the acid detergent fiber, or crude fiber, content 
[F, g (100g)'] was noted for higher plants (eq. 
D = 95 — 1.6 F SSS [3] 
A relationship of higher predictive value was 
noted between jn vitro digestibility (D, %) and 
lignin content [L, g (100g)']. The data are 
shown in Fig. 13; a double exponential line (eq. 
4) was fitted to the data, wz. 
D = 32 @ 9-315 jE + 68e -0.0433 L [4] 
Thus, for plants containing less than 4% 
lignin, digestibility declined with lignin content 
at a substantially faster rate than plants contain- 
ing more than 6-8% lignin. Only one higher plant 
sample did not fit this relationship; the predict- 
ed jn vitro digestibility of the sample of Sa/ix 
lanata (61%) was almost twice the observed 
value (34%). No explanation can be given for 
this apparently aberrant result. However, this 
observation may be significant since S. /anata 
was seldom more than a trace constituent of 
esophageal egesta and rumen contents. 
There are many previous studies in domestic 
sheep and cattle which show that the apparent 
dry matter digestibility of herbage depends on 
its degree of lignification. In wildlife studies, 
browse material consumed by herbivores is fre- 
quently high in chemicals (e.g., tannins) which 
limit digestibility (Longhurst et al. 1968). Thus, 
the general relationship between digestibility 
and lignification is not as common for browsers 
as for the temperate grassland grazers. Present 
evidence suggests that digestive processes of 
caribou in the tundra ecosystem at Prudhoe Bay 
may function under principles similar to those 
noted for ruminants in temperate grassland graz- 
ing systems. 
Energy balance and energy flow through the 
Prudhoe Bay caribou population 
(a) Energy content of forage and intake by 
caribou. From estimates of the energy content 
of forages, dry matter intake (Table 15), and dry 
matter digestibility (Table 18, 19), the amount 
of energy harvested by caribou at Prudhoe Bay 
can be calculated. Individual estimates of the 
energy content of sedges, grasses, and shrubs at 
the study site have not been made. However, 
previous studies (West and Meng 1966) show 
that for the months June through August, gross 
179 
energy of most northern species which have 
been studied is between 4.83 and 5.02 kcal g'! 
dry matter. In the present calculations, a mean 
energy content of 4.9 kcal g' was assumed. It 
was also assumed that the metabolizable energy 
content of forage was 82% of digestible energy 
(Blaxter 1962). 
Table 21 shows a summary of assumed and 
calculated composition of late season (late July) 
forage at Prudhoe Bay. Values for the efficiency 
of utilization of net energy for milk synthesis 
and fattening were calculated from the predicted 
metabolizable energy content of the forage as 
proposed by Blaxter (1962). 
Table 21 
Summary of estimated values for the nutrient status 
of herbage harvested by caribou at Prudhoe Bay. 
Dry matter digestibility = 53% 
Energy digestibility = 55% 
Gross energy content = 4.90 kcal g | 
Digestible energy content = 2.69 kcal g! 
Metabolizable energy content = 2.21 kcal g! 
Efficiency of utilization of net energy for 
(a) milk synthesis = 74% 
(b) body growth and fattening = 39% 
Table 15 lists a summary of expected intakes 
of gross and metabolizable energy by lactating 
females, non-lactating females, and adult males. 
It is clear that the intake of metabolizable 
energy per unit metabolic body size by the 
lactating females is considerably higher than in 
the other cohorts. Maximal intakes of gross and 
metabolizable energy were calculated for early 
to mid-July. 
(b) Maintenance energy requirement and 
energy expenditure of caribou. No estimates 
were available of the daily maintenance energy 
requirement of grazing caribou. However, from 
our modeling efforts below (see Modeling activ- 
ities associated with the study of caribou at 
Prudhoe Bay), we can predict a daily heat 
production of approximately 150 [kcal d"' 
(kg°-7°)"'] which is 1.55 times the fasting 
metabolic rate of caribou-97_ [kcal d-! 
(kg?:7®)'] (McEwan 1970). This estimate is 
low compared with the best empirical field 
estimates of the daily maintenance energy re- 
quirements (or heat production) of sheep at 
pasture, which are approximately 2.0 times the 
