Why We Run: A Natural History - Bernd Heinrich (2002)
Chapter 10. The Camel’s Keys to Ultraendurance
Animals give us solutions to problems that are the product of evolution. They are the results of experiments that have been performed without bias or prejudice for millions of years. It is instructive to examine the results of these experiments, because there are millions of experiments that have yielded many very different solutions, and in this diversity we find both possibilities and pitfalls to specific objectives. Different animals have evolved to optimize different agendas that are not necessarily our own, nor ones to which we may wish to aspire. Instead of running, some evolved such heavy defenses as shells and spines (turtles and porcupines) or they have chemical defenses (skunks). In us, and presumably in other large vertebrate animals, including many dinosaurs, running speed is or was at a premium for both predators and prey. Since a camel’s agenda is to travel long distances, camels provide us with insights into endurance running that antelopes can’t.
Camels are not usually considered ultrarunners. Compared to antelopes, they are phlegmatic, ungainly ungulates. They’ve been of interest more for their marvelous ability to survive in heat and under severe conditions of desiccation. They show us how to handle an oversupply of heat with an undersupply of water. Nevertheless, their problem is precisely what ultramarathoners must frequently deal with. What do camels have that humans and many other animals lack? A wonderful book provides some answers. It is by Hilde Gauthier-Pilters and Anne Innis Dagg, who have observed and studied dromedary (one-humped) camels in the Sahara Desert for many years.
During any race of sufficient distance, all ultrarunners face the same problem, of managing overheating combined with fluid and energy depletion—the problem a camel faces routinely just to survive. The difference between an ultrarunner and a camel is that many camels face the problem almost chronically, while the marathoner has to confront it only for the duration of the race. A runner who races a distance of 1 mile or 1,500 meters would not have the problem at all, although overheating could begin by 10 kilometers. I would be running 100 kilometers, where the two other parts of the problem (fluid and energy depletion) are of critical importance. Camels routinely go far beyond 100 kilometers, and their example could be instructive because the extreme conditions most clearly illuminate problems and their solutions.
In contrast to antelopes and horses, camels are not fast runners over short distances. Their top running speed is reported to be about 10 miles per hour, whereas Secretariat, admittedly running a world record time, won the 1.5-mile Belmont (in 1973) in 2:24, which averages to a speed of 37.5 miles per hour. Nevertheless, camels can reputedly go 100 miles in 16 hours. They can travel the 300 kilometers between Cairo and Gaza in two days. In a one-day race between a horse and a camel over a 176-kilometer course, the horse won, but just barely, since the horse died the next day while the camel kept going. Humans have run over 600 kilometers in four days and lived to run many more. Indeed, Yiannis Kouros, a Greek now living in Australia and possibly the greatest human ultradistance specialist of all time, has run 1,000 miles (1,600 kilometers) in 10.4 days, averaging 153.4 kilometers a day.
These human-camel comparisons do not apply directly, because the human racers were supplied with food and drink along the way, in any amounts that they wished. The camels received no such maintenance. Still, they provide us with a lesson: slow and steady wins the race.
Camels generally walk rather than run. When they run, their foreleg and hind leg on one side move forward simultaneously, and alternately with those of the other side. In this gait, the body swings from side to side, being supported first by the two legs on one side and then those on the other. This gait, which is opposite to our leg and arm movements while running, is economical for the camel, because it reduces the use of antagonistic muscles that would check the sway and provide for greater stability and maneuverability. Stability and quick maneuverability are not important for a large camel on the wide open desert spaces, where there is little danger from predators.
