Devil in a blue dress: colour morph affects fitness in Arctic foxes

This blog post is provided by Cecilia Di Bernardi and tells the #StoryBehindThePaper for the article “Fitness and fur colouration – testing the camouflage and thermoregulation hypotheses in an Arctic mammal, which was recently published in Journal of Animal Ecology.

Living in extreme environments leads to a wide range of adaptations in Arctic and alpine animals, which have drawn scientists’ attention and fascination for a long time. Among these, body colouration and the ability to camouflage – hide – in the snow seem to play a crucial role, with many Arctic species changing their colour to white each year when the snow season comes. For example, prey species such as ptarmigan, snowshoe hare and weasel can benefit from a white body matching the snowy background, to better hide from predators. Camouflage has been widely recognised as a significant ecological driver of animal colouration in the Artic and beyond. Conversely, the relative importance of colouration in thermoregulation has received mixed empirical support, with some studies suggesting a potential thermal advantage for darker individuals in cold environments. The Arctic fox provides an interesting model species for testing alternative hypotheses, because the species occurs in two different colour morphs (types) – white and blue – and both morphs change their fur seasonally. Both morphs occur in most populations, but the blue phase is more common on islands and at coastal sites, and the drivers of variation in their spatial distribution remain unknown. It is also unknown if the fur colour of Arctic foxes influences components of fitness such as reproductive performance or survival.

A mated pair of blue and white Arctic foxes in their winter pelage at the Sæterfjellet breeding station near Oppdal (Norway), a captive breeding and release programme for species recovery in Norway. Photo credit: Nils Kristian Grønvik
Examples of the two most common colour morphs in the Arctic fox: white (left) and blue (right) adults in their summer pelage in the Norwegian mountains. A third colour morph of arctic fox (sandy) occurs at low frequency (<1%) but was not included in our study. Photo credit: Espen Lie Dahl.

In our 11-year study on fitness of Arctic foxes in Norway, recently published in Journal of Animal Ecology, we explored whether camouflage and thermoregulation might explain differences in reproduction and survival between the two colour morphs of the Arctic fox under natural conditions. We took advantage of the long-term monitoring of Arctic foxes in Scandinavia and the Norwegian Arctic fox captive breeding programme to analyse long-term data on demographic performance for 798 Arctic foxes in southern Norway. Our field study provides the first evidence for an effect of colour morph on the fitness of Arctic foxes, where blue foxes had a higher probability of breeding and higher adult survival compared to white foxes. Moreover, winter temperatures had a stronger effect on juvenile survival of blue foxes, with higher survival at low temperatures, but lower survival at higher temperatures. We found no evidence for a camouflage advantage of white foxes in the snow, and a possible explanation might be the relatively low numbers of larger predators in our study areas. What could instead drive the observed advantage of the blue morph? Our results point towards potential physiological adaptations and behavioural traits that might be linked to the genetic basis of fur colouration, but not necessarily connected to thermoregulation. As observed in other mammal and bird species, the darker morph of the Arctic fox could have a fitness advantage via associated physiological and behavioural traits such as stress response, immune function, sexual behaviour, and aggressiveness.

Blue and white morph pups of Arctic foxes at a den site in the Norwegian mountains. Photo credit: Lukas Tietgen.
A white morph adult Arctic fox with its winter pelage in the Norwegian mountains. Photo credit: Arild Landa.

The results of our study add to a growing body of evidence that fur colouration and a suite of associated traits can influence animal fitness under natural conditions. Our paper highlights the need to explore the underlying mechanisms by which the genetics of fur colouration influences the fitness of white and blue Arctic foxes as well as other species with colour polymorphisms. Arctic and alpine species are among the organisms most threatened by climate change and understanding their potential responses to environmental changes is important for species recovery and conservation action.

Other information:
Landa, A., Flagstad, Ø., Areskoug, V., Linnell, J. D., Strand, O., Ulvund, K. R., … & Eide, N. E. (2017). The endangered Arctic fox in Norway—the failure and success of captive breeding and reintroduction. Polar Research36 (sup1), 9.

Blue and white morph pups of Arctic foxes. Photo credit: Ingolf Rotvei.

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