ATLANTA -- Zoo Atlanta officials say the panda cub is a boy and has been given back to his mother.

The only panda born at a U.S. zoo so far this year was listed in 'guarded condition' Tuesday after being placed in an incubator on Monday for closer monitoring by officials at Zoo Atlanta. Officials reintroduced the baby panda to his mother Tuesday afternoon and things seem to be going well, officials said.

The cub was taken from its mother Lun Lun on Monday after zookeepers noticed it was barely moving. After examining the tiny, hairless cub — which is the size of a cell phone — veterinarians determined it was dehydrated.

RAW VIDEO: Panda Lun Lun Gives Birth At Zoo Atlanta

The cub was still in an incubator on Tuesday morning and was being hand fed, but zookeepers returned it to Lun Lun late Tuesday afternoon.

Zoo Atlanta officials said the baby panda was successfully reintroduced to his mother at approximately 1:10 p.m. Tuesday. Officials said, "The reintroduction coincided with the cub’s scheduled feeding, and was chosen so that the cub would show the strongest possible nursing response. Lun Lun showed immediate interest in the cub when it was presented just outside the den, and was very focused on the cub as it vocalized. Based on these positive signs, the cub was placed in the den and Lun Lun picked it up and cradled it. The cub has continued to periodically give strong vocalizations indicating that it is doing well."

Keepers briefly examined the cub again at 4:30 p.m. and noted he had gained a little weight, which is a very good indication. Furthermore, there are no signs of abnormalities or bacteria in the panda cub, due to the hand rearing process, officials said.

SLIDESHOW:Panda Lun Lun Gives Birth At Zoo Atlanta

In other happy news, during the nineteen hours that the cub was being hand-raised, the giant panda nursery expert from China who is assisting Zoo Atlanta staff was able to examine the cub closely. The expert said the baby panda is a boy.

Zoo officials said they were encouraged by Lun Lun's progress with the baby Tuesday but they will continue to keep close watch over the baby's health. Officials said they may periodically separate the cub from Lun Lun to assess his condition.

Zoo spokeswoman Simone Griffin said the Zoo's Panda Cam has been suspended.

"Based upon the cub's behavior we made the joint decision to move the cub to an incubator earlier this evening, where it can be monitored closely," Zoo officials wrote in a news release. "As a reminder, giant panda cubs are born at a very early state of development, and the first days and weeks are a very vulnerable time. This is why we monitor mother and cub continuously during this time."

The Panda cub was born Saturday, just a week before the second birthday of Mei Lan, the zoo's first panda cub.

"You can go to every zoo and see elephants, but you can't go to every zoo and see pandas," said Christy Moonan, whose two children, Mia, 3, and Collin, 5, brought small stuffed panda toys with them to watch the cub on television screens in the zoo's panda habitat.

Three other U.S. zoos house pandas, which are endangered: San Diego, Memphis and Washington, D.C.

Mei Lan and dad, Yang Yang, napped in separate rooms of the indoor part of the panda habitat on Monday. Zoo officials will keep Yang Yang apart from his new cub as they have with Mei Lan, because male pandas in the wild typically don't stay with their offspring.

Zoo officials inseminated Lun Lun with Yang Yang's sperm in April. An ultrasound on Thursday confirmed zookeepers' suspicions that the procedure was a success.

"A lot of people wonder whether this is less exciting for us because it's our second birth," Wilson said. "Every giant panda birth is important. We are just as excited about this birth as Mei Lan's."

Mei Lan's birth was a landmark event for the zoo, drawing in thousands of visitors and generating a huge number of hits on the online panda cam. Weekend attendance figures weren't yet available Monday, but zoo officials said they're expecting them to be higher than normal because of the new arrival.