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new study has found
that female swamp wallabies can mate and conceive while still carrying a
full-term fetus. This allows them to be continuously pregnant throughout their
adult life.
Share on PinterestFemale wallabies can form a new embryo 1–2
days before the end of their existing pregnancy.
These results appear in the journal Proceedings of the National
Academy of Sciences.
Unlike
human females — who give birth after 9 months of pregnancy and are unlikely to
conceive again for another 6 months, if they breastfeed the baby — kangaroos
and wallabies have much shorter pregnancies. They can ovulate and mate again
just hours after giving birth.
This
reflects the diametrically opposite evolutionary strategies of eutherians
(which are mammals with a placenta, such as humans) and marsupials (which are
mammals that lack a placenta, such as kangaroos and wallabies).
Although
humans have evolved to maximize the length of their pregnancy in order to give
birth to well-developed offspring, female kangaroos and wallabies pack as many
pregnancies as possible into their adult life.
They
give birth to tiny young, which must then spend months completing their
development in an external pouch, where they suckle. The mothers can become
pregnant again just a few days after giving birth.
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In
fact, researchers believe that female kangaroos and wallabies can be
simultaneously supporting young at three stages of development: an embryo in
the uterus, a joey in the pouch, and an offspring at her feet.
For some time, however, biologists have suspected
that female swamp wallabies take this evolutionary strategy to the next level —
possibly conceiving during an active pregnancy.
The
evidence for this was that females’ 28-day “estrous” cycles — or the
physiological changes induced by their reproductive hormones — are a few days
shorter than their pregnancy. There were also reports of females mating before
the end of an existing pregnancy.
If
true, this would make them the only female mammals that are permanently
pregnant throughout their reproductive life.
Scientists
at the Leibniz Institute for Zoo and
Wildlife Research in Berlin, Germany, and the University of
Melbourne in Australia decided to settle the matter once and for all.
They regularly examined the urogenital tracts of six
pregnant swamp wallabies and found sperm 1–2 days before the animals gave birth
— but at no other time.
The
researchers also used high resolution ultrasound to monitor the pregnancies of
four other pregnant females. This confirmed that the animals ovulated and
became sexually receptive 1–2 days before giving birth.
All
female marsupials can mate within hours of giving birth. This is only possible
because they have two cervixes and two uteruses, each connected to a separate
ovary.
The
ovaries take it in turn to ovulate, which helps minimize the length of time the
female is not sexually receptive.
Swamp wallabies take this strategy a step further,
the researchers found. While a fetus was finishing its development in one
uterus, ovulation was already occurring in the ovary connected to the other
uterus.
The
swamp wallaby fertilized the resulting egg a few days before giving birth to
the fully grown fetus. The new embryo then remained in a dormant state, known
as “diapause,” until the joey had left the pouch around 9 months later.
“What we found amazed us
— the females come into estrus, mate, and form a new embryo 1–2 days before the
end of their existing pregnancy. The swamp wallaby is the only mammal known to
be continuously pregnant in this way.”
– Lead study author Dr. Brandon Menzies, of the University
of Melbourne
So, a
female swamp wallaby can actually support young at four stages of development:
a dormant embryo in one uterus, a fetus in the other uterus, a joey in the
pouch, and an offspring at her feet.
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