The Feeding Behaviour of Grazing African Ungulates

Abstract

The second to second organization of the foraging behaviour of eleven species of African ungulates is described, with particular emphasis on locomotion while foraging and on the time spent feeding. It was predicted that foraging behaviour should change with the species' body size and stomach specialization and, within a species, with the seasonal or spatial changes in the quality and availability of the food supply. A feeding site was defined as the area which a feeding animal could reach without moving either of its forefeet. Feeding subjects thus encountered a new site every time they took a step with either forefoot and foraging behaviour could be described in terms of the organization of a series of behavioural events (steps) and bouts of activity (feeding). Five indices were used to summarize all records obtained. Only one of these, the proportion of time spent feeding, was significantly correlated with the species' body size. The other four indices include: the rate of movement (in steps per second), the mean feeding time per step, the proportion of sites encountered which carried at least some acceptable food, and the mean time spent feeding from those accepted sites. These last four indices were apparently more strongly affected by the species' stomach specialization than by its body size. The five species with the largest sample sizes (reedbuck, impala, tsessebe, wildebeeste, buffalo) were used to investigate the tendency found within all study species for foraging behaviour to vary seasonally. Firstly, both the mean feeding time per step and the mean feeding time per accepted site were found to be positively correlated with indicators of vegetation bulk and maturity, but negatively correlated with an index of fresh green growth, for all five species: it is suggested that this is due to changes in both the amount of food taken from a given site and in the time needed to ingest a given bulk of food. Secondly, the organization of events within foraging sessions (described in terms of the frequency distributions of step intervals, feeding durations, and the lengths of runs of acceptance or rejection of feeding sites) varied through the year: in particular, food patch size, indicated by the length of sequences of accepted sites, was apparently largest just before the end of the rains, when the vegetation should have been most uniform in quality. Thirdly, the reedbuck is a specialist grazer which, because of its body size, might be expected to feed very selectively within the grass layer: the feeding behaviours of the other four species most closely resembled that of the reedbuck when these four species were feeding off long grass in the dry season, conditions which presented them with much low quality food senescent grass hampering access to very small amounts of higher quality green matter. It is suggested that, as in domestic ungulates, feeding behaviour varies within a species with the proportion of low fibre, high protein, green growth in the vegetation, and in the contrast in quality between the various plant parts. A multivariate analysis was used to identify the foraging characteristics of individual ruminant species. Species which were specialist grass feeders (bulk/roughage feeders) encountered more sites more predictably, and spent more time feeding off those sites, than did species which were known to switch from grasses to other food sources to take advantage of changes in the relative quantity and abundance of food types in the habitat (intermediate feeders). It is suggested that, since pure grazers' foraging is limited to a fairly continuously distributed food supply, their foraging consists mainly of teasing out and biting off grass leaves and the organization of their foraging behaviour is determined by the structure and quality of the grass sward. Intermediate feeders, on the other hand, had the option of taking higher quality but less continuously distributed items, such as fruits, and their foraging may have involved seeking out and moving between such items. Pure grazers' foraging behaviour is thus seen as being dominated by food capture and handling events, while intermediate feeders may be more strongly influenced by food search or pursuit requirements. It is likely that grazers differed from intermediate feeders not only in the basic organization of their foraging behaviour, but also in the way that this organization was affected by the species' body size.