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The pair of male brown bears (who were born in the wild, but were both orphaned as cubs and now live at a sanctuary in Croatia) were seen repeatedly engaging in "vigorous penile sucking" over a period covering six years between 2008 and 2013.

The researchers write, in a paper published in Zoo Biology, that they "observed 28 acts of fellatio between the two male bears in 116 hours of observation... nine acts occurred in various bushes that were abundant in the enclosure, 2 in the open central area, and 17 at three preferred spots along the fence."