Why are giant panda cubs so small? New evidence refutes existing theoration

The giant panda is a popular animal all over the world. However, panda cubs are usually born weighing only 100 grams compared to their mother’s large body. This rare volume difference has been troubling researchers. With the exception of needles and kangaroos, newborns in other mammals are rarely as different from their mothers.

Why are giant panda cubs so small? New evidence refutes existing theoration

Kathleen Smith, a professor of biology at Duke University, and her former student, Peishu Li, studied the bones of bears and other animals and found that some of the current theories were untenable. They published the results in the journal Of Anatomy.

The bones of giant panda cubs are difficult to obtain, but researchers have obtained the remains of the giant panda cubs born at the U.S. National Zoo. Ling Ling and Xing Xing, the first giant panda couple at the National Zoo, gave birth to five full-foot edgy cubs in the 1980s, but none of them survived.

The researchers performed micro-CT scans on two cubs, as well as micro-CT scans of newborn grizzly bears, lazy bears, polar bears, dogs, foxes and other animals at the National Museum of Natural History and the North Carolina Veterinary College. They created a 3D digital model of the inner bones of each cub at birth.

They point out that giant pandas may be an extreme example, but all bears have a disproportionate number of cubs. Polar bears are born underweight mothers by one in four percent of their mothers. For the vast majority of mammals, including humans, the infant weight ratio is close to 1:26 for the mother. However, the weight ratio of the panda cubs to their mother was 1:900.

For years, the perception was that for some species, pregnancy coincided with hibernation. Pregnant animals do not eat or drink during this time, relying mainly on their own fat reserves to survive, while also breaking down muscles to provide protein to the fetus. The solution, therefore, is to give birth early so that the cubs can survive on their mother’s milk without consuming their muscles.

Proponents of this theory acknowledge that not all bear animals hibernate (such as giant pandas do not hibernate). However, the idea is to lock a small birth weight into the bear family tree to prevent non-hibernating relatives from evolving larger babies. “It’s certainly an attractive hypothesis,” Smith said.

Duke’s team, however, doesn’t think the theory holds water. They did not find significant differences in bone growth between hibernating bears and their year-round counterparts. In fact, most bears’ bones are born as mature as their cousins. Giant pandas are the only exception. Even the full-footed giant panda cubs have bones that look like hounds born weeks premature. “It’s like a 28-week-old fetus,” Smith said.

The researchers speculated that the particularly small panda cubs may have something to do with their experiences during pregnancy. All bears experience so-called embryo delays in bed. After the egg is fertilized, the embryo floats in the womb for several months, then goes to bed and resumes development. Other bears usually give birth after two months in bed, while giant pandas only need about a month.

They are also looking for a complete explanation of how the giant panda’s weight differences evolve with geological age. (Bio-tomint Mint)

Original search

Comparative skeletal anatomy y sylwedd and the extreme altriciality of the giant panda

https://doi.org/10.1111/joa.13127

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