Washington - A tiny newborn panda cub is getting extra care at the National Zoo after mother Mei Xiang stopped letting animal keepers swap the smaller cub in for feedings as she cares for the larger twin, the zoo said on Tuesday.Mei Xiang has the larger cub in her possession and appears to be taking very good care of it, the zoo said. Still, the cubs remain in a high-risk period.The panda team is focusing more intensely on the smaller cub because it has been away from its mother. Zoo officials said the smaller cub's behaviours are good, but the team is concerned about the cub's fluctuating weight. Keepers are bottle- and tube-feeding the smaller cub and because it has shown some signs of regurgitation, which can lead to aspiration, they also are administering antibiotics.Because pandas won't usually nurse twins if left to their own devices, officials are trying to switch the cubs every four hours to allow the mother to nurse and bond with one cub at a time. Mei Xiang stopped allowing the cub swap Monday afternoon, but animal keepers will keep trying, the zoo said.The smaller cub weighed 86g at birth, and the larger cub weighed 138g. According to the zoo, bear cubs have the smallest infant-to-mother size ration of any placental mammals at about 1 to 700. Panda mother Mei Xiang currently weighs about 108kg.Additional veterinarian staff members have been brought in to assist from the Smithsonian Conservation Biology Institute in Front Royal, Virginia, along with an additional panda keeper from Zoo Atlanta to work with the team in Washington.Animal keepers successfully hand-reared a sloth bear named Remi at the National Zoo last year.