REGULATION OF A LARGE HERD OF MIGRATORY CARIBOU - SUMMER NUTRITION AFFECTS CALF GROWTH AND BODY RESERVES OF DARNS

The range used for calving and for the first month of lactation by the Riviere George Caribou Herd (RGH), which peaked at over 600 000 indiv iduals in the mid-eighties, showed signs of overgrazing, in contrast t o that used by the adjacent Riviere aux Feuilles Herd. Density of fema les in the tundra habitat below 600 m asl averaged 11.2 animals/km(2) on the overgrazed range in 1988, in comparison with approximate to 0.5 /km(2) on the other range. Inadequate foraging conditions during the f irst month of lactation caused complete exhaustion of fat reserves in females on the poor range; milk production was seemingly insufficient, as calves were lighter and grew at a slower rate than calves born on the better summer range or born in captivity from dams fed ad libitum. Moreover, females on the poor range were shorter and lighter than the ir counterparts on the good range, illustrating that priority goes to reproduction rather than to growth under suboptimal foraging condition s. Accretion of body fat and protein in RGH females was rapid in autum n, but individuals with a calf at yield accumulated smaller fat reserv es than nonreproductive adult females. Condition did not affect ovulat ion. However, interruption of gestation was suspected in females that were unable to accumulate enough body reserves in autumn or early wint er, judging from the progressive decline in the pregnancy rate and in the autumn cow:calf ratio observed for the RGH since 1984, concomitant with the levelling off of the herd. Summer nutrition seemingly regula ted the RGH through a combination of decreased fecundity and survival.