Estimating pregnancy rates and litter size in snowshoe hares using ultrasound

Accurate estimates of pregnancy rate and litter size are essential to many studies of population dynamics. We examined the use of ultrasound for estimating pregnancy rate (the proportion of pregnant females in a population) and litter size and for monitoring fetal development and survival in snowshoe hares (Lepus americanus). We compared ultrasound-based estimates of fetal number in 16 females to either radiographic estimates of fetal number 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 pregnancy rate. Application of the technique to other species would require familiarity with internal anatomy and training in operation of the ultrasound machine.