Physiological and ecological determinants of nutrient partitioning in caribou and reindeer

Abstract

The effects of season, migration, and reproduction on the adipose and protein dynamics of barren-ground caribou were determined from field collections of adult females from the Porcupine Herd. Radio-collared females recaptured over time provided data on animals of known reproductive status. Pregnant females averaged a daily loss of 50g body fat and 15g body protein during the last 60 days of gestation. Between June and September, lactating females preferentially deposited body protein but non-lactating females preferentially deposited body fat. In both cohorts, fat deposition increased relative to protein deposition in fall, but maximum fat deposition occurred in summer. Females that conceived averaged 220% more body fat and 17% more body protein than females that did not conceive. Fetal and birth weight positively correlated with maternal protein reserves, but not with maternal fat reserves. Fieldwork on free-ranging caribou were complemented with nutritional experiments on captive animals to determine the effects of energy intake, protein intake, the dietary protein:energy ratio, date, and body condition on nutrient partitioning between fat and protein deposition, and between maternal tissue deposition and milk production. In both lactating and non-lactating females, the proportion of tissue deposited as fat rather than protein increased between spring and fall but decreased with increasing fatness. Lactating and non-lactating females had comparable efficiency coefficients for net energy retention (60% and 65% respectively), but daily maintenance requirement for lactating females (456 KJ/BW$\sp{0.75}$) was twice that for non-lactating individuals (233 KJ/BW$\sp{0.75}$). Energy intake increased protein deposition in lactating females but increased fat deposition in non-lactating females. Production of milk dry matter, fat, and energy were unaffected by maternal energy intake, maternal protein intake, maternal body condition, or calf age. However, production of milk lactose correlated with maternal energy intake, while production of milk protein correlated with the maternal dietary protein:energy ratio. Prediction equations for body weight and composition of barren-ground caribou were developed using bone, muscle, fat, and organ indices. Prediction equations for body weight were validated with an independent data set.