Aarhus University Seal / Aarhus Universitets segl

PCB threatens polar bear populations

New research from ARC shows that PCB impairs male mating success and population growth

2016.12.14 | Susanna Pakkasmaa

Polar bear. Photo Rune Dietz.

PCB or polychlorinated biphenyls are transported from industrialised areas of the Northern Hemisphere to the Arctic via air and ocean currents. They accumulate in marine food chains, with concentrations peaking in polar bears, killer whales and humans. The use of PCBs started to decline in 1960s, and it was forbidden in the Stockholm Convention on Persistent Organic Pollutants, but the levels in the Arctic are constantly high due to the persistency of these compounds. Today, climate change affects marine food chain dynamics, and polar bears are eating more contaminated food (Greenland seals and hooded seals).

Male polar bears accumulate large amounts of PCB in their body. It affects the bears’ testicles: they shrink and the fertility falls. The more PCB in the body, the smaller testes and weaker penis bone, and thus, lower reproductive success.

Male polar bears peak at the age of 10-14 years. Their mating success is high, and they outcompete younger, smaller, or older males. Unfortunately the largest males have the highest levels of PCB in their body, and therefore many of them probably suffer from reduced fertility.

"This is deeply unfortunate for polar bear populations", says Professor Rune Dietz, who has studied marine mammals for decades.

"First, it takes a long time between the horny polar bear females encounter a fertile male. Second, the biggest males are often able to outcompete other males. And this is quite unfortunate, because the largest males often have poorer reproductive ability due to the endocrine disrupting effects of PCB”, Rune Dietz explains.

The relationship between fertile males and females is crucial for population size and a small number of pregnant females can be explained by the high levels of PCB in males.

The new study shows that the reproductive capacity of polar bear populations is determined by male fertility and population size. The poorer male fertility, the weaker mating success. The researchers suggest that monitoring of polar bears should pay more attention on the reproductive capacity of males, and their fertility should be evaluated together with the load endocrine disruptors and the effects of climate change on their feeding.

Read the article: Pavlova V, Nabe-Nielsen J, Dietz R, Sonne C & Grimm V 2016. Allee effect in polar bears: a potential consequence of PCB contamination. Proceedings of the Royal Society B. DOI: 10.1098/rspb.2016.1883

Arctic Research Centre