The Shanghai Zoo has successfully hatched rare constrictor snakes for the first time. They are expected to go on show to visitors in about a month, the zoo said yesterday.

Constrictor babies started to come out of their eggs on Thursday and, by yesterday, almost 20 had hatched. All were in good health. Another 10 baby constrictors are expected to be hatched in the next few days, officials said.

"They will start eating later this week," said zoo official Wu Weichun.

"We'll feed them with rat cubs which have not grown hair. If they eat properly, they'll have no health problems."

The zoo said the 10-year-old mother, a black-tailed python, laid about 30 eggs on April 22.

The mother displayed strong maternal instincts, thus raisers decided to allow her to hatch the eggs by herself.

The day before hatching, the mother constrictor was disturbed by other constrictors, and keepers had to erect a protective cover over her and the eggs.

On the morning of June 17, they found that two of the babies had emerged.

The Shanghai Zoo has been studying artificial constrictor hatching since 2006, but experienced many failures, Wu said.

"If the temperature is too high, they'll not sleep and this affects their reproductive system," he said. "And if the temperature is too low, they may catch cold."

Another problem is the difficulty of determining a snake's sex and they had been learning about that from their foreign counterparts.

Constrictors are a first-class protected animal in China. Because of urbanization and the destruction of their habitat the numbers of wild constrictors has been declining rapidly in recent years.