Survivor: Winter Wildlife Edition—Outwit, Outlast, Outplay
NOTE: This story was originally published in 2013; a report by Elizabeth Wiles & Pauline Donaldson of the February 2013 MVFN lecture presentation by Patty McLaughlin (nee Summers), Wild Bird Care Centre.
A delightful, clearly delivered talk to the Mississippi Valley Field Naturalists by Patty Summers from the Wild Bird Centre, February 2013 described the varied and intriguing ways wildlife prepare to survive winter. How do they do it? Summers divides wildlife winter-survivor strategies into three categories – outwit, outlast and outplay, with outwit being by far the most widely employed strategy.
Outwitting winter, Summers explained, involves turning the tables, knowing the science of cold and of snow and cold water to find the secret, hidden warmth. Fresh snow can be up to 90-95% air and is a good insulator. In the ‘subnivean’ space 15 cm under the snow, small mammals such as mice and voles inhabit a relatively cozy 0 degrees C space between snow and ground. They are not alone there; in fact an entire foodchain inhabits the subnivean space: bacteria, fungus, springtails, spiders, shrews, weasels etc. Likewise aquatic ‘outwitters’ seek out the relative warmth of deep water zones way below the ice. Cooler water sinks and stabilizes at 4̊ C with no circulation and there it has a higher concentration of dissolved oxygen than surrounding layers. Fish here eat less, move less or, like carp, bury themselves in mud. Some aquatic plants have turions which survive in the 4 degrees C water at the bottom of ponds. These turions, or overwintering ‘buds’, sink, but will outwit winter to rise again in spring, and grow new plants. Dragon flies stay in the water in the nymph stage. Another outwit strategy is ‘Build a four season home’. Bees do this. They consume honey for energy and form tight shivering clusters which are 32 degrees C in the middle. Individual bees regularly rotate position, with bees near the centre trading places with bees on the periphery so there is a better chance for survival. Waste is excreted outside the cluster.
Not surprisingly there are challenges faced by the ‘outwitters’, and some will not survive. Life in the subnivean space is risky. The insulating capability of snow depends on its density. Freeze-thaw cycles compact snow, reducing its insulating ability and allowing dangerous levels of carbon dioxide to accumulate. There is also the threat of hunters of the subnivean space. Foxes can hear prey under the snow and can leap and pounce through. Grey owls can locate prey 2 feet under the snow and plunge through a snow crust that can hold 175 pounds!
While outwit involves taking advantage of subnivean and deep water spaces, or building a four-season home during freezing weather, outlast involves becoming dormant and conserving energy. Or, as Summers described it, “dig deep and stay there”. This is the way of the frog, toad, ant and worm. Earthworms survive 6 feet underground in a slimy membrane. Ants burrow into the soil or under tree bark. Others such as groundhogs, chipmunks, and woodland jumping mice hibernate below the frost. Frogs and salamanders, who can absorb oxygen (O2) and emit carbon dioxide (CO2) through their skin, go deep underwater, as do turtles, who can survive but must dig very deep. Another slogan of the outlast survivors is “It’s better with friends”. Snakes can’t dig but they gather by the hundreds in tree stumps, holes, or in cracks or caves among rocks and share their warmth. Ladybugs do the same under bark and rocks or the south side of a house.
Dormancy or hibernation is another key ‘outlaster’ strategy. In an extreme example, some frogs cryopreserve themselves. As ‘frogcsicles’ their heart is stopped but their organs stay ‘alive’ with no oxygen or nutrients. They survive fatal freezing damage by eliminating water from inside their cells; no ice is formed inside their cells because, instead of water, cells are high in glucose which does not freeze easily. Box turtles and many insects use a freeze-tolerant mechanism; the arctic woolly bear caterpillar may freeze and thaw seven times before finding conditions right for it to pupate, often a matter of years. Some animals have a unique super cooling ability; using high sugars or sugar alcohols and excreting waste, they can lower their body temperature below freezing without becoming a solid. Mourning cloaks, slugs, snails, gall wasp larvae do this but it is risky if they touch ice or if it gets too cold. Perennial plants outlast winter as well, storing nutrients in roots below the frost line. Trees reabsorb valuable nutrients from leaves before the leaves are shed and form buds before winter. Conifers form protects them from snow load and as their roots go past the frost line for water, valves can shut off if ice is present.
Just as there are risks to outwitting winter, there are also risks when attempting to outlast winter. Turtles hibernating under the mud with their hearts beating only once every few minutes are totally vulnerable if they did not dig deep enough. They will be eaten if found because they will not wake up.
A third winter survivor strategy is ‘outplaying’ winter. Dress for winter, remain active and ‘play’ all winter despite the harsh conditions. Birds increase feathers and down layers, lose bright colours, eat more and spend nights in torpor, with lowered metabolic rates and body temperature. They keep their feet warm with extra feathers, and a heat-exchange blood circulation system. Some birds will tuck alternate legs up inside their feathers to keep them from freezing. “Who needs boots?” says Summers. One well-dressed ‘outplayer’ among the winter survivors is the ptarmigan with feathers around its toes and ankles and projections off its feet that look like mittens. Mammals will increase fur, change color to a dull white fur which has more air pockets for better insulation. They will fatten up with brown fat. Some small mammals like chipmunks and flying squirrels are active in their burrows and often emerge on sunny days. Squirrels are active all winter, as are deer, that ‘yard’ in an area of good browsing and shallow snow. They keep the snow beaten down with their trampling for ease of movement.
Another game of the outplayers says Summers is “Cache and Seek”. Birds, mammals, squirrels will hide (cache) extra food to use in winter. Many birds cache food in the fall and find it later by smell and in some cases by their amazing memory. ‘Bird brains?’ Beavers live in their houses with food stored nearby and muskrats make and live in mounds of vegetation called ‘push-ups’. They also establish food caches and bundle together for warmth. Others, such as weasels continue to hunt. Some owls have lopsided ears which allow them to locate prey through triangulation of sound. As mentioned, a grey owl can locate prey under deep snow and plunge through to catch prey. Another strategy is ‘form an alliance’. Crows roost together. Flying squirrels must nest in groups together. Large ungulates will follow group paths through the deep snow. In cities birds flock to roost near warm buildings or chimneys.
Which of these strategies is best? If there was an award for the best winter survivor amongst wildlife, which animal would it go to? At the conclusion of her presentation, Patty Summers, told us that for her, the star of ‘winter survivor wildlife’ is a bird, the golden crowned kinglet. This tiny bird does not enter torpor. It maintains a normal body temperature which is 3̊ C higher than other birds. This ultimate outplayer of winter also manages to find three times its weight in food daily, and may raise two broods per year – a marvel of activity!