Volkswagen's decision to experiment with monkeys in vehicle emissions has drawn wide condemnation, reflecting the growing unpopularity of animal testing.

The New York Times recently reported that the car lobby group European Research Group on Environment and Health in the Transport Sector commissioned the tests in May 2015. During the four-hour-long experiments, 10 Java monkeys were isolated in tiny chambers and exposed to vehicle exhaust, according to the Guardian.

The companies represented by the research group include Volkswagen, Daimler, and BMW.

Following an uproar on social media, Volkswagen told PETA: "The Volkswagen Group clearly distances itself from all forms of animal cruelty. Animal testing contradicts our own ethical standards."

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On Tuesday, Volkswagen suspended Thomas Steg, head of external relations and sustainability, after he said he had known about the experiments, which occurred at the Lovelace Respiratory Research Institute in Albuquerque, New Mexico, the Guardian reported.

The use of non-human primates as lab animals has long been contentious. Animal welfare groups argue that experiments on non-human primates are especially cruel, because of the animals’ similarities to humans.

Animal testing "gives this sense that animals are disposable and commodities for us to use," Kathleen Conlee, vice president of animal research issues at the Humane Society of the United States, said in a previous interview about cloning monkeys.

"Is this appropriate, to have an animal you can do whatever you want to? ... It creates a poor dynamic about how we treat animals overall."

For instance, for over a century chimpanzees were used in U.S. laboratories to test new vaccines and medical procedures. Sometimes housed in small, cement cages, many lived most of their lives in isolation.

But in recent years, the movement to end invasive research on these animals, humans' closest genetic relatives, has gained force. In 2015, the National Institute of Health announced that all federally owned chimpanzees would be moved to sanctuaries, and the U.S. Fish and Wildlife Service officially classified all captive chimpanzees as endangered—effectively ending experimentation.

Although many chimpanzees have yet to be transferred to sanctuaries, an increasing number of facilities are opening their doors to these great apes. They're here to serve the chimpanzees who served humankind as subjects of government-funded biomedical research.