Development of fetal Eurasian beavers (Castor fiber)

Abstract

Mammals are categorized along an altricial-precocial spectrum based on degrees of neonatal development. Altricial neonates are immature at birth and dependent on care, whereas precocial neonates are relatively well developed and independent post-partum. The Eurasian beaver (Castor fiber) is a large, semiaquatic, relatively precocial mammal in the order Rodentia. However, the extended care of kits in burrows begs the question of why beavers have precocial characteristics. A study of fetal development might enable understanding of the traits that are significant for neonatal survival. The present study examined external morphological characteristics, external biometry and the development of internal organs in 80 Eurasian beaver fetuses (crown-rump length [CRL], 2.5–19.3 cm) derived from 33 wild pregnant females hunted in Norway (33 gestational age). We also created equations to predict fetal ages from biometric parameters based on a linear relationship between biometric parameters and fetal ages estimated from fetal weight. Fetal features appeared in the following chronological order; eyelid bulbs and tactile hairs (CRL ≥ 7.3 cm), outer ears, claws, skin differentiation (CRL ≥ 8.9 cm), and hair coverage and tooth eruption (CRL ≥ 9.4 cm). All biometric parameters increased linearly and positively as the CRL increased and did not tend to stabilize. Notably, slopes of hindfoot parameters were higher than those of forefeet, suggesting that the hindfeet of beavers grow faster than forefeet, as they do in other semi-aquatic mammals. Our calculations revealed that dorsal length, CRL, thoracic and abdominal circumferences were candidate predictors of fetal age due to a high coefficient of determination (≥ 0.95) and broad range (≥ 10 cm). Our findings revealed that beaver growth was precocial in terms of differentiated skin, hair coverage, and tooth eruption, but not fully completed, as none of the fetuses had open eyes. The rapid growth of the hindfoot and some precocial traits (skin and hair differentiation) may be involved with the ability of neonatal beavers to move in water.