Antler Loss and Udder Distention in Relation to Parturition in Caribou

Abstract

Abstract: Wildlife biologists commonly assume that retention of antlers and development of distended udders during the calving season are reliable indicators of parturition in female caribou (Rangifer tarandus), but that assumption has not been tested adequately in free-ranging animals. Therefore, I recorded antler retention and udder distention and related those characteristics to parturition in radio-collared females of the Porcupine Caribou Herd in Alaska and Yukon Territory 1983-90 (n = 491). Five percent of cows never grew antlers. Ninety-nine percent of cows with hard antlers at the beginning of the calving season were parturient, whereas 86% of cows that shed antlers were not parturient (P < 0.001). Ninety-six percent of parturient cows developed distended udders 1-21 days before parturition. Cows retained distended udders for 0-27 days after their calves died. I correctly determined parturient status of collared caribou cows with 97% reliability by using simultaneous observations of antler retention, udder distention, and/or presence of a calf at heel in a survey conducted 4-5 days after the beginning of calving. In contrast, I incorrectly determined parturient status in 11.6, 8.2, and 45.1% of the same cows solely on the basis of antlers, udders, or presence of calves, respectively. I found no support for estimating early calf mortality from proportions of cows having distended udders but no calves at heel. Pregnant cows as well as cows with dead calves could have large udders, and parturient cows did not always retain udders after their calves died.