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.

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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]

A brown bear rolls on its back in the Kodiak National Wildlife Refuge in Alaska. Steve Hillebrand / Fish and Wildlife Service

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.

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. Bonobos do it to ease social tension. Chinese fruit bats also perform oral sex.

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. For the provider in this study, fellatio could be a relic of this infantile behavior, the researchers speculated.

— Megan Gannon, Live Science

This is a condensed version of an article that originally appeared on Live Science. Read the entire story here. Follow Megan Gannon on Twitter and Google+. Follow LiveScience on Twitter, Facebook and Google+.