Western Howgills

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Saunterings:  Walking in North-West England

Saunterings is a set of reflections based upon walks around the counties of Cumbria, Lancashire and North Yorkshire in North-West England (as defined in the Preamble). Here is a list of all Saunterings so far.
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113.  White Stoats on Caton Moor

Our local hill, Caton Moor, received its first snow of the winter on New Year's Eve, so, as is almost traditional, we walked up the hill as soon as we could, bright and early on New Year's Day. We thus began the year as we are intended to carry on, that is, by walking from home. I had the ulterior motive of looking for white stoats but I didn't mention this to Ruth, as I didn't want her to feel disappointed when we didn't see any.

The roads were still white and icy but we walked up gingerly, appreciating the views of the Lune valley and the gradual revelation of the surrounding hills. At first, we could see only the Howgills and Barbon Fell up the valley, but eventually Gragareth, Whernside and, after passing Quarry House Farm, Ingleborough came into view. Their tops were smudged by grey cloud but, as far as I could tell, they appeared to have less snow than Ward's Stone, just off to the south. But the highlight was behind us. The Lake District hills appeared over the ridge on the north bank of the Lune and they too were mainly under grey cloud except for a sunny patch that made the Coniston hills and then the Langdales seem aglow.
ld hills

Towards the Lake District hills (about 30 miles away), Black Combe on the left, Coniston hills in sunshine

Now, about those white stoats ... Handel, Vivaldi and other composers of that vintage thought nothing of recycling their compositions to meet their commitments. Writers nowadays are liable to re-use their words in various forms - newspaper columns, books, anthologies, even in films if they are lucky. I have more excuse than them for borrowing from myself. I am supposed to be writing about walking in the northern hills and dales but I can't walk there, for the time being. So, as it happens, I wrote something about white stoats five years ago which I can regurgitate here (slightly edited) ...

You can’t really set out to see a stoat. Stoats are seen by chance, if they are seen at all. But I had resolved that when the first snow of the winter fell on Caton Moor then I would set off in search of a white stoat. By chance I have, during occasional visits in the last 35 years, seen two white stoats on Caton Moor. I don’t know if I am lucky to have seen as many as two, or unlucky to have seen only two. I just don’t know how common white stoats are on Caton Moor. I have also seen several brown stoats not on snow. I’ve never seen a white stoat without snow nor, I think, a brown stoat with snow. These observations, scanty though they are, provoke a number of questions in my febrile mind.

brown stoat As is well known, stoats in places where there is plenty of snow, such as the Cairngorms, turn white in winter and stoats in places where there is little snow, such as Dorset, do not turn white. But how does the mechanism work in intermediate places like Caton Moor, where snow is patchy and unpredictable? What causes the change? Is it in anticipation of snow, or in response to it? Does it occur gradually or quickly (like human hair that turns white overnight as the result of some trauma)? Is it an adaptation to the environment, like that of a chameleon? Does turning white occur once each winter, or could a stoat turn white, then brown, then white in response to snowy periods? If you took a Dorset stoat to the Cairngorms would it turn white? If you took a Cairngorms stoat to Dorset would it turn white? If turning white is such a nifty strategy, then do other species adopt it? Do all stoats in a particular location turn white or do they all not turn white? If not, why not? Are the numbers of white stoats decreasing, in response to climate change?

I have found the answers to some of these questions in the book The Natural History of Weasels and Stoats (King and Powell, 2006). It seems that the white stoat provides a pristine case study on the interaction between genes and the environment.

First of all, some preliminaries. Stoats are members of the Mustelidae family, which also includes weasels, minks, ferrets, martens, badgers and otters. The stoat Mustela erminea and weasel Mustela nivalis are within the Mustela genus of this family. Ermine is an alternative name for the stoat, usually used for the white stoat and for its fur. In the United States, Mustela erminea (our stoat) is called the short-tailed weasel and Mustela nivalis (our weasel) is called the least weasel or common weasel. They also have a long-tailed weasel. In Ireland Mustela erminea (our stoat) is usually called the weasel. There are no Mustela nivalis (our weasel) in Ireland. You could say that there are no weasels in Ireland, but the Irish might say that there are no stoats. Clearly, outside the UK, the weasel is not so easily distinguished and the poor stoat is totally confused.

Stoat and weasel have a huge range, across the whole northern hemisphere from western North America to eastern Asia. Within that range there are many climatic zones with prolonged snow cover. Some stoats and weasels live at 3000 metres in permanent snow. Snow is not a problem for stoats, as it is for many animals. With its long, thin, sinuous body the stoat is well-adapted to burrowing in grass and small tunnels and is therefore equally well-adapted to burrowing within snow, where it may seek prey, find safety from predators, and take refuge from the cold.

stoat and rabbit A stoat has quite an appetite, needing to eat up to one third of its body weight every day. This is because it leads such a frenetic life: it is alert, with rapid movements; its pulse runs at 500 beats per minute; it digests and defecates within two hours; and it doesn’t sleep for long. It can kill rabbits twice its weight. I once saw a stoat doing a strange leaping dance beside a hedge. I then noticed that it had at its feet a dead rabbit. It was leaping up trying to get the rabbit into the hedge but it was too heavy and the stoat lost its grip. It eventually succeeded. On another occasion I saw a stoat disappear into a stone wall. I stood by the wall and eventually the stoat popped its head out, stared at me, squeaked, and went back inside. It repeated this performance every minute or so. The squeaks became gradually more threatening so, bearing in mind what a vicious killer the stoat is, I thought I had better move on.

