Hong Kong (CNN) -- Gorillas put off their food and stressed out orangutans, could nature tourism be bad for wild animals' health?
It's an idea that has been suggested in a recent report that tested for stress hormones in orangutan excrement.
Researchers from the University of Indiana and eco-tourism group Red Ape Encounters spent 14 years studying two apes in Sabah, Malaysia, which were used to seeing humans. By testing the animals' feces they found that the orangutans' stress levels were higher than normal the day after coming into contact with humans.
"As for the unknown wild orangutans that we were also able to gather samples from, we found numerically, but not statistically, higher stress hormone levels in these animals following contact with researchers than in the habituated animals," said Michael Muehlenbein, of Indiana University and one of the authors of the report.
Muehlenbein is keen to point out that there was no indication from the study of any long term changes in behavior of the orangutans, as Red Ape Encounters limits the number of people on their tours to seven and the visits to one hour.
Yet pathological effects like impaired cognition, growth and reproduction could be a consequence of less sensitive wildlife tours, believes Muehlenbein.
However, studying tourism's prolonged effects on primates is made harder by apes' "fight or flight" response, says another of the report's authors, Marc Ancrenaz. Orangutans are easier and safer to study than chimpanzees or gorillas, he says, as they don't have the same potentially aggressive response to intrusive humans.
As the value of eco-tourism increases each year, so too do the chances of money being more important than animal welfare. However Liz Macfie, gorilla coordinator for the Wildlife Conservation Society and co-author of the International Union for Conservation of Nature's "Best Practice Guidelines for Great Ape Tourism" believes that most eco-tour companies are trying to do the right thing.
"More and more sites are trying hard to minimize the impact on the animals," she said, pointing out the success of conservation and tourism projects with mountains gorillas in central Africa.
"Mountain gorilla tourism is one of the reasons they have continued to flourish. They're the only sub-species of gorilla whose number is actually growing and they are visited by tourists on a daily basis.
"The gorillas are worth more to Rwanda, Uganda and DR Congo because of the tourism. Not just the value of the tour, but the money the tourists then put into the local economy. So they have a monetary value."
Eco-tourism generates billions of dollars annually. Across southern Africa nature-based tourism brings in the same revenue as farming, forestry and fisheries combined, and as long ago as 1998 whale- and dolphin-watching generated more than $1 billion, according to the World Tourism Organization.
Yet Macfie is aware that only a limited number of sites could have the success seen with mountain gorillas.
As long as travelers are aware of the environmental impact of the tours they take, and operators are adhering to the principles of the IUCN guidelines, Macfie believes that the growth in eco-tourism generally bodes well.
"Tourists don't all want to drive around a savannah park with hundreds of other vehicles, and I think that's the same with ape tourism," she said.
"But I do worry about sites where they have ex-captive apes. Sometimes there can be over one hundred people crowded around feeding stations, looking at two or three orangutans that have come in. In that situation there's great potential for disease transmission and interaction between humans and orangutans."