Battle of the blood

To generate heat to keep their temperature
constant, all mammals must eat regularly.’
Issue date 16 November 2007

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The Kalahari is a land of extremes. Not only is it a desert that after ample rain can turn into a garden of flowers and a sea of rolling grass, but the sizzling heat of a summer day can turn into a winter night that freezes the bones. In 1972, together with my friend Sarel, I camped in the Kalahari, with the temperature dropping to -9°C at night. For protection we slept under the canopy on the back of my Ranchero bakkie. What a mistake. It was like a freezer and the next morning long icicles were hanging from the roof. The wet cloth on the camp table was rock hard and the cooking oil was solid, white fat. With painfully numb hands and shivering all over, we packed up and prepared to leave. It was too cold to prepare breakfast even though used a lot of energy in the night to keep warm. A mug of sweet, steaming coffee and a handful of rusks however did wonders to thaw me out and replenish my blood sugar. cannot stand such cold, although I’m warm-blooded due to internal combustion. What I, however, did not know was that the Ranchero’s radiator was still solidly frozen and within less than 1km its internal combustion engine boiled. While we waited for the engine to cool, looked around.

On a bush nearby a blue-headed ground agama flattened its body to soak up the morning sun – it was solar-powered and cold-blooded. lthough the blood of these animals is not always cold, as determined by the variable temperature of the environment, they can control their body temperature and metabolism to a limited extent. They do this by alternating the time they spend in the sun or in a cool place, which can either be a shady refuge like a hole or in the water like frogs and crocodiles. Staying too long in the sun can cause brain damage, but when the general temperature drops too low they become dormant and all their bodily functions slow down. If the cold is long-lasting, like in winter, they can fast for months and, like all good loafers, hibernate. Warm-blooded creatures, including humans, however, can only function in a very limited temperature band, which must be maintained at all costs. When their internal temperature drops too low, they become hyperthermic and if too hot, they suffer from heat exhaustion. In both cases they die. To generate enough heat to keep their temperature constant, all birds and mammals must eat regularly.

Diet dynamics While crocodiles and some large snakes can survive on only three significant meals a year, all well-nourished humans eat three times a day. They also celebrate all kinds of events or occasions with food – even funerals. Some people fast for religious and other reasons, but in fact usually only refrain from eating in the daytime. Apart from supper, most people do not eat during the night, and it is therefore considered a daily fast because we call the first meal of the new day “break-fast”. H erbivores, from antelope to elephants, live on relatively low-nutritional diets, have huge stomachs and need to consume enormous quantities of food. They spend most of their lives feeding, both day and night. Warm-blooded animals also need more food when it’s cold and that’s exactly when the food of herbivores becomes even less nutritious.

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They lose condition and weaken, and become more susceptible to predation. In extreme climates or severe winters they often die of starvation. To prevent loss of body heat, warm-blooded animals have fur, feathers and fat for insulation. Fat or blubber is a poor conductor of heat and animals with lots of it, like pigs, hippos and marine mammals, have little hair. Hair and feathers are raised or puffed up to trap a layer of air around the body to prevent the body heat from escaping. For their active lifestyles, birds have a significantly higher body temperature than mammals and can tolerate heat better, but are more susceptible to cold. Because modern humans are almost hairless, they’ve been draping themselves with the skins of fur-bearing animals for millennia. Maybe our nakedness was the result of this habit. We still, however, possess about the same number of hair follicles as chimps or gorillas. The small muscles that used to pull the hairs erect are still functional though, but now only draw the follicles erect to cause goose bumps.

This highly effective insulation, however, has a downside. When it’s hot, some mammals like horses need a lot of water to produce sweat, which is actually a diluted form of urine excreted to cool down the animal. For the same purpose camels spray actual urine onto their legs and storks defecate on theirs. Elephants use their trunks to spray water all over their bodies and flap their enormous ears to fight the heat. Many animals that can’t sweat, like lions, dogs and birds, pant heavily to cool the increased flow of blood through the mucus membranes of their mouths and throats. The higher body temperature of birds (41°C to 43°C) is actually an adaptation to their inability to cool down effectively. When fair-skinned humans do strenuous activity they go red in the face because the capillaries just under the skin widen to allow a higher flow of blood.

This can be cooled down by the evaporation of large quantities of sweat, which is secreted on the forehead and upper cheeks. Energy engineering There is no question that cold-blooded animals are more energy efficient than their warm-blooded counterparts which sacrifice most of their energy for the sake of being warm-blooded – they never grow beyond a certain size but only get fatter. Cold-blooded animals, however, use none of the food they eat to heat their bodies. Instead they use all of it to produce offspring and to build new cells and tissue to grow larger the older they get. For heating they only use the freely available solar energy of the sun. Although there are a few mammals, like bears and bats, and only a single bird species, the poorwill (Phalaenoptilus nuttallii), that do hibernate in winter, it’s not done because they cannot tolerate the cold, but because of the lack of food. Many reptiles and amphibians hibernate in winter simply because, being cold-blooded, they cannot generate their own internal heat and when they cool down their metabolism drops to a standstill.

During this time they can usually fast for months and it is especially true for snakes, which consume large prey that will simply rot inside their stomachs and cause death if not digested. Animals like frogs and crocodiles can also drop their metabolism, including their heart rate, to survive shorter, unfavourable drought periods. This is not hibernation but aestivation. This behaviour enables crocodiles to survive in burrows dug in the banks of ancient seasonal riverbeds in the middle of the Sahara Desert during most of the year when there is no water. And bullfrogs in dry areas like the Kalahari and lungfish in Central Africa can stay underground, encased in cocoons for months and even years if it does not rain. While reptiles in the tropics are active year-round, it is not always so easy to determine whether a reptile in SA is actually hibernating or aestivating during winter. Some, like the blue-headed agama, will emerge on sunny days and even feed on rapidly digestible insects.

On the whole, however, warm-bloodedness has brought great advantages to its possessors. They can inhabit many regions of the earth which cold-blooded animals can’t. Only two reptile species occur inside the Arctic Circle, but hordes of warm-blooded mammals and birds live there. A higher metabolic rate gives animals in general the competitive edge and not only made mammals the dominant animal group they are today, but enabled birds to migrate to any other suitable place on earth when local conditions deteriorate. The learning process also depends heavily upon how recent and frequent a lesson is experienced. Even humans forget new skills quickly if they are not soon repeated. Most of what the slow, chilly brain of a reptile learns is probably forgotten during long periods of hibernation. One group of animals that is the exception to the rule of cold-bloodedness is fish. They are the oldest vertebrates and have through the ages developed the ability to function at much lower temperatures than other cold-blooded creatures.

This ability made them the largest cold-blooded vertebrate group and enables them to live anywhere on the planet where there is unpolluted water. Anglers know that at the onset of cold weather, many warm-water species will go into an “active” dormant state, when they will swim around but won’t eat, but true cold-water species like trout bite better in winter. Humans are the only species that keep other animals like dogs, parrots and snakes as pets. Were it not for our warm-bloodedness, which gives us the capacity for superior learning, it could have been very different – parrots could have kept us as pets and taught us to talk, instead of the other way around. – Abré J Steyn Contact Abré J Steyn on 083 235 4822. |fw