The program follows the trap-neuter-return principle, or TNR, a method of humanely trapping unaltered stray cats, spaying or neutering them, and returning them to the location where they were collected, according to Gao Haiying, a Beijing-based animal protection organization staff member in charge of the museum's program.

Gao said that Chinese pet keepers never had a tradition of neutering their pets as they think it is better for the animals, but the resulting births have led to higher numbers of abandoned pets.

In a densely populated city like Beijing, the capital of China, the control of stray cats has become a controversial issue as people have debated whether neutering and euthanasia are effective and humane methods to curb the stray cat numbers.

Ma said 181 cats in the Forbidden City were neutered in the past five years and the number of stray cats is now steady, adding that every neutered cat has been marked to tell it apart from the others.

Ma even has a ledger in which he has recorded all "personal information" about the cats: names, pregnancy status, neutering operation type and the amount of money spent on the operation.

"From 2009 to 2013, the program cost 18,410 yuan (3,023 U.S. dollars) in total, a small amount compared with the whole sanitation work budget," said Ma.

The museum's money has not been spent for naught. Though rarely seen by the visitors, the cats have played an important role in safeguarding the integrity of the precious antiques.

"They are a powerful deterrent to museum rats and we have not found a single piece of cultural relics damaged by cat claws," said Ma.