The mother, Toma, was to get a thorough examination Wednesday to determine why she will not nurse the newborn, the Pittsburgh Tribune-Review reported.

Veterinarians at the zoo say that the mother might have an infection or that the newborn cub might have had a twin that died in the uterus.

Another possibility is that something is wrong with the newborn that the mother has sensed even though veterinarians have not.

The cub was born Sunday, Mother's Day. Toma stopped taking care of it a few hours later.

On Monday, staffers removed the cub for bottle-feeding. Mother and child were reintroduced Tuesday.

"This is very, very unusual," said Barbara Baker, the zoo's president and CEO. "She's not aggressive toward the cub, she's just not displaying a lot of interest."

Toma had three cubs in 2006. One, suffering from birth defects, died within a few weeks, but the other two are still at the zoo.

Amur tigers, also known as Siberian tigers, are among the rarest large mammals with about 400 surviving in the wild and 200 in zoos.