Why did the arctic fox change its Colour What is the main cause that changes the Colour of the fur?

Fur colour in the Arctic fox: genetic architecture and consequences for fitness (2021) Tietgen et al., Proceedings B, https://doi.org/10.1098/rspb.2021.1452

The Crux

Researchers who try to understand the dynamics of wild populations often look at how different traits affect the survival and reproduction of different individuals within those populations. Usually, the investigated traits are visible and easy to observe, like an animal’s size or their colour. However, there may be cases where the important traits are not as conspicuous or even hidden behind more striking features.

The arctic fox (Vulpes lagopus) occurs with two distinct fur colours, often called morphs. The two most common are the white morph and the blue morph. Which of these morphs is more common depends on the population. In Norway, the white morph is more common but in recent years an apparent increase in foxes of the blue morph has been observed. Previous research has shown that blue arctic foxes are usually fitter, but until now there hasn’t been a good explanation of why.

We wanted to dive a little bit deeper into the differences between the two colour morphs, explore the genetics behind this trait and seeing whether we could find any “hidden” traits connected to fur colour that could explain the difference in fitness between the two morphs.

What We Did

The first thing we did in our study was to perform a genome-wide association study (GWAS). A GWAS makes use of known portions of genetic code that are distributed across the genome of the study species. This allowed us to find associations between different regions of the genome and the specific trait we were interested in, fur colour. In other words, if a trait has a genetic basis and isn’t a product of the environment, the GWAS helps us pinpoint the exact gene controlling the trait.

We also investigated which genes lie in the same genomic region as the fur colour gene, to see whether those genes could help explain any differences in fitness.

After using the GWAS to identify the gene underlying arctic fox fur colour, we had a closer look at how selection might affect arctic foxes with different fur colour genotypes. We used the Norwegian arctic fox monitoring programme to compile a data set containing data on how different arctic fox individuals fared in terms of fitness. This dataset included genotypes, meaning we could find differences between arctic foxes with different fur colour genotypes.

Due to extensive hunting, the arctic fox was close to extinction across Fennoscandia (Norway, Sweden and Finlad) in the beginning of the 20th century. The species was then protected across the region in the 1920s and 30s. In both Sweden and Norway, major conservation projects have been in place since the early 2000s in an effort to protect the arctic fox from extinction and increase its population size in Fennoscandia. The Norwegian Institute for Nature Research (NINA) started the Norwegian Arctic Fox Captive Breeding Programme in 2000 and since 2006 over 400 captive bred arctic fox pups have successfully been released into the wild. In 2020, the Norwegian population was estimated to approximately 300 adult individuals.

What We Found

We were able to identify a gene called MC1R (melanocortin-1-receptor) as the basis for arctic fox fur colours. MC1R comes in two versions, each with different alleles, C and T. A fox with two C alleles (a CC genotype) has the white colour morph, whereas a fox with one or two T alleles (TC or TT genotype) has the blue colour morph.

We also found that foxes with the TC genotype (blue foxes) had higher fitness than foxes with the CC genotype (white foxes). Specifically, theyhad a higher survival probability and produced more offspring. Interestingly, this advantage in fitness was more pronounced in female foxes.

Lastly, we identified a wide range of genes in the same region of MC1R and many of these are known to play a role in vital functions such as immune response, stress resistance and metabolism.

Problems

Unfortunately, we had only a handful of foxes with the other blue morph genotype (TT genotype) in our data set and could therefore not include these in the analyses. It would have been interesting to see whether there are any fitness differences between the TC and TT genotypes as this would have been a strong indicator that these differences are not solely due to the actual fur colour.

So What?

With our study we confirmed previous findings that arctic foxes of the blue genotype have a fitness advantage in the Norwegian population and were able to point to both survival and reproductive advantages. Information like this is especially important in conservation efforts, especially given the arctic foxes status (see Did You Know). It allows us to plan for which families or populations are more likely to flourish in different environments.

Not only did we learn more about the dynamics of the endangered arctic fox population in Norway, but we also showed how knowledge about the genetic basis of the traits that drive fitness can be used to look beyond the trait. This approach will also help unravel the details of how other species’ wild populations work, and why some individuals do better than other. Many previous studies have failed to point out single genes lying behind fitness-altering traits, and it was exciting to show that this was possible.

Lukas Tietgen finished his master’s degree in ecology last year and has recently published the work of his thesis as an article. Since finishing studying, Lukas has contributed to different projects of the Norwegian Institute for Nature Research, including the Arctic fox project. 