Camels are not indestructible. They may die from overwork or undermaintenance, as the French learned in the Egyptian Sudan in 1883 when they created a camel corps that took part in all their military operations until 1921. In 1900, twenty thousand of their thirty-four thousand camels died, apparently from overwork and improper care. Later, the French employed desert nomads as mounted camel men, and these méharistes had better success with their camels in the desert. The méharistes always possessed two camels, working only one at a time and resting the other. When traveling over long distances, they worked half the time. Yet the méharistes, typically led by a French officer, pursued such rebellious nomads as the tough Tuaregs for amazing distances. In one famous raid in March 1932, Captain LeCocq and his méharistes covered 770 kilometers in eight days while pursuing the emir of Adrar, who had killed some of their men. In 1911, Captain Charlet and his camel corps tracked the Tuaregs, who had been illegally trading in slaves, for more than 7,000 kilometers. After such long, ultramarathon exploits, the military camels typically needed six to eight months to rest, and badly overworked camels require a year’s rest to recover fully.
When travel is fast and long as in the above routine camel treks, the high energy expenditure that is required promotes high internal heat production. Additional heat input from the external environment additionally threatens to overheat the animal. Physiologically the camel can counteract overheating by sweating. For a while. Since water is usually a limiting resource in the desert, the animal’s endurance is most conspicuously focused on water economy. I suspect that an overworked camel that died in a race or on a long trek likely succumbed to the effects of dehydration. Perhaps not surprisingly, camels have evolved elegant solutions for water economy that reduce dehydration and its debilitating effects. These were not known until the 1950s, when they were discovered by Knut and Bodil Schmidt-Nielsen and their colleagues in a brilliant series of experiments. It was then already clear that the camel’s legendary endurance was not attributable to diet. Camels make do with what food they can get. They eat practically any vegetation, even tough thorns.
For nearly two thousand years, it was presumed that the camel’s secret to endurance was water storage. Pliny the Elder (c. A.D. 23–79) declared that camels store water in their stomachs, and this was long accepted as fact. A scientific publication in 1950 agreed with Pliny, going into some detail describing water sacs (glands with digestive fluids?) in the sides of the rumen (the first of two stomachs in ruminants). When the Schmidt-Nielsens began their studies to determine how camels could go for weeks without drinking in a hot desert climate (where a person would be prostrate in a day), they discovered no more remarkable water storage capacities than those of cattle, who need to drink daily.
Another myth that the Schmidt-Nielsens exploded with a few simple calculations was that the fat in the camel’s hump is the key to water balance. The hump provides shading, and its fat contributes to water balance, but only in an indirect way. The 10-to-15-kilogram hump does indeed dramatically shrink in a starved and dehydrated camel, but that’s because fat is used up as an energy source.
Metabolism of foodstuffs results in the production of carbon dioxide and water as by-products. The amount of this metabolic water, produced during the combustion of a given weight of foodstuff, is higher when fat rather than protein or carbohydrate is burned. At the same time, the oxygen required for any metabolism necessitates breathing, and breathing results in exhalation of air that is saturated with water. Under most dry desert conditions, more water is lost through the expired air than can be gained if fat were oxidized solely to produce water. There is thus no truth to the idea that camels metabolize the fat in their hump in order to get water.
The camel’s hump is instead like a fanny pack that ultrarunners sometimes use when refueling stations are few and far between. It is not like a load of drink, but more like a load of concentrated food, like the commercial power bars that are currently popular. The advantage of carrying the fat on the back, rather than evenly distributed all over the body, is that it leaves the belly and other shaded areas less insulated and thus more available for heat loss from the body core. Perhaps even more important, the fatty hump serves, like our head hair, as a heat shield from the sun in the middle of the day, so that less water needs to be lost by sweating.
Part of the camel’s secret is just plain toughness and the ability to survive desiccation. We’re near death if we lose water equal to about 12 percent of our body weight, but camels can survive body water loss of 40 percent of body weight. After being dehydrated, a camel can ingest 20 to 25 percent of its body weight in one drinking bout. As in humans, the ingested water reaches the blood plasma from the stomach relatively slowly, requiring about an hour to attain a 25 percent equilibrium. But unlike humans, camels tolerate blood dilution to an extent not tolerable in other mammals. Our blood cells swell and rupture in dilution, and we can become very ill and even die from water toxicity if we drink too much liquid, especially when it is dilute (without salt or sugar) and therefore absorbed more quickly. There is no way to prescribe specific amounts of water, or concentrations of fluid supplement, to drink while running. The amount or dilution of water that is too much varies, depending on running conditions and on the individual. More commonly, runners drink too little and suffer from heatstroke. Camels suffer no ill effects from overdrinking; the camel’s blood cells can swell up to 240 percent without rupturing.
