Trying to ensure the survival of a rare set of twin chimps born at the Detroit Zoo, zookeepers are letting the mother nurse one of the newborns and donning furry costumes to care for her sister.

A 21-year-old chimpanzee gave birth to one female on April 8, and then, startling experts, delivered a second female a half-hour later.Twins occur about once in every 100,000 chimpanzee births, zoo spokeswoman Khadejah Shelby said Monday.

"We're quite excited about it. There has never been any known instance of survival of twins in captivity or wild," she said. "Twins are rare, and survival is known to be zero."

The Detroit twins are healthy. The challenge to their survival is the need to share their mother.

A chimpanzee mother needs her arms and legs to walk and can't hold her newborn, so the offspring clutches to her fur. But there's little mobility when she's got two hanging on, and there's seldom enough milk for both, Shelby said.

The zoo assigned the firstborn to a 24-hour team of seven handlers.