There is a mystery at Zoo Atlanta that has officials watching monitors closely, writing down clues and looking for signs.

The big puzzle: Is giant panda Lun Lun pregnant or not?

Renee' Hannans Henry / AJC

Zoo Atlanta officials are closely observing giant panda Lun Lun to find out if she is pregnant.

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Zoo Atlanta officials artificially inseminated the attraction's favorite mother in April, but won't really know whether the effort was successful until just before she gives birth, said Rebecca Snyder, the zoo's curator of carnivores.

The zoo hopes to know sometime next month or in early September.

"She is doing well," Snyder said. "We are watching her to see if there are any signs. So far, they are inconclusive."

Why so much uncertainty?

A giant panda's uterus is incredibly small and is hard to see because the animal's intestines block the view. The intestines also, to put it politely, are often full from constant eating of panda's favorite food — bamboo, Snyder said.

"It takes a lot of time to break down the bamboo to eat so they tend to have a lot of obstructions," said Snyder.

So Synder and her team are watching for changes in Lun Lun's diet — decreased eating is a strong indicator of panda pregnancy — as well as whether she is sleeping more and lacks the desire to go into the exhibit area. In addition, she said Lun Lun's mammary glands are swelling and officials are keeping an eye on her progesterone level to see if it increases and then falls sharply, another indicator of pregnancy.

If Lun Lun is indeed pregnant, it would be the second birth of a giant panda at Zoo Atlanta and further cement her as a bona-fide star in zoological circles. When Lun Lun gave birth to Mei Lan in September 2006, the pair brought Zoo Atlanta international fame and sent visitation at the Grant Park attraction through the roof.

"There is a fascination with giant pandas," said Steve Feldman, a spokesman for the American Zoo and Aquarium Association. "Some say it is because of the black and white. Penguins are popular, too. I don't know if there is any science to that."

Lun Lun has competition in the baby department, however. Bai Yun, a giant panda at the San Diego Zoo, has four children, the most recent of which — Zhen Zhen — was born Aug. 3, 2007.

While the zoological world has been studying the giant panda for years, its reproductive system still remains a mystery in many respects.

Giant pandas have a gestation period of anywhere from 83 days — the shortest on record — to 197 days, the longest, Snyder said. A dog's gestation by comparison is about 63 days; a cat's 60 to 67 days and an elephant's is two years.

The fertilization period for a giant panda is also different from most species.

"When the egg is fertilized, it doesn't implant for a variable amount of time," she said. "It could be a few weeks, it could be a few months."