Once again signs are showing up at the National Zoo that breeding season is drawing near for the female giant panda. (Smithsonian National Zoo photo)

The road to panda pregnancy is long and arduous, but at the National Zoo, Mei Xiang has apparently embarked once again on the journey.

On March 15, the zoo said last week, she “started exhibiting behavioral signs that breeding season is approaching! “

Analysis of hormones in her urine confirmed that changes in her behavior were due to rising levels of estrogen, the zoo said.

That, according to the zoo, indicated the breeding season for pandas would soon be here.

She has been “much more active,” the zoo said, describing her as eager to play, and engaging in scent-marking around her living quarters.

Scent-marking, it seems is one of the things you do if you are a female giant panda interested in attracting the attentions of a male.

Adult pandas, the zoo said, are solitary animals. This means they do not hang out together outside of breeding season.

So, the zoo said, a scent mark helps make it possible for the animals to locate each other in the brief time available for breeding.

The breeding window, the zoo said, can be as short as 24 hours. On the upside, it stretches to 72.

So far, the zoo said, the male panda, named Tian Tian, has shown great interest in Mei Xiang. But she has indicated, the zoo said, that she is not yet ready.

Such natural handicaps have helped to make reproduction a challenge for the giant panda.

Many setbacks have been encountered on the path to expanding the panda population. Since 2000 when the adult pair arrived here, the number of surviving cubs stands at three.

Correction: This story was updated to reflect that three cubs survive since 2000, not two.