untitled

The gut, for example the rumen, large intestine, and cecum, crops of hummingbirds,

and cheek pouches of heteromyid rodents, has a capacity which is a linear function

of body weight (W1.0) (Clutton-Brock and Harvey 1983; Robbins 1983). Energy require-

ments, however, are a function of metabolic body weight (W0.75). Thus, the difference

between the exponents (W1.0/W0.75=W0.25) means that a larger animal can eat more

food relative to requirement than a smaller animal. This can be expressed in two ways:

(i) on the same quality of diet a larger animal needs to eat less food per unit of body

weight than the smaller; and (ii) a larger animal on a lower nutrient diet can extract

the same amount of nutrient per unit of body weight as a smaller animal on a higher

nutrient diet. Thus larger animals can eat higher fiber diets, a feature that allows resource

partitioning in African ungulates (Bell 1971; Jarman and Sinclair 1979).

Jarman (1974) extended the relationship between body size and diet of African

ungulates to explain interspecific patterns of social and antipredator behavior. We

can identify five categories, from selective browsers to unselective grazers:

1 Small species (3–20 kg), solitary or in pairs, which are highly selective feeders on

flowers, birds, fruits, seed pods, and young shoots. Their habitats are thickets and

forest which provide cover from predators. There is little sexual dimorphism and both

species help in defending a territory. This group includes duikers (Cephalophus species),

suni antelope (Nesotragus moschatus), steinbuck (Raphicerus campestris), dikdik

(Madoqua species), and klipspringer (Oreotragus oreotragus).

2 Small to medium species (20–100 kg) that can be both grazers and browsers, but

are very selective of plant parts as in (1). Habitat is riverine forest, thicket, or dense

woodland. Group size is larger, from two to six, one male and several females. They

are usually territorial and include lesser kudu (Tragelaphus imberbis), bushbuck

(T.scriptus), gerenuk (Litocranius walleri), reedbuck (Reduncaspecies), and oribi

(Ourebia ourebia). There is some sexual dimorphism. Predators are avoided by

hiding and freezing.

3 Medium size species (50–150 kg) that are mixed feeders, changing from grazing

in the rains to browsing in the dry season. Habitats are varied and range from dense

woodland and savanna to open flood plains. There is one male per territory. Female

group size is variable (6–200) and these groups do not defend a territory but

wander through the male territories. Non-territorial male groups are excluded from

territories and behave like female groups. Females have a large home range that is

smaller in the dry season than in the wet season. Species typical of this group include

52 Chapter 4

90

70

50

400 600 800

Energy content of food (J / g)

Rate of intake

Fig. 4.9Relationship
of intake rate to energy
content of food in domestic
sheep. Below an energy
content of 590 J/g, intake
rate falls because of a finite
gut capacity and declining
fermentation rates. Rate of
intake is dry matter/day/
body weight0.75. (After Sibly
1981, which is after Dinius
and Baumgardt 1970.)

4.8 Body size and diet selection