Fitness and fur colouration - testing the camouflage and thermoregulation hypotheses in an Arctic mammal

1. Selection for crypsis has been recognised as an important ecological
driver of animal coloration, whereas the relative importance of
thermoregulation is more contentious with mixed empirical support. A
potential thermal advantage of darker individuals has been observed in a
wide range of animal species. Arctic animals that exhibit colour
polymorphisms and undergo seasonal colour moults are interesting study
subjects for testing the two alternative hypotheses: demographic
performance of different colour morphs might be differentially affected by
snow cover with a cryptic advantage for lighter morphs, or conversely by
winter temperature with a thermal advantage for darker morphs. 
2. In this study, we explored whether camouflage and thermoregulation
might explain differences in reproduction and survival between the white
and blue colour morphs of the Arctic fox (Vulpes lagopus) under natural
conditions. 3. Juvenile and adult survival, breeding propensity, and
litter size were measured for 798 captive-bred and released or wild-born
Arctic foxes monitored during an 11-year period (2007-2017) in two
subpopulations in south-central Norway. We investigated the proportion of
the two colour morphs and compared their demographic performance in
relation to spatial variation in duration of snow cover, onset of snow
season, and winter temperatures. 4. After population re-establishment, a
higher proportion of blue individuals was observed among wild-born Arctic
foxes compared to the proportion of blue foxes released from the captive
population. Our field study provides the first evidence for an effect of
colour morph on the reproductive performance of Arctic foxes under natural
conditions, with a higher breeding propensity of the blue morph compared
to the white one. Performance of the two colour morphs was not
differentially affected by the climatic variables, except for juvenile
survival. Blue morph juveniles showed a tendency for higher survival under
colder winter temperatures but lower survival under warmer temperatures
compared to white morph juveniles.  5. Overall, our findings do
not consistently support predictions of the camouflage or the
thermoregulation hypotheses. The higher success of blue foxes suggests an
advantage of the dark morph not directly related to disruptive selection
by crypsis or thermoregulation. Our results rather point to physiological
adaptations and behavioural traits not necessarily connected to
thermoregulation, such as stress response, immune function, sexual
behaviour, and aggressiveness. Our findings highlight the need to explore
the potential role of genetic linkage or pleiotropy in influencing the
fitness of white and blue Arctic foxes as well as other species with
colour polymorphisms