By Dr. Karen Shaw Becker
Wallabies are marsupial animals native to Australia. Along with other marsupials like kangaroos and wombats, wallabies have very short pregnancies, giving birth to underdeveloped, embryo-like young that climb into their mother's pouch to complete development. It's often said that marsupials lack a placenta (a key difference separating them from so-called placental mammals, like people), but new research suggests a placenta may be present after all, just in a very unique form: milk.
While "placental mammals," otherwise known as eutherians, have complex placentas made from different cells and tissues, marsupials have a simple placenta made of a few layers of cells, researchers wrote in eLife, adding:
"The simpler placenta means that marsupials give birth to young that are underdeveloped compared to eutherians. These young must develop further inside the mother's pouch, where they are fed with milk that changes over time to support the different stages of their development."1
Wallaby Babies Get Placental Support From Their Mother's Milk
Researchers looked into the development of tammar wallabies, which is a small wallaby that grows up to weigh about 6 to 12 pounds. Their pregnancies last only 26.5 days, supported by a yolk-sac type of placenta, followed by a period of lactation that lasts nearly one year, which is necessary not only to promote growth and development but also to provide immune protection.
What's fascinating is that the milk changes dramatically as the wallaby grows, secreting different proteins and amino acids depending on the stage of development.
The way the milk changes over the course of lactation "is so potent," researchers said, "that early pouch young fostered by a mother lactating for an older pouch young results in accelerated growth and development."2 Also interesting, when researchers analyzed the genetic expression of the wallaby's placenta, they found it had "striking molecular similarity" to eutherians' placenta, such as mice, despite it being quite different.
For instance, during the later days of the wallaby's pregnancy, gene expression patterns in the placenta are similar to those seen in the eutherian placenta during the early stages of pregnancy.3 Their analysis also revealed that, once the baby wallaby finds its way into its mother's pouch (which it does by crawling in all on its own) and latches on, the milk it feeds on acts as its placenta for the next approximately 12 months.
In fact, wallaby moms express placental genes in their milk, the same genes expressed by eutherian placentas late in pregnancy. Senior study author Julie Baker, Ph.D., professor of genetics at Stanford, said in a news release:4
"What we're learning is that the marsupial placenta functions much as it does in eutherians in the very early stages of development, but the expression of later-stage eutherian placental genes instead occurs in the mammary gland. So clearly although the placentas of humans, cows or mice are extraordinarily different from those of marsupials, the animals are fulfilling the same necessary functions in different ways."
Wallabies Begin Climbing in Utero
Part of the mystery of marsupials is how the barely formed babies are able to make the trek — independently — to their mother's pouch. Using high-resolution ultrasound, researchers monitored the short in-utero development of a tammar wallaby, detecting strong, rolling movements in the lining of the uterus, which may enhance the exchange of uterine secretions and gases between the mother and baby.
Further, they observed climbing movements by the fetus about three days before birth, which mimicked those necessary to climb to the pouch.5
"These findings emphasize the remarkable adaptations of the [undeveloped] … wallaby young to survive the journey from the uterus to the pouch and successfully attach to a teat from which it will gain all its nourishment for the next nine months," the researchers noted.6 Once in the pouch, the wallaby baby attaches to its mother's nipple, which swells in its mouth. This makes it so the baby can't let go until later in development.
Young wallabies are known as joeys. When they initially leave the safety of their mother's pouch, they stay close by and return if they feel threatened. In case you were wondering, wallabies are very similar to kangaroos, though smaller, and, like kangaroos, can support three babies in different stages of development all at the same time — one developing in utero, one feeding in the pouch and a joey that lives outside the pouch but still comes back to drink milk.
Many Wallaby Species Are Threatened
There are many different wallaby species, including shrub wallabies, brush wallabies and rock wallabies, which are grouped by habitat, and hare wallabies, named for their hare-like behavior.7 According to the Australian Wildlife Conservancy, tammar wallabies are considered endangered in South Australia, where fewer than 10,000 animals are thought to exist in the wild.
They live primarily in coastal heath and scrub and are threatened by predators (including foxes and cats) as well as habitat loss due to clearing for agriculture. In some areas, the animals are at risk due to being perceived as agricultural pests.8 Further, according to the New South Wales (NSW) Office of Environment & Heritage:9
"Two centuries ago there were 21 species of macropod [kangaroos and wallabies] in NSW, now there are only 15. The smallest species, and those with special habitat requirements and restricted ranges, have suffered the most, both from predators and from the destruction of their habitats. A number of species of kangaroo and wallaby are listed as threatened in NSW."