Some animals pause their own pregnancies, but how they do it is still a mystery



Tammar wallabies are one of many species that can pause their pregnancies until the time is right.
LCAT Productions / Shutterstock

Jane Fenelon, University of Melbourne

Putting your pregnancy on pause until the time is right to give birth sounds like something out of a sci-fi novel, but for many mammals what’s known as “embryonic diapause” is an essential part of raising their young.

Although scientists have known since the 1850s that some animals have this ability, it is only now becoming clear how it could teach us valuable lessons about human pregnancy, stem cells, and cancer.

Which animals can do this?

More than 130 species of mammal can pause their pregnancies. The pause can last anywhere between a couple of days and 11 months. In most species (except some bats, who do it a little later) this happens when the embryo is a tiny ball of about 80 cells, before it attaches to the uterus.

It’s not just a single group of mammals, either. Various species seem to have developed the ability as needed to reproduce more successfully. Most carnivores can pause their pregnancies, including all bears and most seals, but so can many rodents, deer, armadillos, and anteaters.




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More than a third of the species that take a breather during gestation are from Australia, including some possums and all but three species of kangaroo and wallaby.

The record-holder for pregnancy pause time is the tammar wallaby, which has been studied extensively for its ability to put embryos on hold for up to 11 months.

Why pause pregnancy?

The main advantage to pausing pregnancy is that it separates mating and birth. There are two main ways in which animals do this.

The first way is to mate soon after giving birth, to have a backup pregnancy in case something happens to the newborn young. The stress of lactating triggers a pause that lasts during suckling, and the pregnancy restarts once the young leave.

The second way is to pause every pregnancy until the time is right (usually depending on the season). For example, minks mate around the start of March but put the embryos on pause until after the spring equinox (March 21), when the days are growing longer in their northern hemisphere homes. This ensures that the young are born in spring when conditions improve, and not in winter.

The tammar wallaby combines these two methods (suckling in the first half of the year, short days in the second) to pause for almost a year and give birth in January. This ensures the young leave the pouch the following spring instead of in the middle of a hot Australian summer.

What can we learn from diapause?

Diapause was first identified in 1854 after hunters in Europe noticed that pregnancy in roe deer seemed to last a lot longer than normal. Since then scientists have been fascinated by this process and it has helped us understand more about basic reproductive processes in all mammals.

But it took until 1950 before our knowledge of pregnancy had increased enough so that we could confirm what the hunters had observed 100 years earlier.

But how the process worked at the molecular level is still a mystery. Until recently, there seemed to be no connection between which species used it and which didn’t and there didn’t seem to be a unifying mechanism for how pregnancy was paused. Even the hormones controlling diapause are different between mammal groups.

However, research now suggests that regardless of what hormones affect the uterus, the molecular signalling between the uterus and the embryo is conserved, at least between the mouse, mink and tammar wallaby.

Furthermore, researchers in Poland paused embryos from sheep (a non-diapause species) by transferring them into a mouse uterus and then back into the sheep with no ill effects.

This indicates the potential for diapause could lie in all mammals, including humans.

So when can I pause my pregnancy?

It’s unlikely that pausing pregnancy will become the norm in humans. For starters, you’d have to know you were pregnant within five days of conceiving to match the time when most species start diapause.

Understanding how mammals pause their pregnancies does have significant implications for our understanding of how to make healthy embryos. The time when the embryo enters into diapause is the same time in IVF when an embryo is transferred into the uterus. Diapause could help us improve how we grow embryos in culture or how to recognise which is the “best” embryo to transfer.




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Diapause could also help create better stem cells and find new cancer treatments. The first stem cells ever isolated by scientists came from a mouse embryo in diapause, when the cell cycle of the embryo is arrested. Stem cells are also remarkably similar to a diapaused embryo.

So understanding how diapause works at the molecular level could lead to new therapies to halt cell division or to identify markers for tumour stem cells, which are thought to be responsible for metastasis in cancer.The Conversation

Jane Fenelon, Research fellow in monotreme and marsupial reproduction and development, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

New research could lead to a pregnancy test for endangered marsupials



Knew you were coming: a koala cub on the back of the mother.
Shutterstock/PARFENOV

Oliver Griffith, University of Melbourne

Many women realise they are pregnant before they’ve even done the test – perhaps feeling a touch of nausea, or tender, larger-than-usual breasts.

