Parturition and perfect prematurity: birth in marsupials

This is a pre-print of an article published in the Australian Journal of Zoology, v.54 (no.3) 2006, reproduced with permission from the CSIRO. The definitive version is available at http://www.publish.csiro.au/nid/91/issue/1435.htmMarsupials are distinguished from eutherian mammals in their mode of reproduction. They give birth to a highly altricial young, which completes its development whilst attached to a teat, usually within a pouch. The marsupial neonate has relatively well-developed digestive, respiratory and circulatory systems but retains its fetal excretory system with a fully functional mesonephric kidney and undifferentiated gonads and genitalia. We have investigated birth in the tammar wallaby (Macropus eugenii) and shown that the tiny (400 mg) fetus determines the time of its own delivery. Although plasma progesterone falls, and oestradiol associated with the postpartum oestrus typically rises, around the time of parturition, neither hormone is essential for the timing of birth. However relaxin may loosen the connective tissue of the cervix and vaginae for birth. Labour starts suddenly and is completed within minutes. Both prostaglandins and mesotocin are essential for the contractions that deliver the young. Prostaglandins from the reproductive tract act via the brain to control parturient behaviour. In the last 2 days of gestation fetal adrenal glucocorticoid production increases, promoting lung maturation and surfactant production and ultimately triggering labour. The accessibility of the altricial neonatal marsupial provides a unique opportunity for experimental manipulation of organ development and maturation