Re: Do polar bears store triglycerides in a liquid from ? Why?

Hi Lizette,

Thanks for asking a great question. The answer to your question touches on many different aspects of biology, and I don't know that much about all of them, but I hope that I can give you a pretty good answer.

First off, I think that your question, "do polar bears store triglycerides in a liquid form?" is a little misleading. This is because triglycerides (or fats as they are better known) are liquid when they are found in the range body temperatures common to mammals. When you hold a pat of butter in your hand it melts and becomes liquid, just like the fat in your body.

Now, triglycerides have two components. First, there is a glycerol molecule, and then there are three fatty acids that are attached to the glycerol. What I think your book is referring to is the nature of the fatty acids that are found in polar bears. While all glycerol molecules are the same, there is a large variety of different fatty acids. These differ in both their lengths, and their degree of saturation, which is a way of describing the number of hydrogen atoms along the carbon chain that makes up most of the fatty acid. When each carbon in the chain has its full complement of hydrogens, that fatty acid is said to be saturated (with hydrogen). When the chain does not have a full complement of hydrogens, carbons are linked by double bonds, and the fatty acid is said to be unsaturated.

Now, shorter fatty acids and unsaturated fatty acids tend to have lower melting temperatures than longer fatty acids and saturated fatty acids, so that long saturated fatty acids are solid at room temperature, and short unsaturated fatty acids are liquid (or even gasseous) at room temperature. When animals make fatty acids, we make saturated fatty acids. This is why bacon fat and butter are solid at room temperature. Plants tend to make unsaturated fatty acids, which is why oils (plant fats) are liquid at room temperature (although some plants do make saturated fats, like palm oil, coconut oil, and cocoa butter, which are solid at room temperature).

A saturated fatty acid will have the formula CH3(CH2)nCOO-, where n is the number of carbon units between the omega methyl group on one end of the chain, and the carboxy terminus on the other end. Unsaturated fatty acids have have many different formulae. For example, the formula for linolenic acid is CH3CH2(CH=CHCH2)3(CH2)6COO-. From this, you can see that there are 3 sets of double bonds along the length of the chain, starting from the third carbon up from the omega methyl group.

So, what about the polar bears?

Okay, to get to the polar bears, we have to start in the ocean depths, where it is very cold. It is so cold down there that the saturated fats we animals make freeze solid. But there are fish that swim down there. If these fish had to get along with nothing but the saturated fats they make, it would be very hard for them to swim, because they wouldn't be very flexible. However, these deep ocean fish eat a lot of plakton, and plankton produce unsaturated fats. The fish eat so much plankton that they are rich in unsaturated fats, and can remain flexible and active in the cold ocean depths. In particular, these fish have a lot of the omega-3 fatty acids, which have their first double bonds between the 3rd and 4th carbons from the omega methyl end of their chains. Linolenic acid is an example of an omega-3 fatty acid found in terrestrial plants, and omega-3 fatty acids like eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA) give fish like salmon their oily flavor and texture.

Now, these fish get eaten by marine mammals, like seals and sea-lions, and whales and such. These marine mammals basically concentrate omega-3 fatty acids, and other unsaturated fats, in their tissues because they eat omega-3-rich fish (which concentrate the unsaturated fats from plankton). Seals happen to be the favorite food of polar bears. The polar bears eat the seals, further concentrating the unsaturated fatty acids in their tissues. In fact, there is a measurable difference between the levels of omega-3 fatty acids in polar bears that are actively hunting and eating seals, and bears that are starving or hybernating.

So with respect to your question, the contrast is between polar bears and other land mammals. Most land mammals have very low levels of unsaturated fats, because we make our own saturated fats and we (land mammals) don't eat a lot of plants that are high in unsaturated fat. The polar bear is arguably a marine mammal, but it is at the top of a marine food chain (plankton -> fish -> seals -> polar bear), which explains polar bears enjoy such high levels of unsaturated fatty acids.

So, polar bears have high levels of unsaturated fatty acids, which are liquid at room temperature, because they eat marine animals that are rich in unsaturated fatty acids. This probably probably has an added benefit in that it helps polar bears survive the cold conditions of the arctic (both on land and in the water).

Here are some references if you want more information:

For deep discussions of fatty acids, I suggest looking through a college-level biochemistry textbook, like Biochemistry by Lubert Stryer.

For more information on polar bears, I suggest, Polar Bear: Habitats, Life Cycles, Food Chains, Threats by Malcolm Penn or The World of Polar Bears by Virginia Harrison and Martin Banks.

I also have two papers from the 1980s that discuss the origins of the omega-3 fatty acids found in polar bears.

Ackman RG. (1988) Some possible effects on lipid biochemistry of differences in the distribution on glycerol of long-chain n-3 fatty acids in the fats of marine fish and marine mammals. Atherosclerosis. Mar;70(1-2):171-3.

Innis SM, Kuhnlein HV. (1987) The fatty acid composition of Northern-Canadian marine and terrestrial mammals. Acta Med Scand.222(2):105-9.