Back to the future: forecasting climate change to predict polar bear litter size

            Do you ever plan ahead for an event to have it suddenly be changed or canceled?  Whether it makes a small or large impact on you, it still did not go the way you had planned. Even though animals like polar bears do not have calendars for planning or a means of predicting the forecast, they still plan ahead for events like bearing offspring and how the weather plays out, affects how their pregnancy goes.

            Climate change is affecting animal populations and habitats all over the world but in the Arctic, polar bears are particularly at risk.  In many polar bear populations, body condition, reproduction, survival, and abundance is declining as the polar bear habitat disappears considerably.  Though it is not certain how polar bear reproduction and survival will change with climatic warming, existing data may help determine future reproduction and survival.  If the processes causing these changes are known and accounted for, models may be derived to determine changes in population; thus, these models can be used to predict future polar bear litter sizes.

            Polar bears from the Hudson Bay area were studied in an experiment performed by Péter Molnár and three other scientists.  For eight months of the year, the Hudson Bay is frozen and polar bears hunt for seals on the ice.  When the ice melts during the summer season, the polar bear population is forced onto land and must rely on energy that they have stored up, as there is little food on land available to them.  Female polar bears that are pregnant enter “maternity dens” in October.  They stay in the dens until February or March when they emerge with one to three cubs.  Food is scarce while the females are in their den so they rely on fat and protein stores that they have built up during the previous hunting season.  The amount of storage energy that is available to pregnant females depends on the length of the last ice season; storage energy influences the number of cubs that a female can raise.

            Due to rising temperatures and melting ice, polar bears have been arriving on shore earlier than previous decades.  This earlier arrival means that on-ice feeding has been shortened, on-land fasting will be longer, and the females will arrive in poor body condition.  Consequently, food energy stores are lower for pregnant mothers, which leads to less energy available for gestation and lactation; thus, litter sizes are declining. 

            Molnár and his team obtained predictions as to how future changes in sea ice melting will affect the litter size of pregnant polar bear females.  They demonstrated that the size of a litter when a female emerges from her den could be predicted from “maternal energy density” at den entry.  Maternal energy density is defined as the relationship between food storage energy and lean body mass.  They also created an energy model to calculate the energy consequences of extended fasting and decreased feeding.  The model allowed the to determine the spread of den energy densities and the expected size of litters and how these litter sizes might change if females are forced onto the land earlier in the summer.

            The scientists considered two feeding scenarios for their experiment: early feeding and late feeding.  Early feeding supposes that bears gather all of their storage energy up until the end of May and that feeding for the rest of the on-ice period decreases significantly.  Late feeding assumes that the bears gather much of their energy storage right before on-shore arrival.

            During the 1990s, the average litter size for a female polar bear was two cubs.  Both feeding scenarios have been comprised by earlier on-shore arrival; therefore, the proportion of pregnant females will decrease and the proportion of non-producing females will increase.  With less polar bears having cubs, the chance of having twins or triplets also decreases significantly more than the chance of having only one cub.  Polar bears that go from having twins and triplets will go to either having one cub or none whereas females that usually have one will go to not bearing any cubs.

            It is sometimes hard to predict how population levels in different species can change because the data required to create population equations cannot be gathered before climatic changes transpire.  Molnar and his fellow colleagues advocate models and equations that consider the mechanisms by which environmental variables impact populations.  Their model suggests that forty to seventy-three percent of pregnant polar bears will be unable to produce a litter (40% corresponds to early feeding and 73% to late feeding scenarios).  Females producing one cub will decline by sixteen percent, two cubs at forty-three percent, and three cubs at one percent.

            It is dispiriting to realize that because of current and future climatic changes, whole populations of animals are negatively affected.  In the meantime, it is important to evaluate the ecological impacts of climate warming in order to plan for species conservation.  Although polar bears may not understand the consequences of litter size declines or be able to express how they are affected, it is critical that our society at least is aware of what is happening.  Imagine future populations not co-existing with polar bears because polar bears have become extinct.            

For more information about polar bears and climate change check out:

-A video of the real dangers of changing climate for polar bears in the Hudson Bay by clicking here.

-Another study on polar bear maternal dens.    

Reference: Molnár, P.K. et al. Predicting climate change impacts on polar bear litter size. Nat. Commun. 2:186 doi: 10.1038/ncomms1183 (2011).  


First photo: Zoo Borns
Second photo: Animals of the Arctic - Norbert Rosing