"He's like a rambunctious little toddler that loves to get into everything," said Dr. Suzan Murray, the zoo's chief veterinarian.
Murray has monitored every developmental benchmark for the black and white panda since his birth on July 9, 2005. Those included the opening of his eyes, the development of his distinctive markings, his heartbeat, his mobility, and even the frequency of his squeals, grunts and barks.
The cub still nurses, but he has advanced from milk provided by his mother, Mei Xiang, to eating bamboo. The woody perennial grass is the staple of adult pandas' diets, accounting for more than 90 percent of their nutrition.
"My next challenge is weaning Tai Shan from his mom," said Lisa Stevens, the zoo's curator of pandas and primates.
For his birthday, staffers prepared a giant fruit-sicle for the cub, a frozen mélange of apples, yams, carrots and fruit juices. That is a favorite of Mei Xiang's, but this was the first prepared for the growing cub.
Tai Shan, whose name means peaceful mountain, routinely awakens before daybreak. After a meal, the mother and cub are often seen wrestling. Tai Shan also rolls around in the outdoor paddock and climbs trees, delighting zoo visitors.
More than 1.2 million have visited the panda exhibit since the cub went on display last December, and more than 21 million people have linked to the panda cam Web site.
Tai Shan is aware of the scent of his father, Tian Tian, in a separate exhibit, and they are often in visual proximity of each other, but they do not look at each other at all, said Stevens.
"In the wild males play no role in the rearing of cubs, so there's no reason for there to be any recognition of who this strange neighbor's scent mark belongs to," said Stevens.
Under the agreement with China that allowed the National Zoo to bring the two adult pandas to the U.S., the cub must be available for shipment to China at 2 years of age to become part of breeding efforts to preserve the endangered species.
Only about 1,600 giant pandas remain in the wild, and fewer than 180 live in captivity. Zoo officials hope the Chinese will allow the cub to remain in the U.S. until it nears breeding age of 5 or 6.
Tai Shan was the product of artificial insemination. Zoo officials hope to breed the adult pandas again in the next year.