This Mom Is Still Pregnant. But She’s Having A Baby!

Should you be fortunate enough to hangout in Kangaroo country you are apt to hear the tale of how the word got into our lexicon. The belief that it means “I don’t understand” is a popular myth that is also applied to any number of other Aboriginal-sounding Australian words.
Search for the drawings and descriptions of Joseph Banks. You’ll be amazed.

The word kangaroo is said to derive from the Guugu Yimidhirr (an Australian Aboriginal language) word gangurru, referring to the Grey Kangaroo. The name was first recorded as kangaru by Joseph Banks on James Cook’s first voyage of exploration, when they were beached at the mouth of the Endeavour River in the harbour of modern Cooktown for almost 7 weeks repairing their ship which had been damaged on the Great Barrier Reef. Kangaroo soon became adopted into standard English where it has come to mean any member of the family of kangaroos and wallabies.
Male kangaroos are called bucks, boomers or jacks; females are does, flyers, or jills and the young are joeys. The collective noun for kangaroos is a mob.

By – Science – NY Times

“Female kangaroos and wallabies are known to use both of their uteruses, but the swamp wallaby uses both at the same time.

Kangaroos and wallabies don’t reproduce the way most of their fellow mammals do — they keep their pregnancies short and to the point, with young crawling out of the womb and up to their mother’s pouch after just a month’s gestation. Once there, the tiny joeys spend about nine months nursing and growing before they’re ready to actually climb out of the pouch into the world.

This is the kind of thing you’ve probably been hearing vaguely on nature programs about Australia for years. But what you might not have heard is that the joey in the pouch is not the only offspring in its mother’s body. Almost all kangaroos and wallabies have two separate uteruses, and they usually contrive to have extra, undeveloped embryos waiting in the wings — or rather, in whichever uterus was unused in their most recent pregnancy. Often they get pregnant again within days of birth, and their bodies keep the new embryo from developing for months at a time, until its older sibling has reached sufficient maturity.

But researchers report in a study published in the Proceedings of the National Academy of Sciences on Monday that the swamp wallaby, a small, dark-furred creature, has an even more peculiar way of doing things. It gets pregnant again before the first pregnancy is even over, suggesting that female swamp wallabies may be pregnant continuously for their entire reproductive lives.

Swamp wallabies are delicate, skittish creatures, said Brandon Menzies, one of the paper’s authors. While researchers had suspected for decades that they were doing something unusual, answers were not forthcoming until he and his co-authors, Thomas Hildebrandt of the Leibniz Institute for Zoo and Wildlife Research in Germany and Marilyn Renfree of the University of Melbourne, managed to use ultrasound scanners on pregnant females.

The researchers monitored 10 pregnancies in the University of Melbourne’s captive wallaby colony. They gently sedated some creatures and scanned their pouches and uteruses, while they peeked into others’ pouches regularly for new young, and they swabbed the females for sperm to pinpoint when mating had occurred.

Based on traces of sperm found in the days before the birth of the first joey, the researchers found that the wallabies’ estrus, or mating period, began before the pregnancy was over. What’s more, in the case of two females who lost their young in the final day or so of gestation, an ultrasound 10 days later showed they had already grown a 12-day embryo in the other uterus. That implied that fertilization had occurred two days before the losses.

“Potentially, these animals are always pregnant,” said Dr. Menzies, with not even a day or two between pregnancies.

There may be good evolutionary reasons to have backup embryos available. In another species the group has studied, the tammar wallaby, the backup’s development is paused to allow its older sibling to grow. The restarting of the embryo’s development is later triggered by changes in day length during the Australian summer, previous research has found. As a result, the second joey, after it is born and has grown in the pouch, will be exiting the pouch around the time that spring grasses burgeon, when there is plenty to eat.

Australia is a place of wild extremes of climate and environment, Dr. Menzies said: “It is boom or bust cycles here, famine or flood.

The ability to control exactly how many offspring one has at a time and replace any lost to the elements very swiftly may be quite useful for kangaroos and wallabies.

But why the swamp wallaby would get pregnant again before the first pregnancy is even over is a puzzler. It may be that having two embryos at once allows the female’s body to assess the differences between two different mates, Dr. Menzies suggested, or there may be some other as-yet-unknown benefit. One answer might lie in their social lives, or lack thereof.

“Swamp wallabies are actually very solitary. Kangaroos hang around in mobs together,” said Dr. Menzies. “We thought, maybe the swamp wallaby is pushing estrus back into pregnancy so it has a longer period of receptivity to find a male in the wild.”

 

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