As if polar bears didn't have it hard already, what with climate change expected to reduce ice coverage to below one million square kilometres by 2050, a new study has found that the chemical pollutants we're putting into the world's oceans are significantly reducing the density of their penis bones. So much so, that they're at risk of fractures, which would be devastating for both the individual polar bear and its contribution to the survival of the species.

But before we talk about that, you might be thinking, "Penis… bones??" Besides the spider and woolly monkeys, humans are the only primates that don't have a penis bone. Even our closest relative, the chimpanzee, has a penis bone. But no one's entirely sure why we lost them. Richard Dawkins suggested that it evolved as a result of sexual selection - only the males who could actually 'get it up' would be chosen by the females. "That is, having a penis that relies on ' hydraulics' to become erect (rather than a bone) means there will be some males with poor erectile function," writes Lauren Reid from Durham University in the UK at The Conversation.

But humans are certainly not the norm when it comes to mammals. Dogs have them, cats have them, walruses, weasels and bats have them. Known as the baculum, it's not clear why this peculiar appendage has persisted so vehemently in mammals, when humans have shown how easy it is to get along without them, but they have, and the survival of polar bears, quite literally, depends on them.

Back in 2006, Christian Sonne at Aarhus University in Denmark and her team had found that polar bears that had unwittingly ingested high levels of organohalogen pollutants (OHCs), an industrially produced chemical used in pesticides, metals, food additives and personal care products, had smaller testes and penis bones. Also referred to as endocrine disrupting chemicals (EDCs), the World Health Organisation says that in humans, these chemical pollutants are suspected to alter reproductive and immune function, increase the risk of breast cancer, and disrupt growth and brain development in children. 

And now, Sonne's team has found that a specific class of EDCs, called polychlorinated biphenyls (PCBs), are actively reducing the density of the polar bear penis bones to the point where too vigorous use could result in a fracture. "PCBs were used industrially for several decades from the late 1920s onwards. They had hundreds of applications, including in production of paints and rubber products," Penny Sarchet reports at New Scientist. "Then evidence emerged that they can harm health and cause cancer, and were banned by a UN treaty signed in 2001. But they are slow to break down, so can accumulate in the environment."

Just as apex predators in the oceans end up with the highest concentrations of mercury in their systems, so too do polar bears end up with very high concentrations of EDCs in their tissues. 

The team used hospital X-ray machines to analyse the penile bone mineral density of 279 polar bear samples born between 1990 and 2000, which spans eight polar bear subpopulations around the world. While their original owners were long-gone, the penis bones have stuck around thanks to collectors, so they were easy to come by. The researchers compared the penis bone density measures with known concentrations of EDC in the areas where the polar bears were found, and they found a clear correlation between high levels of EDC and low penis bone density in the polar bears. 

Publishing in the journal Environmental Research, the team found that the polar bears of Western Hudson Bay in Canada had the best health in this regard, with the highest penis bone density and the lowest levels of EDCs, while the Northeast Greenland polar bears faired the worst, with the lowest penis bone density and highest levels of EDCs. The team suggests that the reduction in penis bone density is a serious threat to the survival of this species in such regions because the weakening of this vital structure could see reduced mating and fertilisation. "If it breaks, you probably won't have a bear which can copulate," Sonne told New Scientist.

Declared an endangered species in 2008, and with a worldwide population of between 20,000 and 25,000 individuals, the fate of the polar bear is particularly uncertain, due to the ups and downs of regional populations. But what we do know, says Jane Palmer at the BBC, is if the trend of sea ice decline continues as it has been - about 13 percent each decade - penis bones are going to be the least of these charismatic creatures' problems.

Source: New Scientist