The arctic fox (vulpes lagopus) is a small fox found in the northern hemisphere. They live in cold climates and are well adapted to frigid environments. It’s a well-known fact about arctic foxes that they are white to blend in with the snowy white arctic landscape, but what happens to them in the summer when parts of the tundra melt. Do arctic foxes change color?

Yes, arctic foxes change the color of their fur depending on the season which makes it harder for predators to spot them. The arctic fox’s coat color will be white during winter months to blend in with the snow, then they will switch to earthy tones such as brown or grey during summer months.

Keep reading to find out why and how they change the color of their fur coat and some other adaptions they make to help them survive.

Does an Arctic Fox Change Color?

Watch the video version here or scroll down to keep reading:

Arctic foxes are best known for their thick white fur that helps them blend in with the snow of the arctic and ice floes. This provides camouflage to protect them from their predators.

However, as the snow melts in the warmer months of the year and they migrate to areas with less snowfall, they don’t keep their white fur as this would make them too obvious to predators.

Instead, arctic foxes shed their white coat for more earthy tones such as brown, grey, or sometimes even red fur. This helps it camouflage against the vegetation and rocky terrain in the tundra and the edges of forests where they can be found in summer (source: National Geographic YouTube Channel).

As well as changing the color of their coat, they also change the thickness to help with insulation, but more on this later.

However, one variant known as the blue morph arctic fox does not grow a white winter coat. Although it does still change color slightly, from a dark gray/blue coat in summer to a lighter gray coat in winter (source: PubMed, DI Våge, et al, 2005). The blue morph arctic fox is very rare with estimates putting the population of blue morphs between 1-3% of the total arctic fox population (source: Norwegian Polar Institue).

Most of the arctic animals you know will keep the color of their fur year-round such as the polar bear and snowy owls. However, arctic foxes aren’t alone in changing color between seasons. Both the arctic hare and the stoat have a white coat in winter to better camouflage against the snow (source: National Geographic).

How Do Arctic Foxes Change Color?

Seasonal molting occurs in around 21 different species from 5 families of birds and mammals and all work in a similar way whether it is feathers or fur.

The color variation is determined by melanin pigments which are responsible for giving arctic foxes their brown/grey color during summer (source: Italian Journal of Animal Science, J Bao, et al, Vol 14, Issue 3, 2015).

You’ll notice variations in the summer coat color of arctic foxes depending upon where they live. For example, those in wooded areas may be more red/brown whereas those in rocky areas may be more grey to help them blend in.

This is because there are two types of melanin pigment – eumelanin and phaeomelanin.

Eumelanin results in black/brown colors whereas phaeomelanin results in yellow/red colors. Arctic foxes in different areas will have a different ratio of the two which has evolved over time (source: Biological Reviews, M Zimova, et al, vol 93, Issue 3, Aug 2018).

When Do Arctic Foxes Change Color?

Arctic foxes molt their white winter coat around May in favor of their thinner brown/grey summer coat. This lasts them through the summer months until they begin growing their white winter coat again in September (source: PubMed, DI Våge, et al, 2005).

It can take several months for them to fully grow their winter coat and it may not reach full length until early December.

Why Do Arctic Foxes Have Thick Fur?

Arctic foxes have adapted to life in the extreme conditions of the arctic tundra by being able to live with little food and water, having thick fur for insulation, and white fur so they can blend into their surroundings. This means that arctic foxes do not need to hibernate or migrate.

Their winter fur is 200% thicker than their summer fur (source: PubMed, DI Våge, et al, 2005) and at a microscopic level contains more air which makes them more effective at keeping the fox warm.

There have been suggestions that the white fur loses less heat via radiation although this has never been proven (source: Biological Reviews, M Zimova, et al, vol 93, Issue 3, Aug 2018).

They also grow additional hair on their feet for better insulation and traction during snowy seasons, having lots of hair on their feet helps to prevent slipping as they race across the icy terrain (source: National Geographic).

Their thick busky tail is also useful for keeping warm in winter and similar to cats is used to help them maintain balance.

The Effect of Climate Change on Arctic Foxes

Climate change is having an impact on the habitat of the arctic, notably the melting of snow which has resulted in arctic foxes becoming endangered in some regions of Fennoscandia. This can result in a mismatch between the color of their winter coat and their surroundings.

This makes their camouflage less effective and can result in them becoming more susceptible to predators. It also has an impact on their ability to catch food as they use camouflage to sneak up on their prey. This is one of the reasons why red foxes are flourishing in areas previously dominated by arctic foxes (source: Climate Change Biology, L Hannah, 2011).

Scientists expect that arctic foxes will adapt the color of their winter coat over time (by natural selection) or may even lose their winter coat altogether (source: Biological Reviews, M Zimova, et al, vol 93, Issue 3, Aug 2018).