It was a love story that touched the heart of New Yorkers. Two gay penguins at Central Park Zoo who - after trying unsuccessfully to hatch a rock - were given a fertilised egg and raised their own little chick called Tango.

The tale of Roy and Silo was even made into a children's book called And Tango Makes Three. But, while liberal Manhattanites may have sighed at the sweetness of it all, not every American seems quite so pleased. The book has caused controversy in a number of small towns in the American heartland, where teachers and parents have complained that it is not suitable for children. In Shiloh, Illinois, some parents insisted that the village school library restrict access to the cartoon tome.

Shiloh resident Lilly Del Pinto felt upset when her five-year-old daughter brought the book home. She was reading it to her when she got to a point where the zoo keeper says Roy and Silo must be in love. Then she realised it was not quite the straightforward animal tale she had expected. 'That's when I ended the story,' she said. Now Del Pinto wants the book kept in a more mature section of the library or for parental permission to be sought for it to be taken out.

So far the requests have not been heeded by the school district, though a panel appointed by Superintendent Jennifer Filyaw did agree with Del Pinto's suggestions. However, Filyaw considers the book 'adorable' and fears censorship issues if any action is taken. 'My feeling is that a library is to serve an entire population,' she said.

Parents in Missouri had the book moved to the non-fiction section of the school library, arguing that it described a real story and genuine penguin behaviour.

But the twist in the tale - which is not touched on in the book - would please social conservatives who see gay marriage as a threat to society. In real life the gay penguins of Central Park split up last year after Silo took up with a female penguin called Scrappy.