According to some reports, there are now more tigers in captivity in the US than in the wild in the whole of Asia, as well as large numbers of lions, bears and chimpanzees.
Travelling to America's heartlands, Louis Theroux spends time with an Oklahoma man who has bred and collected over 150 tigers, visits the woman who privately owns one of America's largest collections of chimpanzees, and finds himself in uncomfortably close contact with a number of big cats and dangerous primates
Travis, a 14-year-old, 200lb (91kg) chimpanzee, had once been the star of several TV commercials. He lived in suburban Connecticut with his owner Sandra Herold, sipping wine from a stemmed glass and occasionally popping a Xanax. One day in 2009, he viciously attacked a human neighbour, ripping off her face and chewing off several of her fingers.
Chimp owners have been fighting an uphill publicity battle ever since.