388 G. O. Batzli et al. 



year and estimates based on total available area may give a better indica- 

 tion of the long-term population levels. The average resident caribou 

 density in the Prudhoe Bay region was similar to recent estimates of the 

 density of the Porcupine herd in 1972 (0.004 caribou ha''; Calef and Lor- 

 tie 1973, Calef 1978), and the eastern Canadian Kaminuriak herd in 1973 

 (0.002 caribou ha"'; Parker 1972). These densities seem small, but the 

 biomass they represent in the region is greater than that of lemmings, 

 particularly when large numbers of migratory caribou move into the area 

 (Table 10-3). 



In response to photoperiod, breeding activity commences with ag- 

 gressive behavioral displays between adult males in mid- to late Septem- 

 ber. Peak rutting occurs in late October to early November (Kelsall 1968, 

 Whitehead and McEwan 1973). Estrus in female caribou begins in late 

 September, and estrus cycles recur at 10-day intervals (McEwan and 

 Whitehead 1972). The date of peak calving varies from year to year, 

 which suggests that secondary factors such as nutrition and climate may 

 modify the timing of both rut and parturition. Gestation lasts about 210 

 days. The effects of winter nutrition on the gestation period are uncer- 

 tain; however, nutrition can affect birth weights of calves and milk pro- 

 duction in lactating females (Skjenneberg, pers. comm.. White and Lu- 

 ick, unpubl. obs.). Normally one calf is born; twins are rare. 



The age at which caribou first breed varies (Kelsall 1968). Female 

 calves rarely breed, and frequently females are as old as 3'/2 years when 

 they breed for the first time. Under good nutritional conditions up to 

 30% of females will conceive as yearlings, and caribou older than 4 years 

 have peak pregnancy rates of 78 to 90% (Kelsall 1968). Female caribou 

 breed until they are at least 16 years of age with little decline in fertility. 



Mature female caribou generally breed annually. However, when se- 

 verely undernourished some females do not come into estrus, and lacta- 

 tion continues through January (Reimers, pers. comm.). Under these cir- 

 cumstances breeding in alternate years would be expected. Disease, as 

 well as nutrition, may affect fertility. For example, brucellosis probably 

 caused lowered pregnancy rates in the Western Arctic herd in 1961 (Lent 

 1966). 



In the Prudhoe Bay region, when calves were 4 to 6 weeks of age, 

 67% of non-yearling females had calves with them in 1972 and 31% had 

 calves in 1973. Few caribou were observed in 1973, and the estimate may 

 not be representative (White et al. 1975). The estimated number of fe- 

 males with calves in the Porcupine herd to the east of Prudhoe Bay was 

 50% for 1972 (Calef and Lortie 1973). Caribou calves have a high mor- 

 tality rate, which can be attributed to inclement weather, predation and 

 accidents. By the end of the first year the cohort has normally been 

 reduced 40 to 50% (Kelsall 1968, Parker 1972). Lack of data on age- 

 specific mortality precludes the construction of life tables, but survivor- 



