Noninvasive monitoring of fetal growth and development in the Siberian polecat (Mustela eversmanni)

The Siberian polecat (Mustela eversmanni) is the preferred species to assess procedures and establish normative values for application in the related and endangered black-footed ferret (Mustela nigripes). This study was undertaken to physically, ultrasonographically, and radiographically evaluate fetal development in a spontaneously breeding captive Siberian polecat population. Ultrasonographically, fetal sac enlargement allowed presumptive preg nancy detection as early as 12 days of gestation, the fetal pole was the first definitive sign of pregnancy at about 18 days of gestation, when the fetal heart beat also appeared, and definitive pregnancy detection by ultrasound was essentially 100% accurate after 18 days. The estimation of fetal number by ultrasound was less reliable than by radiography, as it is in other litter-bearing species. Crown-rump growth, organ differentiation, and calcification patterns resembled those of domestic carnivores except that comparable developmental stages in polecats occurred at dispro portionately later times, suggesting that young Siberian polecats are delivered in a less developed state. Careful palpation permitted detection of pregnancy after day 17 but with less certainty than with ultrasound. Radiographic evaluation was insensitive and of limited value for pregnancy detection until near term. Litter number and fetal detail were difficult to assess until ossification could be observed, 3-6 days before parturition.