For a long time, biologists had thought most marsupials lacked a way to recognise a pregnancy.

But new research published today shows a marsupial mum knows – in a biological sense – when she’s carrying a young one before they make their journey to the pouch.




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This knowledge changes how we think pregnancy evolved in mammals. It may also help in breeding programs for threatened or endangered marsupials by contributing to new technologies such as a marsupial pregnancy test.

Marsupials do things differently

When people think of marsupials – animals that mostly rear their young in a pouch (although not all marsupials have a pouch) – kangaroos and koalas tend to spring to mind. But marsupials come in a range of shapes and sizes.

A red-necked wallaby with a joey.
Pixabay/sandid

Australia has about 250 species of marsupials, including wombats, possums, sugar gliders, the extinct Tasmanian tiger, and several endangered species such as the Tasmanian devil.

In addition to Australia’s marsupial diversity, there are also 120 marsupial species in South America – most of which are opossums – and just one species in North America, the Virginia opossum.

One thing all marsupials have in common is they give birth to very small, almost embryonic, young.

An opossum with two day old young.
Oliver Griffith, Author provided

Because marsupial pregnancy passes so quickly (12-40 days, depending on the species), and marsupial young are so small and underdeveloped at birth, biologists had thought the biological changes required to support the fetus through a pregnancy happened as a follow on from releasing an egg (ovulation), rather than a response to the presence of a fetus.

Marsupial pregnancy is quick

One way to explore the question of whether it is an egg or a fetus that tells the marsupial female to be ready for pregnancy is to look at the uterus and the placenta.

In marsupials, just like in humans, embryos develop inside the uterus where they are nourished by a placenta.

Previously, biologists thought all of the physiological changes required for pregnancy in marsupials were regulated by hormones produced in the ovary after ovulation.

If this hypothesis is right, then the uterus of pregnant opossums should look the same as the uterus of opossums that ovulate but don’t have the opportunity to mate with a male.

To test this hypothesis, my colleagues at Yale’s Systems Biology Institute and I examined reproduction in the grey short-tailed opossum.

Grey short tailed opossum with young.
Oliver Griffith

Signs of pregnancy

We looked at two groups of opossums: females that were exposed to male pheromones to induce ovulation, and females that were put with males so they could mate and become pregnant.

We then used microscopy and molecular techniques to compare females from the two groups. Contrary to the current dogma, we found that the uterus in pregnancy looked very different to those females that did not mate.

In particular, we found the blood vessels that bring blood from the mother to the placenta interface were only present in pregnancy. We also noticed that the machinery responsible for nutrient transport (nutrient transporting molecules) from the mother to the fetus was only produced in pregnancy.

While hormones may be regulating some aspects of maternal physiology, the mother is certainly detecting the presence of embryos and responding in a way that shows she is recognising pregnancy.

How this knowledge can help others

Given that recognition of pregnancy has now been found in both eutherian (formerly known as placental) mammals like ourselves and marsupials with the more ancestral reproductive characters, it appears likely that recognition of pregnancy is a common feature of all live bearing mammals.




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But this knowledge does more than satisfy our curiosity. It could lead to new technologies to better manage marsupial conservation. Several marsupials face threats in the wild, and captive breeding programs are an important way to secure the future of several species.

Two Tasmanian devils.
Pixabay/pen_ash

One such species is the Tasmanian devil, which faces extinction from a dangerous contagious cancer. Captive breeding programs may be one of the only mechanisms to ensure the species survives.

But management can be made more difficult when we don’t know which animals are pregnant. Our research shows that maternal signals are produced in response to the presence of developing embryos. With a bit more research, it may be possible to test for these signals directly.

New reproductive technologies are likely crucial for improving outcomes of conservation programs, and this work shows, that to do this we first need a better understanding of the biology of the animals we are trying to save.The Conversation

Oliver Griffith, ARC DECRA Fellow, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.