People who consider homosexuality to be "unnatural" might be forced to reconsider after a visit to Zurich zoo.
As part of the city's "Warm May" festival, the zoo is offering guided tours dealing with homosexuality among animals.
The crowd that gathered for the first tour comprised a predictable mix of homosexual humans and their curious heterosexual counterparts.
"I have kids at school who think that homosexuality is unnatural," one young teacher told swissinfo, "so I wanted to come along and get some facts to prove them wrong."
"I'm just interested in animals and nature, and of course I'm gay," said one of the men waiting for the tour, "so I just wanted to hear the arguments and of course see the gay animals."
Unfortunately the decision to schedule the tour for the early evening meant that most of the zoo's inhabitants were favouring sleep over any form of amorous activity. But, despite the lack of first-hand evidence, tour guide Myriam Schärz assures her audience that gay and lesbian activity is a common part of animal life.
"I don't know of any species that is exclusively heterosexual," says Schärz. "There are studies of this going back hundreds of years, although scientists were previously reluctant to explore the matter further for fear that they would themselves be branded gay."
During the entertaining one-hour tour, Schärz recites evidence from American biologist Bruce Bagemihl's groundbreaking 1999 study "Biological Exuberance" which documented homosexual activity in more than 450 animal species.
Visitors to the zoo learn about the indiscriminate and almost insatiable sexuality of bonobo apes. And re-runs of 'Flipper' might never be the same again after hearing how gay male dolphins use their lovers' blowholes for sexual gratification.
But there are also anecdotes from closer to home.
"Right here in Zurich we once had a gay flamingo couple who remained partners for life," recalls Schärz. "In Cologne zoo they have a pair of lesbian penguins who each year steal an egg from one of their neighbours and treat it as their own."
Some conservatives and religious groups now admit that homosexuality is common in the animal kingdom, but many of them have also put forward theories to explain the phenomenon.
"Some argue that homosexuality only occurs when animal populations become too large," says Schärz, "or that animals only turn to homosexuality when they have no other alternative - for example, when they are living in a harem-based society with only one dominant male.
"But there is no evidence to back up the population theory, and there is plenty of proof against the harem argument. Dominant silver-back gorillas, for instance, have frequently been seen engaging in homosexual activity and deliberately shunning available females."
Whatever the reasons for homosexuality in the animal kingdom, Schärz argues that humans can learn a good deal about the apparent lack of prejudice among beasts.
"Humans seem to be the only species where homosexuals are not readily accepted in society. Animal societies tend to stay together and accept each other. Of course, animals do get excluded occasionally but that tends to happen if they get injured or if they are not liked, rather than because of their sexuality."
It's a message which goes down well with the first group of tourists. But Schärz is well aware that she's generally preaching to the converted.
"I think most of the people who come on this tour are already pretty open-minded about the subject and keen to find out more, but it would be great if we could also present the evidence to people who think that animal homosexuality is just a question of exuberance or casual play.
"I don't think we can claim that tours like this are doing anything to improve gay rights unless we can also persuade sceptics to come along."
Sceptics and believers alike will be welcome at the zoo when the last two tours take place on May 28.
swissinfo, Mark Ledsom in Zurich
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