dissertation, University of Chicago, Chicago.]. Since those data had been investigated with animals without water access, we intended to measure MRT in hippos which were allowed to enter water pools during the night. MRT parameters as well as dry matter (DM) digestibility were determined in four common (Hippopotamus amphibius) and four pygmy hippos (Hexaprotodon liberiensis) on two different diets each using cobalt ethylendiamintetraacetate (Co-EDTA) as a fluid, chromium (Cr)-mordanted fibre (<2 mm) as a particle and acid detergent lignin (ADL) as an internal digestibility marker. Four of the animals additionally received cerium (Ce)-mordanted fibres (2-10 mm) as particle markers. Total MRTs for fluids and particles ranged between 20-35 and 48-106 h in the common and between 13-39 and 32-107 h in the pygmy hippos. The difference between fluid and particle retention was greater than usually reported in ruminants. Excretion patterns of the markers differed from those usually observed in ruminants but resembled those reported for macropods (kangaroos), indicating a plug-flow reactor-like physiology in the hippo forestomach (FRST). This finding complements other described similarities between the macropod and the hippo forestomach. The measurements of larger particle retention profiles suggest that in the hippo, larger particles might be excreted either faster or at the same rate as smaller particles, indicating a general difference between ruminants and hippos with respect to differential particle retention. The digestive physiology of hippos is characterised by a generally low food intake, long ingesta retention times and dry matter digestibilities lower than reported in ruminants. Moderate digestibilities in spite of long retention times might be the result of the generally high average ingesta particle size in hippos. The comparatively easy management of pygmy hippos, together with the significant correlations between food intake, MRT and digestibility in the pygmy hippos of this study, recommends this species for further studies on the interplay of these parameters in herbivore digestive physiology.

Using stable isotope analyses to identify allochthonous inputs to Lake Naivasha mediated via the hippopotamus gut

The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the ingested material must therefore be defecated directly into the aquatic system and can thus be considered an allochthonous resource available to aquatic consumers. The utility of stable isotope analyses of carbon and nitrogen to distinguish hippo faecal matter from other potential basal resources was tested at Lake Naivasha, Kenya. Hippopotami proved faithful to a short grass diet although supplementary grazing of aquatic macrophytes was observed. The typical isotopic ratios of C4 grasses ingested were not altered substantially by gut processes, and were clearly distinct from algal and aquatic macrophyte isotopic ratios. However, marginal plants such as Cyperus papyrus exhibit C4 ratios, and so the technique is suitable only for use in localities where 'contamination' from such sources is negligible.