Polar bear penises not breaking due to PCBs – new paper full of “coulds” and “maybes”

This is a short discussion about research on polar bear penis bones, regarding a new paper that sent several of the Internet’s most juvenile science headline writers off the deep end.1

Marine mammal penis bones_not labeled_PolarBearScience_2015_sm

Short quiz first. Above are the penis bones (each called a baculum, plural bacula) of three marine mammals – one is from a bearded seal, one from a polar bear and another from a Steller sea lion. Which one do you think is the polar bear? Answer below.

Beaufort Sea male polar bear USGS_2005 Amstrup photo

The polar bear baculum (top) is from a young male (you can tell because the ends are not fully formed). These are all casts of the real thing, purchased from professional bone casters Skulls Unlimited.

The polar bear baculum (top) is from a young adult male (that is, not a fully mature adult, you can tell because the ends are not fully formed). These are all casts of the real thing, purchased from Skulls Unlimited.

Now — the paper:Penile density and globally used chemicals in Canadian and Greenland polar bears.” (Sonne et al. 2015).

Highlights (from the authors):
– Bacula bone density was measured in eight polar bear subpopulations.
– Canadian bears had the largest bacula and the highest density.
– Bone density was negatively correlated with estimated PCB concentrations.
– T-scores and risk quotients indicate that PCBs may induce osteoporosis.

The authors found that the penis bones of polar bears with relatively higher levels of organic chemicals (PCBs, polychlorinated biphenyls, of environmental origin) were somewhat less dense when examined with a specialize bone density measuring x-ray (a densitometer) than were penises of bears with lower levels of PCBs. In other words, they found a correlation between reduced bone density and increased PCBs they measured in the bear’s fat.

They assumed causation between the two (high PCBs cause low penis bone density), but were not able to suggest a mechanism (why there was a correlation). They assumed this lessened bone density would make the penis bones weaker and more likely to break but they did not test this assumption.

They did not offer any evidence that penis bones of wild polar bears had been breaking due to this lessened bone density. They did not cite any previous papers or studies in which broken penis bones have been found and studied. And they did not attempt to find samples of broken penis bones in polar bears where the penis bone density could be tested.

Here, for example, is the last sentence in their paper:

“Furthermore, Risk quotient estimations and T-scores showed that PCB may be in a range that may lead to disruption of normal reproduction and development that could lead to lower BMD and increased risk of fractures. Based on this we suggest that EDC exposure may negatively affect reproductive health of especially East Greenland polar bears.” [my emphasis]

Their conclusions are merely possibilities, not certainties of any kind. In short, there is nothing definitive in this study that demonstrates an immediate cause for concern.

Footnote 1. What is it about a research topic like this that causes science headline writers to sound like giggling eight-year-olds? Such as Grist “We are literally breaking polar bear penises now” [no, we’re not]. Lesser-knows offered “Polar bear dicks are BREAKING” (caps in original) [no, they’re not], and “Polar Bear Penis Bones Are Snapping In Half Due To Pollution. [no, they’re not]
Do they know the difference between “could” and “are”?

More nuanced reports: NewScientist “Polar bear penis bone may be weakened by pollution”; Washington Post “Pollution is weakening polar bear penises”

Reference
Sonne, C., Dyck, M., Rigét, F.F., Jens-Erik Beck Jensen, J.-E.B., Hyldstrup, L., Letcher, R.J., Gustavson, K., Gilbert, M.T.P., Dietz, R. 2015. Penile density and globally used chemicals in Canadian and Greenland polar bears. Environmental Research 137:287–291.
http://www.sciencedirect.com/science/article/pii/S0013935114004770

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