http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2003/02/19/waa19.xml says:

The promiscuous sex life of lesbian Japanese monkeys is challenging one of the central tenets of Charles Darwin. He argued that females are coy, mate rarely and choose mates to ensure the best genetic inheritance for their offspring, while males are promiscuous and fight among themselves for female partners.

But after studying Japanese macaques in the wild, Dr Paul Vasey, of the University of Lethbridge, Canada, begs to differ. He found that bisexuality is common in females and that they often compete with males for sexual partners.