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Why giant panda cubs are so incredibly small?

The size of a newborn giant panda cub is 900 times smaller than an adult panda

Giant pandas are not only icons of species conservation, they also still pose some puzzles for researchers. One of them is the unusually small size of their newborns: at only 100 grams, panda cubs weigh 900 times less than their heavyweight mothers.

Other bears also have unusually small offspring. Biologists have now used skeletal analyses to find out what is the reason for this. They concluded that contrary to previous assumptions, most bear cubs are by no means underdeveloped and born too early - they are just as developed as the newborns of other mammals. Only the pandas are an exception.

Among mammals, giant panda (Ailuropoda melanoleuca) cubs are the smallest and lightest newborns in relation to the mass of their parents. While the newborns of most mammals, including humans, weigh about 26 times less than adults, the ratio for pandas is 1:900. Other bears, such as polar bears, also have unusually small offspring, with a ratio of 1:400. But why?

A common hypothesis is that this is related to hibernation, a common feature of many bear species. Due to the fact that pregnant females do not eat during the hibernation period, their energy reserves are limited.

Are bear cubs born prematurely?

Hibernation is the reason biologists have assumed that bears shorten their pregnancies to prevent complete emaciation of their muscle mass. Instead, the young bears are born premature and then fed by the fat-rich mother's milk. According to this hypothesis, it does not matter that not all bear species hibernate today. The shortened pregnancy could be explained by the fact that it is an original characteristic developed at the root of the Ursidae evolutionary lineage.

According to the common explanation, it has therefore been preserved in species that have lost their hibernation in the course of evolution. If this hypothesis is correct, newborn cubs would have to show typical characteristics of an premature newborn. At first glance, the almost naked, blind offspring of a giant panda seem to resemble newly born marsupials.

Researchers now used micro computed tomography to investigate how newborn cubs of various bear species and pandas look on a skeletal level. For their study, the team examined the maturity and calcification of the skeleton and teeth of stillborn, but fully-matured giant panda cubs from the collection of the Smithsonian Museum of Natural History in Washington DC.

The scans revealed that contrary to previous assumptions, the newborns of most bear species are by no means immature. Researchers have found that most bear newborns are very similar to other mammals in terms of ossification of their skeletons. Even within the Ursinae, a subfamily of bears, there are hardly any differences in the degree of skeletal development.

Giant pandas are different

According to the researchers, despite their small size, the cubs of polar bears, brown bears or grizzlies are by no means premature babies - they are fully developed, but particularly small, as the researchers explain. In their view, this contradicts the hibernation hypothesis as an explanation for the enormous size difference between adults and newborns in these bear species.

Instead, they think another explanation is more likely. According to this, the height of bears has only increased in the course of their recent evolution. The extreme mass ratio of newborns to mothers could be related to this development. While the adult bears became bigger and heavier, the embryonic development of the offspring remained at the same level - and therefore the newborns are disproportionately small today.

However, this is not true for giant pandas. Giant panda cubs actually show characteristics of preterm babies, as the CT scans revealed. The pandas have a skeletal maturity at birth that of a puppy on the 42nd to 45th day of pregnancy, according to the researchers. At this point, only 70% of the puppies would have been delivered. In humans, this would correspond to a fetus in the 28th week of pregnancy. Giant panda cubs are similarly immature at birth. But why?

A possible explanation for this could be the late implantation of the fertilized egg in the placenta of the mother. It floats around in the womb for several months without developing further.

Although such a delayed implantation occurs in all bears, it is particularly long in pandas, as the researchers explain. This leaves the fetus only around a month to develop after implantation, compared with around two months for the other bears. The result is that giant panda cubs are born underdeveloped since their development has been shortened. To find out why this is the case with the pandas, however, and what biological purpose this fulfils, requires further research, according to the researchers of the study.