Pregnancy of adult caribou (Rangifer tarandus): evidence for lactational infertility



To examine the relations between pregnancy, body condition, and lactation, 106 adult female caribou Rangifer tarandus granti) of the Porcupine Herd were captured, weighed, measured, and scored for body condition in November 1990–1992. Blood samples were drawn for determination of pregnancy, and body fat content (kg) was estimated from the product of body weight and the body-condition score. Pregnant females were significantly fatter and heavier than non-pregnant females, and non- lactating females were significantly fatter, but no heavier, than lactating caribou. Probability of pregnancy was positively correlated with body weight and fat content, but prediction of pregnancy was improved when multiple independent variables, including skeletal dimensions and lactation status, were included in logistic models. Non-lactating females were more fertile than lactating females at a particular body weight and metatarsus length; but, at the same fatness, they were not more fertile than females that had lactated through the summer and were ceasing lactation in November. However, females that were extending lactation in November were least fertile, and this reduction of fertility was not explained by differences in body condition or skeletal dimensions. We suggest that the extended lactation group is exhibiting lactational infertility. Finally, unexplained differences in the relation between pregnancy rate and body condition exist among years, suggesting that interannual patterns in the nutrition and ecology of caribou affect the physiological relation between pregnancy and body condition.