Sammendrag
1. Selection for crypsis has been recognized as an important ecological driver of
animal colouration, 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.
apparent survival, Arctic fox, camouflage, capture–mark–
recapture
models, colour
polymorphism, fitness, reproductive performance, snow cover
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