Climate Change Causing Polar Bear Birth Rates to Decline

Written by on February 9, 2011 in Marine Life
Polar Bear Cubs

Polar Bear Cubs

New research, published in Nature Communications on February 8, from the University of Alberta has linked declining number of cubs in polar bear litters with loss of sea ice. 

Peter Molnar, Andrew Derocher and Mark Lewis say projections of declining litter size is a significant threat to the western Hudson Bay polar bears.  If climate change continues, all polar bears in the Arctic will be in danger.

They studied data collected since the 1990s and looked at the changing (or shortening) length of time the Hudson Bay is frozen over.  This corresponds directly with the polar bear hunting season.  They also looked at the amount of energy pregnant females can store before hibernation and birth.

Pregnant females remain in their maternity den for up to eight months with no access to food.  If the ice breaks up in early spring, these mothers will have a difficult enough time trying to eat enough to support themselves, much less give birth to and raise cubs.

According to data from the early 1990s, 28 percent of energy-starved pregnant polar bears in the Hudson region did not give birth to a single cub.  Some don’t enter a maternity den and some naturally abort the birth.

Using mathematical models, the researchers calculated possible scenarios of a shorter hunting season:

  • If the spring break up of ice in the Hudson Bay comes one month earlier than in the 1990s, 40 to 73 percent of pregnant polar bears will not reproduce.
  • If the ice breaks up two months earlier, 55 to 100 percent of all pregnant females will not have a cub.
Polar bears rely on this ice for hunting

Polar bears rely on this ice for hunting

The population of polar bears in the Hudson Bay is estimated to be around 900.  This has declined from 1200 bears in just the past decade.  The total number of polar bears across the Arctic is estimated to be between 20,000 and 25,000. 

The team says that because the polar bears of the Hudson Bay are the southernmost population they are the first to be affected by global-warming.  They continue to say that if the warming trend continues, polar bears across globe will be at risk.


Copyright ©  2011 by Marine Science Today, a publication of OceanLines LLC

About the Author

About the Author: Emily Tripp is the Publisher and Editor of She holds marine science and biology degrees from the University of Miami's Rosenstiel School of Marine and Atmospheric Science and a Master of Advanced Studies degree in Marine Biodiversity and Conservation from Scripps Institution of Oceanography. When she's not writing about marine science, she's probably running around outside or playing with her dog. .


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