Marsupials represent approximately 6% of all mammal species and include iconic pouched mammals like the kangaroo, wombat and koala. Compared to eutherian mammals like the dog, cow and human, marsupials have ultra-short pregnancies and give birth to very immature, almost embryonic, young that complete most of their development attached to the teat, usually within a pouch.Even kangaroos, the largest extant marsupials, give birth to a newborn that weighs less than one gram and must climb blind and unaided from the birth canal to the pouch. 

Using high-resolution ultrasound, researchers have visualized the short pregnancy of a small species of the kangaroo and wallaby family of marsupials, the tammar wallaby (Macropus eugenii) and found specialized movements of the endometrium that roll the embryo around the uterus prior to attachment, and climbing movements of the tiny fetus up to three days before birth.

This latter finding is one of the earliest developmental behaviors observed in a mammal and prepares the immature young for the journey to its mother’s pouch.

The tammar wallaby is one of the smaller species within the genus Macropus, which includes 14 species of kangaroos and wallabies, of which all but one species have the ability to halt the development of an embryo (embryonic diapause). It is a model species for understanding marsupial reproduction and evolution and its biology, especially its embryology and pre-natal development, is very well described.

Tammar embryos in embryonic diapause can be reactivated during the breeding season (Jan-April) by removal of the sucking pouch young, which results in birth approximately 26 days later. While the hormonal and morphological changes surrounding this process have been intensively studied, it was not known how the marsupial embryo and fetus interacts with the mother in vivo. With this objective in mind, scientists from the Leibniz Institute forZoo and Wildlife Research (IZW) in Berlin, Germany, the University of Melbourne and the University of Sydney in Australia examined the prenatal development of tammar wallabies using high-resolution ultrasonography.

Wallabies give birth to very immature, almost embryonic, young that complete most of their development attached to the teat, usually within a pouch. Photo: Kathleen Röllig

The short intra-uterine development of the tammar was monitored from reactivation of the 100-cell blastocyst in diapause to birth. After reactivation, the blastocyst could be visualized upon reaching a diameter of 1.5 mm. From at least halfway through pregnancy, there were strong rolling movements of the endometrium that massaged the expanding vesicle against the highly secretory endometrial surface. These unique movements possibly enhance exchange of uterine secretions and gases between the mother and embryo. This finding was surprising since in eutherian pregnancies uterine movements are usually suppressed so as not to disrupt implantation.

Once the reactivated embryos were detected by ultrasound, their developmental progress was constant until birth. Thus, blastocysts in diapause that reactivated first were born first and the birthing sequence amongst mothers could be predicted from the size and stage of their embryos.

Interestingly, the tiny tammar fetus displayed preparative climbing movements up to 3 days before birth that appeared as alternating grasping actions of the left and right forelimbs mimicking those required to climb to the pouch. The fetal forelimbs are well developed at this stage whereas the hindlimbs are still only buds. In eutherian mammals such highly coordinated movements occur much later during development.

The study has highlighted some fundamental adaptations of marsupials that are specific to their unique mode of reproduction. Future work will determine how widespread these adaptations may be within the Marsupialia.