How do Deer keep warm in winter?

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by Helga Guderley

How do deer keep warm in the winter?

Winter is a hard season.  While we relish going for a walk or ski in the snow, enjoying the changes in our familiar landscapes, most of us can warm up in our cozy homes afterwards.  We enjoy our parkas and warm boots, but how do deer manage?  

Deer maintain their body temperature at 37C, just like us.  To keep their core at 37C when experiencing subzero temperatures, they use several approaches.  First, they increase their insulation.  In preparation for winter, deer gradually change their coats to have thicker, longer and darker hairs and grow a thicker undercoat.  If needed, they make “their hair stand on end” to trap more air in their coats and insulate better.  Oil glands help their coats shed water. Choosing protected areas as dens is crucial to overwintering survival.

Deer use counter-current heat exchangers to prevent blood from the extremities from decreasing core temperatures.  Deer legs are primarily composed of bone, tendons and skin; tissues that can withstand cooler temperatures than the vital organs.  Blood destined for the legs leaves the central core in arteries that run right next to the veins that return blood from the legs to the body. The blood that has cooled in the legs warms up before entering the central circulation, whereas the outgoing blood is cooled.    
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Deer reduce their activity in winter, but they do browse on available plants (the yew and rhododendrons in my garden are very popular!) to obtain energy and nutrients for their organs and for their microbiome.  Under harsh winter conditions, wild ruminants may break down their organs (muscle in particular) to provide nitrogen to the microflora in their rumen.  This ensures that when food becomes available in the spring, they can digest it properly.  

​Even though it looks uncomfortable to spend winter out in the elements, deer have clearly figured out how to do it!

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Helga Guderley is a retired professor of Biology, having worked at Université Laval for 30 years.  She has served as an adjunct at Dalhousie for 14 years. She is a founding member of the Healthy Forest Coalition, and has taken the health of Nova Scotia's natural environment, in particular its biodiversity and forests, to heart since retiring here in 2010.  ​