180 
fasting metabolic rate (Young and Corbett 
1972). Therefore, we have assumed a value of 
190 [kcal d!' (kg®-7°)'] (or 1.97 x fasting 
metabolism) for caribou. The maintenance 
energy requirement, or daily metabolic rate, of 
caribou would be higher under conditions of 
insect harassment; hence the following estimates 
should approach favorable grazing conditions. 
For lactating animals, the amount of energy 
secreted in milk and associated with milk syn- 
thesis (i.e., the efficiency of milk synthesis), 
was also estimated as a component of the main- 
tenance energy requirement of lactating females. 
Estimates of the amount of milk synthesized 
were taken from estimates for reindeer given by 
equation 1 (White, Holleman, and Luick, unpub. 
obs.), and an efficiency of 74% for synthesis was 
assumed (Blaxter 1962). The predicted main- 
tenance energy requirements as shown in Table 
5: 
(c) Net energy and predicted body-weight 
gain. The amount of energy available for synthe- 
sis of body tissue (i.e., net energy) was estimated 
as follows: 
Net energy = Metabolizable energy intake— 
Maintenance energy requirement——————— [5] 
Calculations from the present data suggest that 
lactating females were in positive energy balance 
for only periods 3 and 4 (11-31 June, approxi- 
mately). For the non-lactating female and adult 
male segments of the population, a positive 
energy balance was noted for periods 1-5 (i.e., 
for the entire 50 days). 
If it is assumed that net energy is used in the 
proportion of 80% for fattening and 20% for 
growth, then tissue would be synthesized at 
between 77 and 173 g d'', which amounts to 
cumulative body weight gains of 1.9, 5.3, and 
4.2 kg for the lactating female, non-lactating 
female, and the male segments, respectively. 
It was found in this study that the digestibil- 
ity of the diet was an important determinant of 
the amount of food retained for fattening. For 
example, in the non-lactating female cohort, if 
the digestibility of the diet is increased by 10 
units from 55 to 65% (i.e., by 18%), the amount 
of energy retained daily for fattening increased 
from 467 to 1,402 kcal (i.e., by 200%). This 
effect has previously been reported for domestic 
animals (Blaxter 1962); the powerful multiplier 
effect highlights the requirement for an accu- 
rately determined forage digestibility. 
Modeling activities associated with caribou 
studies at Prudhoe Bay 
It is clear from estimates of food intake, 
energy balance, and energy flow that many 
variables influence the final results..In the above 
calculations, it was necessary to assume values 
for variables (e.g., time spent grazing, eating 
rate, digestibility, caribou biomass) based on 
limited empirical estimates. It was not possible 
within the constraints of time to investigate the 
sensitivity of the calculated end product (e.g., 
food intake and body growth) to small changes 
in the magnitude of these variables. To investi- 
gate these limitations, a modeling effort was 
initiated in January 1973. The objectives of the 
modeling activities were to: 
(a) determine the average daily metabolic 
rate, or heat production, of caribou cohorts 
based on (i) estimates of energy costs of activ- 
ities such as standing, walking, grazing, etc.; (ii) 
time budgets of these activities, and (iii) changes 
in behavioral activities in response to abiotic and 
biotic variables. This submodel was termed 
ACTIVE. 
(b) determine daily amounts of forage con- 
sumption and plant community/species selec- 
tion, apparent digestibility, and metabolizable 
energy intake based on (i) estimates of eating 
rate as a function of plant live biomass; (ii) 
eating time as a basis of rumen fill, (iii) daily 
food intake as a function of eating rate and 
eating time, (iv) plant selection based on a 
matrix of chemical composition and biomass of 
plant types, and (v) digestibility and metaboliz- 
ability of forage based on its chemical composi- 
tion (% lignin). This model was termed GRAZE, 
and 
(c) interface models ACTIVE and GRAZE 
to predict net energy available for growth 
and fattening of both adult and juvenile co- 
horts. Growth and fattening were calculated 
based on (i) estimates of daily net energy avail- 
able for growth and fattening (i.e., metaboliz- 
able energy intake — average daily metabolic 
rate = net energy), and (ii) efficiency of growth 
and fattening based on the metabolizability of 
forage and milk. This interface model was term- 
ed GROWTH. 
