The club of fellatio-loving animals just gained a new member: bears.

Scientists have observed a pair of male brown bears in captivity in Croatia that regularly engaged in oral sex over several years. While the creatures in this case study likely do it for pleasure, their fellatio habits might have started because they were forced to wean too early, the researchers suspect.

The two, unrelated male bears in the study were orphaned soon after they were born in 2003 and put in captivity at a sanctuary in Kuterevo, Croatia. Over the course of six years and 116 hours of observation time, scientists led by Agnieszka Sergiel, of the Polish Academy of Sciences' Department of Wildlife Conservation, witnessed 28 acts of fellatio between the two male bears. [Image Gallery: See Photos of Amazing Bears]

The study, published online earlier this month in the journal Zoo Biology, isn't shy about the details. The larger bear was always the one to receive fellatio. In many incidents, he appeared to reach orgasm from the sex act (which lasted for just a few minutes), as evidenced by muscular contractions and, well, fluids on the muzzle of the provider. When the deed was done, he often pushed the provider off with his hind legs or turned away.

The scientists billed their study as the "first observations of long‐term, recurrent fellatio in captive brown bears kept in proper conditions after being orphaned." Researchers had previously seen bears in captivity performing fellatio, or auto-fellatio combined with masturbation, but these were viewed as abnormal incidents, where the bears may have been living in substandard conditions.

Why do it? Sexual reward seems to be a motivation for many species that engage in non-reproductive sex acts. Female cheetahs and lions lick their partners' genitals as a courtship ritual. Famously kinky bonobos do it to ease social tension. Female Chinese fruit bats perform oral sex, seemingly to get the males to prolong the deed.

These two brown bears might engage in fellatio for sexual satisfaction. But the team of researchers was puzzled by one recurring theme: The provider always instigated oral sex.

The bears were very young when they were forced to stop suckling their mothers — an activity that not only provides milk, but also bonding and comfort for at least the first year of life, the researchers wrote. Orphaned cubs have been known to suck their own or their sibling's paws, ears and other body parts as a substitute for their mother's nipples. For the provider in this study, fellatio could be a relic of this infantile behavior, the researchers speculated.

But there are still a few mysteries. The researchers don't know why only one of the bears would perform fellatio or why it has persisted for so long. They acknowledge that the behavior could simply be a weird consequence of life in captivity.

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