COLD HEDGEHOGS, WARM HONEY BEES
WE ALL LEARNED this in our biology lessons in school. Apart from all the other divisions you can make, the animal world can also be divided into warm-blooded and cold-blooded creatures. Yet more compartments and, as you will see, animals don’t fit neatly into these, either. But first let’s get to the scientific categories. Warm-blooded animals regulate their own body temperature and keep it constant. Humans are a good example. When we get cold, we shiver to generate the heat we need. When we overheat, we sweat and cool down as our sweat evaporates. In contrast, for better or worse, cold-blooded animals are dependent on external temperatures. When it gets too cold, they can forget about physical exertion. And that’s why, every winter, I find flies in my wood pile that just don’t have the energy to get up and go. When temperatures drop below freezing, they are not up for much more than crawling around excruciatingly slowly among the logs. Helpless, the most they can hope for is that no birds track them down when it’s cold. And that’s how it is for all insects. All of them? Actually, no, that’s not how it is for my bees (or any other bees, for that matter).
I used to not like bees. It’s difficult to forge a relationship with insects, and if those insects sting, the almost automatic response is aversion. Apart from that, I rarely eat honey. Not an auspicious start for a beekeeper, which is what I’ve become. It’s really only because of the apples. We had hardly any bees in our orchards in spring. To change that, I acquired two hives in 2011. Since then, pollination has not been a problem, and we’ve had plenty of honey. But, most of all, I’ve learned that there are ways in which bees are not the same as other insects. For example, in some respects they are similar to warm-blooded animals, and that is the main reason they are so eager to forage. Nectar, processed into honey and stored in combs, serves as a winter fuel for bees, for they love to be cozy and warm. Their comfort zone lies between about 91 and 96 degrees Fahrenheit, just a little cooler than the body temperature of mammals.
In summer, that’s not a problem. Quite the opposite, in fact. The muscle action of up to fifty thousand busy bees creates a decent amount of heat, which must be dissipated so the hive doesn’t get too hot. This is a complicated task. Worker bees carry in water from the closest source so it can evaporate and cool the interior of the hive. Thousands of wingbeats circulate air in the hive, creating a cool breeze between the honeycombs. Too much disturbance overwhelms this communal effort. If the hive is attacked from the outside or if it is handled incorrectly when it is being transported from one place to another, the agitated bees heat up so much that the combs melt and the insects die of heatstroke. The technical German term for this translates as “death by buzzing.” The expression comes from the noise the colony makes as the bees panic and beat their wings faster, heating up the hive and causing their own demise.
In the normal course of events, however, the bees’ thermo-regulation processes work perfectly. For my hives, most of the year it’s too cold rather than too hot, and creating heat is most important. Vibrating muscles means calories spent, and bees take in the energy they need in the form of honey. Honey is basically a thick, highly concentrated sugar solution with vitamins and enzymes added. Every month, primarily in winter, my bees burn through more than 6 pounds of honey per colony. Honey serves the same function for bees as fat does for overwintering bears, and just as bears emerge from hibernation much skinnier than they were in the fall, so the size of colonies can shrink tremendously in cold weather. If it gets really cold, the insects huddle together and form a ball. It’s warmest and therefore safest in the middle—and, of course, this is where the queen must be. And what about the bees on the outside? If the exterior temperature drops below 50 degrees Fahrenheit, they would die of cold in just a few hours, so bees inside the ball are kind enough to spell them off to give them the opportunity to warm up again in the dense, seething mass.
