Seasonal coat color distribution of
Mustela frenata (Long-tailed Weasel) in
North America
Kelly Matheson
Winter Ecology – Spring 2006
Mountain Research Station – University of Colorado, Boulder
What influences the color of long tail weasel individuals
found in an area of transition?
The long tailed weasel is a very successful small
carnivores. In North America they exploit habitats ranging
from prairies to forests to arctic tundra and are
geographically distributed throughout the conintental U.S.
Seasonal coat color distribution of
Mustela frenata (Long-tailed Weasel) in
North America
1. Seasonal coat color change
2. Physiology of seasonal molt
3. Advantages of seasonal molt
4. Distribution of Mustela frenata
5. Factors influencing the molt
Seasonal coat color change
• All three species of N. Amercian weasels and other species of
mammals from arctic foxes, hares and lemmings, molt to a white
coat in the winter and brown coat in the summer
Seasonal coat color change
• In species that do change and have been
studied, the change from one color to another
is the result of a molt, the loss of fur, of one
color, and the growth of fur of a different color.
Fig. 1 Overview
of physiology of
molt and color
change in
weasels.
Feder, S. 1990
Why molt to white?
• White coat offers a thermal advantage •Feist & White suggest that a white coat may be better suited in cold,
sunny and windy conditions at wind speeds in excess of 3 m/s,
thereby allowing more incident UV radiation to the skin.
•But, insulative qualities of fur are dependent on length and density
of fur.
•According to King (1989), pelage depth and density do not
significantly change in weasels.
•And, smaller mammals like weasels have a larger surface area to
volume ratio, demonstrating an increased mass specific rate of
heat loss.
Why molt to white?
• White coat offers a thermal advantage •Length of fur and fat accumulation are limited by hunting behavior of
weasels.
•This coupled with the long thin shape of the weasel does not
appear to support the theory that a seasonal molt to white offers
any thermal advantage to the long tailed weasel.
Why molt to white?
• White coat offers a thermal advantage - Not
likely
•White coat serves the purpose of camouflage
•Weasels do not practice the sit and wait strategy of hunting.
•Solitary searchers, specializing in the ability to explore
every nook and cranny for small prey, cannot camouflage
movement.
Why molt to white?
• White coat offers a thermal advantage - Not likely
•White coat serves the purpose of camouflage
A white winter coat is not an adaptation to cold but
rather for camouflage, both to avoid predators and
to aid in the pursuit of prey.
Fig. 2 Limit of
winter whitening
in long tailed
weasels (Mustela
frenata).
(map redrawn from Hall 1951)
Suggested influences of north/south
distribution
• Temperature - populations differ in response to
temperature (King 1989)
– Pop. Living in climatic extremes do not need flexibility of
those found in area of transition. A temp. switch is what
determines color of molt.
– Studies by Rothschild(1942), Feder (1990)…etc
demonstrated coat color is directly related to area of origin
for individuals.
– Unlikely that mechanism determining color change would be
present at the individual level rather then population as a
whole.
Suggested influences of north/south
distribution
• Assuming an individual changes color to escape
detection, then it makes sense that the transition
zone is maintained by natural selection.
• The disadvantage of remaining brown must be
severe because all Arctic weasels turn white in the
winter
Suggested influences of north/south
distribution
• Hewson & Watson (1979) looked at the relationship
between winter whitening, snow cover, and
temperature in Scotland and Yorkshire
• Found that differences in proportion turning white
between different regions and altitudes within regions
were positively associated with number of days of
snowfall and snow lie and inversely with monthly
minimum temperature.
What we do know?
• The fall molt is initiated by photoperiod.
• Individual fall molt color change appears to be genetically
determined.
• Days of snow lie, not temperature, influence proportion of white
individuals found.
• The color of hair growth following molt appears to be determined
by the presence or absence of MSH - acts as an intracellular
blocking factor
Where are we going?
• Would transition zone outlined by Hall (1951) be located in
same areas today?
• DNA and breeding experiments over several generations.
• May provide an opportunity to study natural selection in short
time frame.
• Is new white growth the result of the prolactin inhibition or the
high levels of melatonin that cause inhibition of MSH?
References
•Bissonnette, T. and E. Wilson. 1939. Shortening daylight
periods between May 15 and September 12 and the pelt
cycle of the mink. Science. 89:418.
•Feder, S. 1990. Environmental determinants of seasonal
coat color change in weasels (Mustela erminea) from two
populations. M.S. Thesis. University of Alaska Fairbanks.
•Feist, D. and R. White. 1989. Terrestrial mammals in cold. In
advances in Comparative and Environmental Physiology, vol.
4, edited by L. Wang, Springer-Verlag Berlin Heidelberg.
•Hall, E.R. 1951. American Weasels. University of Kansas
Museum of Natural History. 4:1-466.
•Hewson, R. and A. Watson. 1979. Winter whitening of stoats
(Mustela erminea) in Scotland and Northeast England.
Journal of Zoology. 187:55-64
References
•King, C.M. 1989. The Natural History of Weasels and
Stoats. Comstock Publishing Association., Cornell University
Press, N.Y.
•Rust, C.C. 1965. Hormonal control of pelage cycles in the
short tailed weasel (Mustela erminea bangsi). General and
Comparative Endocrinology. 5:222-231.