Estimating pregnancy rates and litter size in snowshoe hares using ultrasound

Abstract

Accurate estimates of pregnancy rate and litter size are essential to many studies of popu- lation dynamics. We examined the use of ultrasound for estimating pregnancy rate (the pro- portion of pregnant females in a population) and litter size and for monitoring fetal devel- opment and survival in snowshoe hares (Lepus americanus). We compared ultrasound- based estimates of fetal number in 16 females to either radiographic estimates of fetal num- ber for the same females or to the number of newborn hares counted after parturition. With ultrasound we detected fetuses as small as 4 mm and at least 25 days before parturition. Ultrasound-based estimates of fetal number were correct for 8 hares, low by 1 fetus for 6 hares, and high by 1 fetus for 2 hares. Sequential ultrasound examinations detected change in fetal size and position, and detected ceased or abnormal fetal development in 2 hares. Ultrasonography is an effective, minimally invasive tool to monitor fetal number and preg- nancy rate. Application of the technique to other species would require familiarity with internal anatomy and training in operation of the ultrasound machine.