The Herbivore-Based Trophic System 389 



ship curves have been compared. In reindeer herds with low early mortal- 

 ity, the mean expected life span may be as high as 4 years, but a value of 

 2.5 to 3.0 years has been reported for the Kaminuriak caribou herd west 

 of Hudson Bay (Parker 1972, White et al. 1981). 



Nutrition and Energetics 



During the summer months caribou graze on both graminoids and 

 dicotyledons while moving slowly at 0.5 to 1.2 km hr''. Bouts of grazing 

 are interspersed with periods of rumination during which they lie down. 

 There are four to six grazing periods daily with a high degree of syn- 

 chrony within each group. 



When not harassed by insects caribou spend 48 to 53% of the day 

 grazing. Lactating females spend more time eating and less time search- 

 ing and walking during a grazing period than do adult males, non-lactat- 

 ing females, and yearlings (White et al. 1975, Roby 1978). The activity 

 cycle is highly modified on days of heavy insect harassment when as little 

 as 30% of the day may be spent eating. On days of intense warble fly ac- 

 tivity, trotting and running may take up 25% of the day. From late June 

 through early August caribou suffer insect harassment for up to 25% of 

 the entire period, and attacks may last for over an hour (White and Rus- 

 sell, unpubl. obs.). Thus, grazing periods are often interrupted, and 

 trampling of the vegetation is increased. 



Although the mouth parts of the caribou are large, grazing is selec- 

 tive. Rejected plant parts, particularly dead and coarse material, are ex- 

 pelled from the rear of the mouth and drop back to the tundra almost 

 continuously while the caribou is eating. On summer range, rejection 

 may be as high as 20% of all vascular plants clipped; foraging and 

 trampling may waste considerably larger amounts of lichens (Gaare and 

 Skogland 1975, White and Trudell 1980). When feeding on willow, cari- 

 bou nip leaf parts, buds and some current year's stems, but they exclude 

 older stems and twigs. Dead material of low nutritional value forms 15 to 

 20% of the diet in the Prudhoe Bay region (White et al. 1975), indicating 

 some inefficiency in the selection and sorting processes. 



Some selection of food results from selection of habitat type. Within 

 the vegetation types there is further selection of plant species and parts. 

 Early in the season an obvious preference is shown for the inflorescences 

 of some dicotyledons, e.g. Pedicularis sudetica and Saxifraga spp. Be- 

 cause of their low availability, inflorescences do not make up a large 

 component of the caribou diet, but selective grazing on flowers may be 

 important because of its influence on plant populations. 



Analysis of forage consumed by caribou in the Prudhoe Bay region 

 showed that the dominant plant species in the diet were those that are 



