According to a new study, male bats perform oral sex on female bats - a behavior previously uncommon in mammals apart from humans.

In the study, researchers observed some 57 cases of bats engaged in oral sex in a colony of Indian flying fox, Pteropus giganteus. The colony of bats that was observed for the study had 420 individuals.

Researchers aren't sure why bats perform oral sex. One theory is that oral sex makes sex last longer. Another theory is that the male bats lick away whatever sperm is left from another male's encounter with the female bat. However, whether the bats mate for a short while isn't known yet. Eliminating sperm of the competitor ensures that the male bat has increased chances of paternity.

The study team spent 13 months observing the bats and they found that initially male bats groom their penises and then approach the female bats. The males then lick the vagina of the female bats - a process called as cunnilingus - to initiate lubrication. These foreplay sessions lasted on an average for about 50 seconds, researchers reported.

"Apart from humans, bats also exhibit oral sex as a courtship behavior," said Ganapathy Marimuthu, a bat researcher at Madurai Kamaraj University in India, reports Livescience.

Oral sex was found to be occurring after copulation and lasting another 98 to 188 seconds in the wild, the study found. Oral sex isn't common among mammals, except humans.

"It is possible that prolonged copulation enables the mobility of sperm. Such mobility of sperm increases the chances of conception," Marimuthu told LiveScience.

The study is published in the journal PLOS One