Bees prove that not all insects are exactly cold-blooded, and as you’ve probably guessed, not all mammals are exactly warm-blooded, either. Maintaining a constant body temperature is supposedly a mammalian (and avian) specialty. Supposedly. The little hedgehog is the exception that proves the rule. While the similarly sized squirrel bounces through the branches every once in a while in the cold season, even in the snow, this spiny ground dweller spends its winters fast asleep. Its spines don’t insulate as well as a squirrel’s thick fur, and therefore it uses a great deal of energy when temperatures fall. Apart from that, its favorite foods—beetles and snails—have made themselves scarce and are no longer to be found above ground. So what better idea than to take a break as well? The prickly little guys roll up into a comfortable ball in a cozily padded nest that is often buried deep beneath a pile of leaves or brush. Here, they fall into a deep sleep that can last for months. In contrast to many other mammals, instead of keeping their body temperature at a hedgehog-appropriate 95 degrees Fahrenheit, they simply shut off their energy intake, which means their body temperature falls to match the ambient temperature and sometimes drops as low as 41 degrees. Their heartbeat slows from up to two hundred beats to only nine beats per minute, and they breathe only four times a minute instead of fifty. Dialed down like this, a hedgehog uses hardly any energy at all and can make it through to the next spring on its reserves.
Hedgehogs don’t mind the cold—quite the opposite, in fact. The strategy I have just described works best when it is cold and frosty outside. It becomes dangerous only when winter temperatures rise above about 43 degrees Fahrenheit. Then the hedgehogs slowly rouse, and the deep sleep of hibernation turns into a lighter sleep during which considerably more energy is used but the animals are not awake enough to move. If such conditions persist for a long time, some of the little sleepers starve. It is only when temperatures reach about 53 degrees that the hedgehogs can really get moving and nibble on something—that is, if they are able to find anything because their prey are still hiding out in their winter quarters. With any luck, a number of early risers in this predicament will be found and fattened up in hedgehog rescue centers.
And what do hibernating hedgehogs dream of? During the really deep phase of sleep, metabolic rates are extremely low and there’s hardly any dreaming going on, because the brain uses a lot of energy in the highly active state of dreaming. Therefore without metabolic activity, there are definitely no mental movies being played. But what about that light sleep above about 43 degrees Fahrenheit? If hedgehogs can dream then (after all, energy use rises), the pictures might be more like nightmares from which the hedgehog would like to wake but can’t. Whatever is going on inside its head, the situation is life threatening and perhaps the animal realizes this in its drowsy state as it struggles in vain to wake up. Poor little thing. Climate change will, unfortunately, bring more of these warm winter interludes.
Things are somewhat better for squirrels, at least in the dream department. They don’t hibernate, they just doze for two or three days at a time before they wake up again and feel hungry. Although their heart rate drops during these down times, so they use fewer calories, their body temperature remains high. This means they need regular servings of energy-rich foods such as acorns and beechnuts. If there are none or if the squirrels cannot find them, they starve. The strategy of red deer is much more like that of hedgehogs, because, surprisingly enough, they, too, can lower their peripheral body temperature. They do this multiple times during the course of a day, meaning that their periods of winter “hibernation” last only a few hours. Even such short down times allow them to restrict their use of valuable body fat. Despite low exterior temperatures, their metabolic rate is then up to 60 percent slower than in summer.30
Now another problem arises: red deer have to rev up their metabolic rate when they are digesting food, but going through the winter without eating anything is usually not an option, either. When a red deer eats in winter, it often extracts less energy from the food than it uses to digest it. And that is why, paradoxically, when hunters feed red deer in winter, the animals can end up starving in droves. This is what happened in 2013 in my home district of Ahrweiler, where there was an indignant outcry from hunters who wanted to continue feeding despite a local ban. Almost a hundred red deer died from starvation, a number of which would likely have survived if they hadn’t been physically stressed after digesting the hay and sugar beets hunters had fed them. That is why, left to their own devices in winter, red deer mostly live off the body fat they accumulate in the fall.
