Like the mammals they’re attached to, polar bears' penis bones—also called bacula—appear to be facing considerable danger.
PCBs (polychlorinated biphenyls) were banned by a UN treaty in 2001, after evidence was found linking them to cancer and other health problems. But they remain in the environment, as the New Scientist explains:
The Arctic has particularly high concentrations of pollutants like PCBs, says Margaret James at the University of Florida in Gainesville. "These chemicals enter the atmosphere at lower latitudes where they were used, and are then deposited down from the cold polar air, so Arctic animals are more highly exposed than animals in more temperate or equatorial regions."
To gather data on the effect of the chemical on polar bear bacula, the team of researchers, led by Christian Sonne at Aarhus University in Denmark, evaluated 279 penis bone specimens using an X-ray technique to determine density. James told the New Scientist that “the analysis was not strong enough statistically to prove that PCBs are the cause of lower bone densities.” But the research does indicate that chemical pollutants have the potential to further threaten a species already made vulnerable by the loss of habitat and food sources caused by climate change.
The exact purpose of the baculum is as yet unknown, though many other mammals have them, including certain monkeys, rodents and bats. It may be used for mechanical support, female stimulation or as a mode of ensuring sperm success. Either way, they are (unsurprisingly) related to mating, and a broken penile bone can impede or prevent reproduction. This is particularly concerning since polar bear populations are already in decline, with the World Wildlife Federation recently logging the loss of about 40 percent of one regional group.
Hunters do have a particular use for bacula, though: after killing a polar bear for either subsistence or recreation, they often collect the bear's baculum as a trophy. That’s why the research team chose to focus on this bone in particular—because, as New Scientist writes, “it is so easy to come by.”