Oh deer, even a mild winter is tough

By all accounts the winter of 2016 was very mild, but even the mildest winters in northern climates pose a significant challenge for animals. Some animals migrate, others hibernate, while a select few tough it out. White-tailed deer are a prime example of this last strategy.

Young white-tailed deer walking through the forest at University of Toronto Mississauga

In spring, summer and fall, times are good for deer. As herbivores, they feed on plants where favourite food include the tasty green leaves and stems of evening primrose (Oenothera spp.), the fruits of Trillium or the berries of poison ivy - yes, they eat the berries of this toxic plant!

In winter, life is a different story. Getting enough food makes the difference between life and death. Fresh leaves are gone and fruits are in dwindling supply. What is a deer to eat? Twigs from shrubs and trees are always an option, but a toothpick-like diet leads to the desire for a little diversity.

On my walk today I couldn't help but notice the signs of a deer's winter diet in abundant evidence. The first obvious winter food I saw was that of staghorn sumac (Rhus typhina),

Fruits of staghorn sumac. A favourite food of American Robins in early spring

or more specifically the bark and inner cambium (the living part of the trunk that move water and sugars throughout the plant) of this plant. The deer had used their lower incisors (they don't have upper incisors) to dig deep (~0.5cm) into the trunk on a number of sumacs.

Sumac bark and cambium stripped away by deer in winter

There are sumac relatives (Toxicodendron spp., like poison ivy and mango) that are highly toxic (and deer eat some of these species' fruits!), so I was naturally interested in how the bark tasted. Now I've tried bark and cambium before and typically it is a tough, tasteless and somewhat bitter affair. To my surprise the bark of R. typhina isn't that bad. It is soft, chewable, filled with a fair amount of water and it doesn't taste like much of anything. That is a lot better than being poisonous - a check on the edible index.

As I walked down into the valley (the Credit Valley of Mississauga, ON, Canada) a second major winter diet item of deer was evident - white cedar (Thuja occidentalis). 

Heavy deer browse on white cedar. The height of the browse is set by the upper reach of the larger deer

The deer in our area are so abundant that their effects on these cedars could be confused for a talented landscaper. All of the white cedars' leaves are stripped bare below a certain level, the height that is exactly as high as they can reach - hence the beautiful straight line trimming in the photo above. This diet option is a lot harder for me to understand. 

Have you ever tried to eat cedar leaves? Well, I did today - imagine the strong bouquet of a beautifully rich Christmas tree exploding in your mouth. That beautiful smell is bitter in the mouth and my lips were ever so slightly numb on the walk home - needless to say, don't try this at home. This taste (and the Xmas tree smell) is due to the rich diversity of terpenoids that cedars and other conifers produce. Actually many plants produce terpenoids, some are defences against herbivores, others are attractants for pollinators, but in this case cedar terpenoids are almost certainly defensive against herbivores. Those defences are evidently not good enough to protect cedars against deer in winter. 

I don't think the cedar leaves, the sumac bark or the twigs are the desired choice for deer at any time of year - indeed, deer will rapidly eat dried and partially rotten apples when offered them in winter. But, when options are few and life is hard, you eat what you can when you can, so long as it doesn't kill you.