What Makes a Polar Bear…a Polar Bear?

Polar bears are universally recognized the world over. Their large body size, white fur, hooked claws, and small ears are all defining features of a predator that is highly adapted to the Arctic environment. But how did these features evolve? In short, it all comes down to genetics.

A primer in genetics

One of the single greatest discoveries made in the fields of biology and medicine in the 20th century was the structure and function of DNA. James Watson, Francis Crick, Rosalind Franklin, and Maurice Wilkins are all credited with putting together the pieces of the puzzle to identify the elegant double helix structure of DNA. Collectively, they identified both the structure of DNA and how it replicates itself.

DNA is like the blueprint to a house, where specific sequences of nucleotides, called genes, lay out the plans for constructingproteins, which go on to combine, like bricks and mortar, to form all the structures in a cell and body. Genes and gene expression control everything that happens in life. They form the foundation for the biological diversity that exists today.

Moreover, these blueprints are handed down from one generation to the next. The long strands of DNA are organized into chromosomes. Children inherit these from their parents, combining the blueprints from mom and dad, sometimes with slight modifications. Some traits are controlled by a single gene, but many are influenced by multiple genes. Much of the field of genetics is focused on identifying the genes that control various traits of an organism and understanding how they are inherited and expressed from one generation to the next.

The polar bear genome is approximately 3.5 billion base pairs in length, with all those base pairs packaged into 37 pairs of chromosomes (humans have 23). And that is what makes a polar bear … a polar bear!

How did polar bears evolve?

At its most basic level, the process of evolution by natural selection involves individuals with beneficial traits leaving more offspring than individuals who lack those traits. If the traits are controlled by genes, then the frequencies of the advantageous genes will increase in a population in response to selective pressures. Although Darwin knew that traits were inherited from parent to offspring, it was Watson and Crick who described the molecular basis for that inheritance, later winning the Nobel Prize for their groundbreaking work.

In the 70 or so years since the discovery of DNA, science has advanced to the point where it is now possible to determine the entire sequence of DNA of any species, its genome sequence, for less than the cost of a plasma screen television. By sequencing the genomes of polar bears and brown bears we know that polar bears shared an ancestor with brown bears roughly 500,000 years ago. This is a relatively recent branching off in the bear evolutionary tree and has become a well-known example of rapid evolution.

So, what can genetics tell us about how polar bears have adapted to the Arctic in such a short time frame? Recent research suggests that both pre-existing genetic variation in the genome and novel mutations (random changes/errors that are made when DNA is being copied) are likely to have played an important role in the rapid emergence of polar bears.

Adapting to Arctic life

For instance, there are signs of selection in genes associated with the most obvious and striking adaptation in polar bears: their coat color. Brown bears can have fur ranging in color from dark brown to blonde, but polar bear fur lacks pigmentation, making it appear white. You can imagine how a white bear would have a greater chance of sneaking up on prey on the sea ice and therefore may have been more successful in passing on their genes to the next generation.

Polar bear skulls and dentition also changed significantly as they adapted to life in the Arctic. Adaptations include sharpened molars, which allow polar bears to shear off pieces of frozen seal. Brown bears, on the other hand, have flat molars that allow them to grind up the vegetation and berries that form a large component of their omnivorous diets.

In addition, polar bears had to adapt to the high fat content of their marine mammal prey. Because of their fat-rich diet, polar bears have high levels of LDL cholesterol (the “bad” cholesterol), which in humans represents a significant health risk. However, because of their genetics polar bears do not have fatty deposits in their arteries despite their fat-rich diets. These are just a few examples of how genes have influenced polar bear evolution. Genes controlling body fat, fatty acid metabolism, heart function, and fur pigmentation may have all played significant roles in the adaptation of polar bears to the Arctic marine environment.

Insights into polar bear ecology and the future

What else can genetics tell us about polar bears? Having a DNA sample from an individual bear gives us their unique genetic fingerprint. Like DNA profiling on your favorite crime investigation series, DNA profiles of polar bears allow scientists to track individual bears through time to look at variations in their survival and reproduction. The data gained helps inform our understanding of changes in population size and growth.

Having individual DNA profiles also provides detailed insights into the polar bear mating system. Observations of polar bear mating are extremely rare. However, by using individual genotypes researchers can build pedigrees that contain both maternal and paternal assignments. As a result of long-term research in Western Hudson Bay led by Environment and Climate Change Canada, researchers have been able to develop a pedigree containing over 4,300 individuals spanning six generations of bears.

Delving into the pedigree, researchers have found several interesting things. For instance, the pedigree has provided evidence of identical twins in polar bears, the first and only case of identical twins in any bear species. Researchers have also identified several cases of cub adoption: Females were observed in the field taking care of cubs that ended up being genetically unrelated to them. This unique behavior was first described in polar bears in the mid-1990s and nobody is sure as to why it happens. Polar bear mothers may be so primed to look after their cubs that they are willing to adopt cubs that appear orphaned or are on their own.

The pedigree has also provided valuable insight into male mating success and has shown that prime aged males between 10-18 years of age do most of the mating. In addition, by looking at litters with multiple cubs we know that some cubs in the same litter have different fathers.

So why is understanding polar bear genomics important? We know from many species that genetic diversity enhances the probability of population survival over time. Thus, understanding how their genetic diversity is distributed among the world’s polar bear populations is an important first step to assessing the potential ability of the species to adapt to environmental change, including ongoing climate warming. Although random mutations can result in adaptation to novel new environments, the standing genetic variation in populations forms the bulk of the raw material for adaptation and change. Assessing and conserving the genetic variation that exists among the world’s polar bear populations is an important first step for the long-term conservation of the species.

We hope all of this has left you with a new appreciation of what makes a polar bear a polar bear and the processes that resulted in evolution of this Arctic icon.

Dr. Evan Richardson is a polar bear research biologist with Environment and Climate Change Canada. Dr. Joshua Miller is a postdoctoral researcher funded by Polar Bears International and the San Diego Zoo Wildlife Alliance.