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n unusual study explores a question that has puzzled
researchers for a long time: Why are giant pandas so tiny when they are born?
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pandas only weigh about 100 grams at birth.
Kathleen Smith, a biology professor at Duke University in
Durham, NC, and her former student Peishu Li, conducted the new research, which
appears in the Journal of Anatomy.
There are a lot of intriguing — and somewhat endearing
— facts about
baby pandas.
For one thing, the giant panda newborns are particularly
"helpless." They are born blind, pink, and hairless. They do not open
their eyes until they are 6–8 weeks old, and they cannot move before the age of
3 months.
The cubs do not leave their mother's side until they are
between 1.5 and 3 years old — out of a lifespan of about 20 years.
Furthermore, giant panda babies are 900 times smaller in
size than their mothers. They only weigh about 100 grams at birth.
With the exception of opossums and kangaroos, giant panda
newborns are the smallest mammal babies compared with their mother's size.
But why is that? To find out, Smith and Li examined
skeletons from baby pandas that had been born at the Smithsonian's National Zoo
in Washington, D.C.
Studying
pandas' skeletons
The prevalent theory for explaining small birth size relies
on the fact that pregnancy occurs at the same time as winter hibernation in
some species.
During hibernation, pregnant mothers rely on fat reserves to
survive, so they do not eat or drink. They also break down muscle mass to feed
protein to the fetus.
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However, such a process can only be short-lived before it
poses a threat to the mother's health. In other words, the energy resources are
limited, so the babies must be born prematurely, resulting in small cubs.
Although pandas do not hibernate in winter, those who
support this theory contend that small birth weight is a common trait that
genetics predetermines in the so-called Ursidae family
— a family that comprises eight species of bears across five genera, ranging
from brown bears to giant pandas.
To test if this theory was correct, the authors of the new
study set out to compare several species.
Giant
pandas' bones not mature enough
The researchers took micro-CT scans of giant panda babies,
as well as other related animals, including baby grizzlies, sloth bears, polar
bears, red pandas, domestic dogs, an African wild dog, and an arctic fox.
They then used the micro-CT scans to create 3D digital
models of the animals' skeletons. The researchers looked at how much of the
skeleton had ossified before birth, whether teeth had started to erupt, and
examined the fusion between the neural arches, that is, the bony plates that
make up the skull.
Even though Smith agrees that the prevalent theory is
"an interesting hypothesis," the researchers' findings did not seem
to support it.
The scientists failed to find any
differences between hibernating bears and their nonhibernating relatives when
it came to bone growth. Despite the small size, most bear skeletons showed a
similar degree of maturity at birth as their relatives, with giant pandas being
the only exception.
Full-term baby pandas resemble a "28-week human
fetus" in terms of bone density and maturity at birth, says Smith.
'Development
is just cut short'
The scientists do not yet know the answer to this question,
but they do know that baby pandas seem to mature at the same pace as other
mammals, at least judging by their skeletons.
The only difference is that "Development is just cut
short," according to Smith. Giant pandas' relatives gestate for 2 months
after the egg's implantation, whereas panda bears only do so for 1 month.
"We really need more information about their ecology
and reproduction in the wild," the author adds, pointing out that their
findings only involve the bones. Looking at other organs, such as the brain,
might reveal new and different theories.
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