Dunnart neonates actively climb to the pouch after birth

Abstract How tiny neonatal marsupials make their way to the pouch after birth was debated in the scientific world until at least the 1700s, when the reproductive anatomy of the opossum was clearly described. While details of the crawling process are now prevalent for a few marsupials, including diprotodontids, dasyurids and peramelids, there are few observations in other species, many of which are small or highly endangered. Here, we describe the crawling movements of young from the fat-tailed dunnart (Sminthopsis crassicaudata) during a chance observation of birth in our research colony. Incredibly, the tiny young in this genus appear to actively crawl to the pouch and locate the teats even though they weigh approximately 5 mg at birth and are among the smallest neonates of any mammal. This is very different from quolls, a species within the same family as the dunnart, in which the young swim through a column of birth fluid to traverse the region between the urogenital sinus and the pouch, and more similar to kangaroos, which are at least 100 times larger at birth. The observation suggests that the act of crawling is an important fitness test for dunnart young that provides an initial test of post-natal competency.