The 15-year-old panda went into labor Monday afternoon and gave birth to the first cub at 6:21 p.m. EDT and the second two minutes later. Hines said zookeepers who had been anticipating only one cub based on a recent ultrasound were surprised by the first-ever twin panda births at Zoo Atlanta.
"We have twins!" Zoo Atlanta announced on its website.
An ultrasound on June 30 determined that Lun Lun was pregnant with her fourth cub. Hines says zookeepers weren't expecting the tiny duo, however.
The twin cubs are the first giant pandas to be born in the United States in 2013, Hines added.
Zoo officials are working with animal care workers from the Chengdu Research Base of Giant Panda Breeding, where Lun Lun and her 15-year-old partner, Yang Yang, were born. There was no immediate report on the size or weight of the cubs at birth, nor the condition of the offspring and mother.
But zoo officials were in a celebratory mood.
"This is a success we share with all of our fellow zoological organizations working to understand and protect this iconic species, and we share our joy with our local community and with our colleagues in China," said Raymond B. King, Zoo Atlanta's president and CEO.
"Twins are an entirely new scenario for Lun Lun, Zoo Atlanta and our animal care teams, who will no doubt be extremely busy over the next few months," his statement added.
Lun Lun had been artificially inseminated in March and has been under 24-hour observation by a camera providing a live video feed from her den. Officials said the newborns' father is 15-year-old Yang Yang. Zookeepers weren't immediately sure of the genders of the cubs.
Zookeepers said Lun Lun is caring for one of the cubs while animal management and veterinary teams from the Arthur M. Blank Family Foundation Giant Panda Center care for the other in a nursery center. Officials said they will likely rotate the cubs between the nursery and Lun Lun's care to ensure they each get enough maternal nurturing without overexerting the 15-year-old giant panda.
Giant pandas typically only care for one cub when twins are born in the wild — which usually leads to only one of the twins surviving, Hines noted.
Giant panda twins have survived in zoos by officials rotating the cubs with the mother during their first few months of life. Hines said giant pandas have a high mortality rate in the first few months after birth, and the risk is greater when twins are born.
Visitors to Zoo Atlanta will likely be able to visit the cubs later this year, Hines said.
She added that Yang Yang and the cubs' older brothers — Xi Lan, 4, and Po, 2 — remain on exhibit and will not be housed with Lun Lun or the cubs. The separation is normal and occurs naturally in the wild, Hines said.
Less than 1,600 giant pandas are believed to be living in the wild, Hines said. Zoo Atlanta was awarded the International Conservation Award from the Association of Zoos and Aquariums in 2012 for its work to preserve the species.