By Steve Lowe, Head of Conservation, Northumberland Wildlife Trust
For a majority of Northumberland’s mammals winter brings a new dimension to their constant struggle for survival. Evolution allows this to be tackled in one of two ways; either adopt physical and behavioural changes to deal with the impending foul weather or find a quiet, secluded spot and go into hibernation.
The distinction between the two is a hazy one, as no resident UK mammal goes into a true hibernation. For example, it is not uncommon to see bats or hedgehogs out of hibernation to feed if a run of warmer weather occurs in January or February.
For those species that do not enter any degree of hibernation, survival usually involves making a physical adaptation to deal with the adverse conditions. For most this will mean a thicker coat of fur. However some species take this change a huge step further and actually change colour to adapt to the white and snowy conditions.
Perhaps the most famous example of this is the Stoat (Mustela erminea). The reduction in temperature and daylight hours triggers a change in the Stoat known as ‘winter whitening’ in which its coat changes from the usual rusty brown to almost completely white, a state known as “ in ermine”. This adaptation gives the stoat greater camouflage from predators in snowy conditions but is also insulating and also provides added stealth when the stoats itself is hunting.
Traditionally this ermine coat was used to line the ceremonial gowns of kings and queens and was once a highly lucrative trade. Thankfully this trade is no longer viable!
A change in colour is also the preferred tactic for one of Northumberland’s most illusive mammals. The Mountain Hare (Lepus timidus) is the UK’s only native member of the rabbit family, and will change the colour of its coat from grey-brown to snow white (except the tips of the ears). This again is a method of camouflage, allowing the Hare to blend in on the exposed mountain sides it inhabits, safe from sharp eyed buzzards and other predators.
Both species turn white because traditionally their chosen habitats will be snow covered for the majority of the winter. However as winters become warmer and the trends in snow coverage decrease, this adaptation goes from help to hindrance. To see a Stoat in ermine is now a rarity, although 2011-12 seemed to be an exception, with over 80 records being received by Northumberland Wildlife Trust. It is simply no longer a viable strategy for stoats when their natural colour affords greater protection from predators. Just as a cold but snow-free hillside is no place for a brilliantly white hare.
It is too soon to say if this will be a permanent change in survival techniques, however studies in Shetland have shown that over the last few years Mountain hares have retained their natural grey-brown colour over winter, showing a potentially strong link between warmer winter weather and animal behaviour.
For anyone interested in learning more about Northumbria’s mammals I would recommend a small £10 investment in a copy of “Mammals, Amphibians and Reptiles of the North East”, produced by the Northumbria Mammal Group and available from the Natural History Society of Northumbria or the Northumberland Wildlife Trust.
Category: Northumbria Wildlife