At some point, I began to worry whether red deer constantly feel hungry in winter, which is a distressing thought. To stand in cold snow with a rumbling stomach and supercooled extremities is surely very unpleasant—at least it would be for a human being. It has now been proven that animals can turn off the sensation of hunger. Hunger is, after all, a signal from the unconscious that it’s time to eat. And this feeling should only trigger the desire to eat when adding calories would be beneficial. Take hedgehogs, for instance. Even when they are hungry, they refuse smelly, rotten food. The unconscious part of their brain temporarily shuts down their hunger pangs and replaces them with a firm resolve not to eat any of the proffered sustenance. We don’t know whether red deer experience an aversion to buds and dry grass or whether they simply feel full. But we do know that the animals don’t feel hungry in winter despite their fast, because at the end of the day not eating makes fewer demands on their energy reserves.
The combination of lowering body temperature and metabolic rate that I have just described doesn’t work equally well for all red deer. How well it works depends on the character of the individual deer and, equally importantly, on its rank and position in the herd. Winter is particularly dangerous for red deer that have strong personalities. As leaders in the herd, they have to be constantly alert. This means their heart rate is constantly elevated, and their energy use is correspondingly high. It’s true that herd leaders have preferential access to good feeding grounds, but that is of little use to them. Meager winter offerings of dry grass and tree bark don’t deliver sufficient calories to build up the fat reserves these deer are burning through at a higher rate than their lower-ranking herd mates, which spend the cold winter nights standing around calmly and dozing. The lower-ranking deer are eating less than their leaders, but they are also using far less energy. And so, at the end of winter, they have more reserves than their superiors. After watching deer in large natural enclosures, foresters in Vienna made the surprising discovery that being the leader of a herd reduces a deer’s chance of survival, despite the fact that leaders always get to help themselves to food first. According to the researchers, in future it will be more important to consider the life histories and personalities of individual animals rather than the average for the species. After all, that is exactly how evolution works—using deviations from the norm.31
Warm-blooded and cold-blooded, then, are fluid categories that can merge into one another. What about feeling like you’re freezing? This sensation signals to the body that its temperature is falling dangerously low and measures must be taken to counteract this. For us, it’s lights out when our body temperature dips lower than 93 degrees Fahrenheit. Before that, we start to shiver and we try to get somewhere warmer. It’s the same for our horses. On cold, windy winter days, our older horse, Zipy, is particularly prone to shivering, and she takes refuge in the pasture shelter. As the mare has less fat and muscle mass than her companion, and as her body is less well insulated despite her winter coat, that’s sometimes not enough. On really cold days, we put a rug on her to warm her up until the shivering stops and she feels better. It’s quite clear that feeling cold is as unpleasant for Zipy as it is for us.
But what is it like for insects? Their body temperature fluctuates along with the air temperature, and they don’t have a mechanism to keep it at a certain level. In fall, insects burrow into the ground or under tree bark or into plant stems so they don’t freeze solid. To ensure that any ice that forms in their cells doesn’t cause the cells to burst, they store substances like glycerin, which inhibits the formation of larger and sharper ice crystals. What might that feel like? Do such creatures even experience cold? When I watch frogs and toads jumping into ice-cold ponds in late fall in order to doze in the mud at the bottom, I can’t imagine that they feel freezing cold. We find cold water so unpleasant only because it draws off our body heat far more efficiently than air. But if your body temperature is the same temperature as the water in the pond, it can’t feel too bad to jump in. And so the frogs and toads down there are probably not feeling like they’re freezing after all.
But if insects, lizards, and snakes don’t know what it’s like to feel uncomfortably cold, does that also mean they don’t know what it’s like to feel pleasantly warm? I don’t believe that. After all, in spring, these creatures love to seek out sunny spots. The more their small body heats up, the more swiftly they can move. Therefore, they perceive being warm as something positive, a condition that can be costly for some. Take slow worms, for example. (Slow “worms” are actually a kind of legless lizard.) Roads warm up particularly quickly in the sun. Asphalt stores heat and radiates it, even at night, so roads are a good place to fill up on warmth. Unless, that is, a car runs over the little sun worshipper, which, unfortunately, happens quite often. Drama aside, it is clear that, like us, coldblooded animals must also have a feeling for temperature, although they probably sense it differently than we do.