The milk that a mother tammar wallaby provides its joey is more than meets the eye. Credit: Dave Watts/NPL

Wallabies are kicking over scientific conventions surrounding mammalian placentas, the organ responsible for protecting and nourishing a developing fetus. A study1 finds that contrary to what scientists thought previously, mother tammar wallabies (Macropus eugenii) have both a functioning internal placenta and milk that performs some of the organ’s functions.

Taxonomists usually separate marsupials — including kangaroos, wallabies and wombats — from placental mammals, also known as eutherians, such as mice and people. The separation is based partly on a perceived lack of a placenta in marsupials. But some researchers think that this distinction is incorrect, noting that marsupials develop simple, placenta-like structures during the end of pregnancy, just before the underdeveloped baby crawls from the uterus into the mother’s pouch. These structures, just two cell layers thick, provide oxygen and nutrients to the fetus while protecting it from the mother’s immune system.

Marsupial pregnancy is remarkably short for a mammal. Tammar wallabies are pregnant for just 26 days — barely longer than rats. Yet the baby, or joey, spends nearly a year continuing to develop and nurse inside the mother’s pouch: a long time compared to other mammals. This developmental mismatch led researchers to suspect that the majority of the baby’s development was driven by specialized features of the mother’s milk. The current study suggests that the milk consumed by the joey acts as a kind of late-stage placenta.

Mother's milk

To determine whether the marsupial placenta functions like a mammalian one, evolutionary biologist Julie Baker and evolutionary developmental biologist Michael Guernsey at Stanford University in California, analysed the collection of genes expressed from the tammar wallaby’s placenta and compared it to those of mice and humans. They found that in the final days before the foetus is born, the tissue expressed the same genes as eutherian placentas do in the early stages of fetal development.

The researchers then analysed the genes expressed in the mammary glands of tammar wallabies that were nursing joeys. They found that this tissue expressed the same genes as eutherian placentas do in late fetal development. The mammary tissue also permanently shut off one copy of a gene called Igf2 through a process called imprinting, which has never before been seen in any tissue other than the embryo and placenta.

“This is beautiful work,” says Anthony Carter, a developmental biologist at the University of Southern Denmark in Odense. He says it provides a convincing argument that marsupials, contrary to common belief, have fully functioning placentas.

The finding suggests that placentas across the animal kingdom could express the same suite of genes, which is surprising given that even closely related species’ placentas may look very different anatomically, says Guernsey. Those differences, however, could be because the placenta evolves rapidly compared with other organs.

Baker thinks the rapid evolution could be due to the fact that the placenta shields the fetus from the mother’s immune system, which treats it as a foreign invader. “The placenta is evolving trying to evade the mom, and comes up with these really bizarre strategies”, including taking liquid form in the mother marsupial’s milk, she says.

The finding also shows that placentas could be even more diverse within the animal kingdom than previously thought. Knowing more about its development could not only help researchers understand animal evolution but also the possible functions of the human placenta, which is impossible to study in real time since it would be supporting a foetus.