(RT) - Tiger wine, an aphrodisiac which sells for more than $500 a bottle, now faces backlash from animal rights activists who say the big cats are being kept in shocking conditions before their bones are eventually mixed in as the special ingredient.

Popular with many Chinese men who believe drinking the wine will enhance their sex drive and make them stronger, the bones are soaked for up to eight years and then mixed with snake extract and herbs.

This growing demand has led to an increase in tiger farming, even extending to neighboring Laos and Vietnam.

Although breeding them for their body parts is illegal, there are an estimated 6,000 tigers in captivity in China, which is more than double the amount of wild tigers in the world.

China is a signatory of the international wildlife treaty, which bans tiger breeding, but makes an exemption for doing it in captivity under the belief that the parks reduce poaching.

The tigers in captivity are reportedly emaciated and overcrowded. In the Xiongsen Bear and Tiger Mountain park, there are more than 1,800 tigers, and a further 1,000 live in the Siberian Tiger Park in northern China.

The farms are open to the public as tourist attractions. At Xiongsen, there is a circus performance held each day, where tigers do tricks.