Kangaroos and other marsupials: Climb to the pouch begins in utero

Marsupials represent approximately 6% of all mammal species and include iconic pouched mammals of Australia such as the kangaroo, wombat and koala. Compared to eutherian mammals, which include more familiar animals such as 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. Kangaroos represent the largest extant marsupials and also give birth to the largest young. However, the newborn kangaroo still weighs less than one gram at birth and must climb blind and unaided from the birth canal to the 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.

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 visualised 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.