The stoat does not hibernate in winter. With its slim, fat-free body, it needs twice as much energy to retain its body heat in winter as it does in summer. It is therefore essential that the stoat be adapted to survive harsh winters. Now we can consider the change to white. Stoats moult twice a year, in spring and autumn. The new fur, replacing the old, is brown except for autumn moults in cold climates, when it is white. The moult does not occur instantaneously and therefore stoats may be seen at an intermediate brown-white stage. In mild climates (such as here) the moult can take a month; in the Arctic it takes a few days.

The moult is triggered by the hours of daylight. This is easily demonstrated by manipulating the lighting over the cages of captive stoats. They can be induced to moult at any time of the year, even if the temperature is not consistent with the apparent sunlight. In this respect, the stoat is similar to other animals that moult. If the temperature or some other environmental factor were the sole determinant then stoats transferred from, say, the Cairngorms to Dorset (or vice versa) would turn white or not according to the conditions in their new home - but they don’t. They moult at the usual time but into the ‘wrong’ coat. So the colour of the new fur is controlled mainly, if not entirely, by heredity.

British weasels do not turn white. Why not? If it's a good idea for stoats to turn white, why isn't it for weasels too? Swedish weasels do turn white - or at least those in north Sweden do while those in south Sweden stay brown (rather like the British stoat divide). However, the two sets of Swedish weasels are two different subspecies (the two sets of British stoats are not). British weasels belong to the same subspecies as the south Sweden weasels. Therefore the reason that British weasels don’t turn white may be more to do with their genes and evolutionary history than the climate.

In the United States it was found that the boundary between white and brown winter stoats was at points where there was an inch of snow for fifty days of winter. In Britain stoats whiten in somewhat milder winters (Caton Moor normally has an inch of snow for only a few days of winter). The boundary line divides Wales, Scotland and parts of northern England from the rest of England - but of course it is not a precise, single line, as mountain-top stoats are more likely to whiten than low-level ones. Caton Moor, with a highest point of 361 metres, is hardly a mountain-top.

The boundary between white stoats and brown stoats is not a line but a transition zone. Within that zone (which includes Caton Moor) more or less stoats turn more or less white. Transition zone stoats are usually pied, rather than fully-white or fully-brown (actually, fully-white stoats are not fully white: they retain the black tail tip). This suggests that our stoats are a hybrid of northern fully-white genes and southern fully-brown genes. The colder or snowier a region normally is, the more pied stoats there are (and, I would guess, the more white they are). In the transition zone female stoats are more likely to turn white than male ones. Perhaps the gene that determines whitening is dominant in one sex and recessive in the other. This would ensure a genetic polymorphism so that the population always has some individuals with every combination, in which case some will benefit whatever the weather conditions turn out to be.

white stoat The reason that stoats turn white is obvious. It confers a clear evolutionary advantage. Stoats have much to gain by being able to live in snowy conditions but the penalty for being brown against a white background or white against a dark background is large. They become much more visible to their predators - hawks, owls and foxes. Other species, such as the mountain hare, arctic fox, ptarmigan and caribou, also turn white in winter. It is also clear how this is a self-regulating mechanism. Those stoats that moult to an inappropriate colour are more likely to be predated and therefore less likely to pass on their ‘inappropriate’ genes.

I have found no discussion of the effect of climate change on the transition zone for stoat-whitening - but I would expect the zone to be moving north. Forty years ago there were ski-orienteering events organised on the Howgills. The organisers could be confident that there would be enough snow. Today, the odds are that a winter date would see no or little snow on the Howgills. Ski-orienteers are now more commonly found further north. I expect white stoats are too.

So my expedition to snowy Caton Moor was made not in the belief that the brown stoats would suddenly have turned white in response to the fresh snow. My hope was that, after this exceptionally mild and wet winter (so far), those stoats unfortunate to have genes that have turned them white would have sufficient self-awareness to have hidden themselves away over the last few weeks in order not to make an easily-predated spectacle of themselves but that, now that the snow has fallen, they would be gambolling about, in their element. Unfortunately, it proved not to be the case. I saw no stoats, white or brown. On reflection, given the decreasing occurrence of snow on Caton Moor, I think it unwise for any stoat to turn white there. I suspect that white stoats have disappeared but I will keep looking and if I see one I’ll let you know.

... Now, back in 2021, we saw no white stoats on our January 1st walk and I haven't seen any in the intervening years either. Not to worry, it was an invigorating start to the New Year. We walked up to beyond the windmills, which had just begun to be stirred into action by a light breeze, and admired the snowy hills all around, with only a few in the glow of sunshine. By the time we set off down, a thaw had set in, which made the roads more slippery, but we made it safely back.

Clougha Pike and Morecambe Bay from above the windmills

    Date: January 1st 2021
    Start: SD543644, Brookhouse  (Map: OL41)
    Route: SE, E on Quarry Road – picnic site – along the windmill track, to the highest corner of the field – and back
    Distance: 5 miles;   Ascent: 250 metres

     114.   Never Mind the Danger
     112.   Walking around Pilling with Pink Feet
               A list of all Saunterings so far

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    © John Self, Drakkar Press, 2018-


Top photo: The western Howgills from Dillicar; Bottom photo: Blencathra from Great Mell Fell