Nutritional and ecological determinants of growth and reproduction in Caribou

Abstract

I investigated the mechanisms by which differences in body weight and body composition (fat, protein) of female caribou (Rangifer tarandus granti) from the Central Arctic and Porcupine herds might determine changes in pregnancy rate and calf growth. Allometric relations between chemical components and body weight variables were highly significant, despite tremendous seasonal changes in composition. Between October 1989 and May 1990, body fat and body protein of adult females of the Central Arctic Herd declined by maxima of 45 and 29%, respectively; an additional 32% of fat was lost by July. Extensive mobilization of fat and protein indicates winter undernutrition. Marked hypertrophy of liver and kidneys in summer suggests the presence of mobilizable protein reserves. Birth weights of calves were similar between sexes, but male calves grew relatively faster during summer and were significantly heavier than females in autumn. Both fat content and growth rate of calves declined between 4 and 6 weeks post-calving, perhaps in response to insect harassment. Weight gains of wild calves were greatly reduced or absent after 100 d of age, while captive calves continued to grow until 175 d, suggesting that first-summer growth of caribou is determined in part by nutrient availability. Birth weight and growth rate of wild calves from birth to 3-4 weeks of age accounted for nearly 79% of the variability in autumn weights, again implying summer nutrient limitation. Female caribou were unable to entirely compensate for the metabolic and ecological costs of lactation: in autumn, lactating females had 42% less fat and 9% less protein than nonlactating females. Unlike females from the Central Arctic Herd, those from the Porcupine Herd did not demonstrate compensatory weight gains over summer; instead, autumn weight was highly correlated to June weight. Probability of pregnancy was positively correlated with body weight and fat content in early winter. Females that extended lactation into November were less fertile than predicted by body size or condition. I believe that these females were exhibiting lactational infertility.