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Ermines turn white during the winter, with only the tip of the tail remaining black. However, in the southern regions of the ermine's range it clearly doesn't get so white. Encyclopedia of Life says that:

However, in the Netherlands, the animals are often only partially white.

Ermine's range is clearly much more southern than the Netherlands:

image of the range of the ermine

Wikimedia, stoat range

So I was wondering where does the border go, north of which the ermines are completely white and south of which they remain more brown? I understand it necessarily is not a clear cut line, mountains have impact and subspecies may differ, but I would assume someone must have mapped this. Or is it just weather related, will a southern ermine turn white if the winter is really cold?

I'm just asking this out of curiosity, but the question is rather relevant. Ermine pelt has a long history as an important trade item, and the most valued pelts must have originated on the white side of this line.

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The color change of Mustela erminea (more commonly known as ermine/stoat/short-tailed weasel) is both temperature-dependent and photoperiodic (depending on the length of the day). Bissonette 1943 and King and Moody 2012 show that the molting of the ermine happens as a result of the change in daylight lengths. The King and Moody paper addresses the temperature dependence, using stoats (ermines) in New Zealand as an example:

In temperate countries such as New Zealand, stoats frequently fail to complete the full change to white. Such animals are not necessarily all caught in the process of moulting. If ambient temperature fell below the threshold for whitening of the lower part of the body but not the rest, new hair would grow white on the tail and flanks and brown on the head and back.

This is a chemical effect of lower temperature (King and Moody):

Experimental work has established that production of brown hair depends on melanocyte-stimulating hormone (MSH); in cold climates in autumn the synthesis of MSH by the pituitary is inhibited.

They don't provide a specific temperature at which the white color change does or doesn't occur, but I suspect that in nature it'd be very difficult to create a map since temperature is not constant year-to-year. There would most likely be an intermediate zone of partially white animals (both in terms of a mixture of brown and white animals, and a single animal that is not fully white).

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