In the Planet Earth episode “From Pole to Pole,” we are shown a courageous mother who guides her two cubs into the unprotected outside Arctic for the very first time. Upon becoming pregnant, female polar bears build their own maternity dens, where she will birth and nurse her babies until the spring, by excavating a cave in the snow. Most wild polar bear cubs are born in December, in sets of two or three.
After the cubs are born, they remain in the den with their mother until the weather begins to warm up in the spring. As winter turns into spring, polar bear mothers and their cubs emerge from their homebase exhausted and desperately hungry. During maternity hibernation, the mother sacrifices as she does not eat for months after birth [1]. Instead, she loses half her body weight by converting mass reserves into milk for her cubs. This period of maternal fasting is supported by previously eating “high-energy prey,” mainly seals [2].
As the mother and her cubs begin their journey into the open Arctic, the family faces cold temperatures, severe wind, melting ice, and the risk of predation. The melting ice is an important issue as the mother needs stable sea ice in order to hunt for seals to feed herself and her cubs. As the climate changes, sea ice diminishes, which results in a more limited ability to hunt and eat seals. This is detrimental to polar bears reproductive rates, as the mothers will have less adipose tissue stores, limiting her ability to support cubs through the winter maternal hibernation [3]. An additional consequence of less sea ice is “nutritionally stressed” polar bears, which can result in cannibalism and starvation [4].
The cubs will remain with their mother for two to three years until they begin to venture on their own. This allows ample time for her to teach them how to survive in the Arctic through hunting, feeding, and swimming. Many cubs will never even make it to adulthood due to the many factors that make their survival so difficult, including starvation and predation.
by Amelia Anderson, Carly Zaladonis, Amber Gonzales and Kimberly Williams
Planet Earth, Season , Episode 1, starting at 3:51
References
- Nelson RA, Wahner HW, Jones JD, Ellefson RD & PE Zollman.1973. Metabolism of bears before, during, and after winter sleep. American Journal of Physiology-Legacy Content 224: 491-496.
- Atkinson SN & MA Ramsay. 1995. The effects of prolonged fasting of the body composition and reproductive success of female polar bears (Ursus maritimus). Functional Ecology 559-567.
- Derocher AE, Lunn NJ & I Stirling. 2004. Polar bears in a warming climate. Integrative and comparative biology 44: 163-176.
- Richardson E, Thiemann GW, Derocher AE & I Stirling. 2008. Unusual predation attempts of polar bears on ringed seals in the southern Beaufort Sea: possible significance of changing spring ice conditions. Arctic 14-22.
The Evolution of Planet Earth 