Birds do it. Bees do it. Bears do it too--but some bears do it a bit differently.

Researchers from Poland are reporting that a pair of male brown bears at a zoo in Croatia have been engaging in oral sex--and lots of it. Over the course of 116 hours of recent observation, the animals engaged in fellatio 28 times. That's about one act of fellatio for every four hours, with each act ranging in duration from 1 minute to 4 minutes.

The researchers, from the Polish Academy of Science in Krakow, gave the "grizzly" particulars of the aberrant behavior:

All cases appeared to be initiated by the provider, who approached the receiver while he was resting on his side or with part of his abdomen exposed. If the receiver's genitals were not exposed, the provider would push his head into the pelvic region or use his paws to separate the hind legs. After accessing and initial licking of the penis, the provider would find a more comfortable posture, such as sitting or lying ... once actual sucking started, neither bear changed position.

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Other captive bears have been observed engaging in oral sex, but these tended to be animals kept "in substandard conditions with inadequate behavioral stimulation," the scientists wrote in a paper describing their study of the bears. But the Croatian bears seem to have been "kept in proper conditions."

So why would these animals be engaging in this kind of sexual behavior? A likely explanation, the researchers wrote, is that the bears were orphaned at an early age:

We suggest that the behavior may have started as a consequence of prematurely curtailed maternal suckling when these bears were orphaned as cubs. Brown bear cubs suckle their mother for milk, bonding, and comfort ... Orphaned bear cubs may suck their own or their sibling’s body parts, such as paws or ears, as a substitute for their mother’s nipples. In the case reported here, the provider may have found a substitute for teat‐sucking that also resulted in a let‐down of substitute “milk.”

The researchers note that fellatio has been documented in other mammals, including primates, spotted hyenas, goats, sheep, cheetahs and the short-nosed fruit bat. While other bears have also engaged in various genital-stimulating behaviors, including auto-fellatio and masturbation, the actions have generally been interpreted as "abnormal" and prompted by poor living conditions.

The report doesn't offer any commentary on the fact that both bears are male, except to note that the oral sex "was not influenced by the presence of a female bear" who was introduced to the sanctuary in 2010.

Whatever the explanation for the bears aberrant behavior, one thing's for sure: these are not your average bears.