By Fred Bunnell
Barefoot on ice
The fool ducks were standing on the ice in their bare feet. I’ve done that, coming out of a sauna. I don’t recommend it. Walking through the crispy snow and slippery ice during our uncommonly cool period I enjoyed the crispness of the air and the brilliance of sun on the snow.
Walking inspired two thoughts.
The first was how I used to play in the snow most winters as a kid. Trout Lake was frozen most years. Some years, we’d run a hose across the street and flood the tennis court to make our own rink in Garden Park. They closed some hilly roads for us to ride our sleds down. Luckily, bright sun banished thoughts of climate change and the unpleasantness it portends.
The second thought was that different creatures were doing different things to cope with the cold. It began with the ducks, but there was evidence everywhere.
Ducks use two mechanisms to keep their feet from freezing. We employ only one of those reasonably well.
That is varying the amount of blood flowing to extremities by varying the diameter of arterial vessels. Our blood is warm. In the cold, blood flow is reduced to lose less heat; when we’re hot, flow is increased to reduce overheating. We do that well enough that our hands and feet become white when cold and pink when unduly hot. Every warm-blooded animal we see is doing that.
We’re not adept at the second mechanism, although many warm-blooded animals are.
For our machinery to run well, our core body temperature cannot vary much. Counter-current heat exchange helps keep the core temperature near constant. It does that by warming the blood entering the body core and cooling the blood to the tail, flippers or feet (see Figure).
The heat exchanger is simply a group of arteries from the core interspersed with venous vessels from the extremity. Warm arterial blood flowing to the feet passes close to cold venous blood returning from the feet. The arterial blood warms up the venous blood, dropping in temperature as it does so.
Figure 1 – ‘Heat exchanger’ diagram.
The blood coming back to the heart is warmed; blood flowing to the feet is relatively cool.
The feet are supplied with just enough blood to provide tissues with food and oxygen, and just enough warmth to avoid frostbite. At 0o C, mallards lose only about 5% of their body heat through their feet. Although the rest of the duck is covered with feathers and in contact with air, not ice, 95% of the heat loss is from the head and body. Meanwhile, cool feet sit on ice and give up only 5%. That’s because the body is much warmer.
Snuggling up to air
Feathers themselves are poor insulators, but the wee chickadees and various sparrows reveal their utility in the cold. The wee birds hunkered down out of the wind are about twice their normal size. That increase in size is all air, an effective insulator.
That is exactly what is going on in my down jacket. Fur plays the same role. The sea otter is the smallest mammal to spend all year in the water (beaver and muskrat dens are dry).
Chilly golden-crowned sparrow
It‘s too small to pack on fat like whales or seals, but sea otters were hunted to near extirpation because they have an incredibly dense fur, up to one million hairs per square inch. A dog has about 60,000 hairs per square inch. We humans have about 100,000 hairs on our entire head.
To work effectively that fur has to be clean and fluffed full of air. In her surveys Anthea Farr found that sea otters spend about 2.5 hrs a day grooming, more than any other mammal. All that fluffed in air explains why they bob around so effortlessly. Moreover, unlike most furry mammals, sea otters do not have a seasonal molt but replace wee bits continually.
Sea otter pup napping on sea otter mum.
For arctic and temperate mammals, the seasonal molt is not because they got the current coat dirty. It’s all about thermoregulation. The winter coat looks different because it is.
The pelage always consists of two layers – the outer guard hairs for protection and shedding rain and the undercoat that is much curlier and fluffier and better at trapping air and keeping heat in. In the summer much of that underfur is shed to avoid overheating. In the winter it is back acting like the feathers in my down jacket.
Let’s just sleep through it
This day is sunny and without wind, so I am struck by something we often forget. We tend to think warmth is all about temperature and forget about radiation (the weather person’s thermometer was in the shade receiving no radiation). The sun on my face is surprisingly warm even though the air temperature is cold. That’s radiation.
So I ponder Anna’s hummingbird – they wouldn’t have made it through this cold without a clear concept of radiation and some dedicated feeder tenders.
We know of only one bird that hibernates – the Common Poorwill. It breeds in BC, but shifts south come winter. Many more birds, and especially hummingbirds, use torpor to survive cold. Torpor differs from hibernation primarily in the length of time it lasts. Consider it a very deep, hypothermic sleep and you won’t be far wrong.
Anna’s hummingbird is tough. Besides being among the smallest of all warm-blooded animals, hummingbirds also lack the insulating downy feathers that are typical for many other bird species. Nor can they pack a lot of fat. They’ve chosen a life style that involves constant tearing around after food.
During summer they eat 2 to 3 times their body weight daily. You try that. With its tiny body and no insulation a sleeping hummingbird has very high energy demands.
Torpid Anna’s hummingbird at feeder, exposed to sun and near food. Wally Williams photo.
On cold nights there is nothing for them but to lower their body temperature until they are hypothermic or torpid. On cold day they enter torpor whenever fighting the cold becomes too much.
Torpor is a kind of sleep so deep the metabolic rate is lowered by as much as 95%. During that sleep, a torpid hummingbird uses up to 50 times less energy than when awake. The lowered metabolic rate causes a cooled body temperature.
At night, body temperature is maintained at a hypothermic threshold barely sufficient to maintain life. The trick is waking up. Waking up takes about 20 minutes.
Fortunately, there’s radiation. You can find torpid hummingbirds in the most peculiar places, sometimes hanging upside down. There is only one near consistent feature – they are where the sun’s radiation will hit them and warm them. In winter the sun’s warmth won’t be enough so the bird shivers to generate heat.
Where’d you put the antifreeze?
Every living thing contains a heap of water. Our bodies average about 65% water by weight, with males being more watery. Ice expands as it forms and grows, bursting cells. An obvious question is: where’s the antifreeze?
The question is particularly apt because bacteria, fungi, plants, insects and a few fish species in Arctic and Antarctic water have evolved antifreeze proteins. They are different but effective in each group.
Water has the unusual property of not being happy once it forms a crystal. Some crystals dominate and grow larger and larger, stealing water molecules from the surrounding small crystals. Antifreeze proteins counteract this recrystallization effect by binding to the surface of the small ice crystals and slowing or preventing growth into larger dangerous crystals.
The process is a form of supercooling. Antifreeze proteins lower the freezing point of water by a few degrees, but don’t change the melting point (thermal hysteresis). Insects build the most effective antifreeze proteins and can lower the freezing point by about 6 oC. It seems that warm-blooded creatures simply cannot let their internal temperature get that low. They lower their internal temperature by hibernation or torpor.
Don’t drop the frog
One frog species in BC survives freezing temperatures by simply allowing parts to freeze. Nucleating proteins in their blood cause the water in the blood to freeze first. As this ice grows it sucks most of the water out of the frog’s cells. But the frog keeps pace as it’s liver builds large amounts of glucose to act as antifreeze and keep the cells from collapsing.
Parts of the frog can actually freeze, like their bladder, but their blood and vital organs do not freeze. The heart can stop beating and the frog can stop breathing, but it when it thaws out, it will still be alive. This simply won’t work in a warm blooded animal, but the frog can survive multiple consecutive Freeze-thaw cycles with no adverse effects.
When frozen, it becomes hard and crunchy. I wondered if it would shatter when you dropped it. It didn’t but it does make a metallic ‘clink’. The frog was fine.