Tummy pouches as child-rearing tools are the bailiwick of all marsupials species — from opossums to koalas to bandicoots. The most famous of the mamas wielding baby holsters, however, is the kangaroo.
Kangaroos are native to Australia and New Guinea; the kangaroo is so archetypally Australian that it graces the country's coat of arms. One of the first things any human toddler with an alphabet board book learns about kangaroos is that the name starts with a "K" and that a small kangaroo head often sticks out of the adult kangaroo's abdomen.
Well, both of those things are true enough, but there is some context to consider. First of all, kangaroos don't have two heads, one at belly button level and one on top of their shoulders; the mother kangaroo sports a big pocket essential to the nursing, protection and ferrying around of her baby, called a joey. Secondly, not all kangaroos have a pouch — just the females, who do 99 percent of the child-rearing. Kangaroos live in large groups called mobs, and the males contribute by acting as bodyguards for the moms and babies.
Nothing Like a Nice Warm Pouch
Kangaroo pouches function exclusively as baby containers. They are very warm inside — about 105 degrees Fahrenheit (40.5 degrees Celsius) — and contain four nipples and two milk ducts on each side. They're expandable and can make space for two joeys of different ages at the same time, as joeys stay with their mothers for about five months, and a female can give birth to up to four joeys per year.
"Normally a kangaroo's pouch is dry, lightly furred and has a dry, flaky, rusty/brown colored substance that is a naturally occurring anti-fungal and anti-bacterial substance," says Katrina McCauley, assistant curator in the Columbus Zoo and Aquarium's Australia and the Islands region.
The pouch is also essential for joey gestation. The mother kangaroo gives birth vaginally to an unbelievably small and underdeveloped baby, after gestating only about 33 days. The baby is pink, entirely hairless and only about the size of the nail on your pinky finger, and it crawls up its mother's stomach and into her pouch, where it will remain for between four-and-a-half and five months.
"When joeys are first born they attach to one of the teats and fuse to it until they are more developed," says McCauley. "Kangaroo females produce different milk compositions as their joeys age, plus the females can produce two different compositions of milk based on the ages of her young. A female can have a joey 'at foot', a younger joey in the pouch and hold a blastocyst in embryonic diapause [hold a dormant fertilized egg in waiting] after mating."
A female kangaroo is extremely efficient when it comes to reproduction. She can control when she gets pregnant, and needs only mate once a year, fertilizing up to four eggs in one go. One of the eggs can implant, while the other three stand by and wait their turn. During times of drought or low food supply, the dormant eggs can bide their time until the mother decides conditions are favorable enough to raise another joey, and allows one to implant.
What About Housekeeping?
Babies do everything in the pouch, including use the bathroom. That's a lot of action for one marsupium — the technical name for a kangaroo's pouch — to endure, and it takes a bit of housekeeping to keep it fresh and clean.
"Kangaroos clean their pouch, but I personally have not noticed this on a regular basis," says McCauley. "When a kangaroo is preparing to give birth, we see an increase in females tending to their pouch. We also see an increase in cleaning once a joey is developed enough to come out of the pouch. In general, we see them clean it when they are standing, by sticking their snout into the pouch."