In humans and camels, the body’s blood plasma holds about 16 percent of the body water. When a camel is dehydrated to about 25 percent of its body water, the blood volume drops by only 1 percent or less, whereas in a human it drops more than three times as much, thereby thickening the blood, since the red blood cells remain. Blood that is thickened becomes viscous, like cold molasses; this greatly compromises its flow and strains the heart, reducing the blood’s capacity to circulate easily and to carry heat to dissipate at the skin surface. Death from heatstroke becomes possible. The camel’s erythrocytes (red blood cells) are uniquely oval shaped and small, reducing blood viscosity and permitting circulation through the capillaries despite fluid loss.
Water balance in camels is enhanced by their ability to reduce urinary water loss. We can produce only urine that is more dilute than seawater. In contrast, the camels’ powerful kidneys can make the urine twice as concentrated as seawater, allowing them to get rid of much waste using little water. Camels can rehydrate even when drinking brackish or salty water that would dehydrate us since we’d expend water to flush out the ingested salts. Camels further reduce urea output (and hence the need to lose water through urine) by use of microbial flora in the rumen. Their gut microbes recycle urea wastes from protein metabolism back into protein that is later reabsorbed as a nutrient rather than being flushed out as a waste product. By a combination of these mechanisms, camels can save more water for sweating, and this enables them to travel farther in the heat.
In common with a variety of other desert mammals, one of the camel’s main adaptations for ultraendurance on limited food and water supplies is its ability to regulate the rate of its metabolism through body temperature. The higher the body temperature, the higher the metabolic rate and the rate of heat production. Reducing both metabolic rate and body temperature is crucial when heat input is excessive and the body must hold the line at some critically high temperature that requires expending valuable water through sweat.
Desert animals in direct sunshine must resolve a paradox. They must try to increase metabolic heat loss, which favors reducing fur thickness, and they must decrease solar heat input, which favors increasing fur thickness to reduce heat coming in. They solve that problem by regionalizing insulation, having very thin or no fur in shaded body parts and very thick fur in areas most exposed to the sun. As already mentioned, the camels’ solar heat input is reduced by the insulating hump and thick fur on the back. The surface temperature of fur on a camel’s back may reach 70–80°C (158–176°F) in sunshine without harm, but that is because such temperatures are confined to where the sun hits the outside fur layers. Such temperatures cannot be tolerated by the skin itself, and left unregulated, bare skin would reach similarly high temperatures in the direct rays of the sun. Camels don’t allow their skin surface temperature to go much above 45°C (113°F). There is only one means of reducing skin temperature: sweating, which in hot desert conditions is the primary water drain for camels and men. By comparing water loss of naked (shorn) and unshorn unaltered camels, Knut Schmidt-Nielsen found that in summer a shorn camel lost 50 percent more water than a fully furred one.
A major source of heat during exercise is, of course internal, that produced by the body’s metabolism. Although air temperatures in the daytime desert may regularly be 40–45°C (104–113°F), there is usually a precipitous drop at night to the low 30s. Camels deprived of water take advantage of the night’s low temperatures by allowing core body temperature to decline to as low as 34°C (93°F). In the day, they let body temperature rise up to 40.4°C (104.7°F). The low body temperature at night has the effect of reducing resting metabolic rate, and hence internal heat production. A relatively low body temperature in the morning also means that when camels start off, they go slowly but can travel for some distance before encountering elevations of body temperature that might require sweating or stopping. Further, the higher the body temperature they can later tolerate, the longer they can delay expending water for thermoregulation. (Hydrated camels, on the other hand, regulate their body temperature within the much narrower limits of 36–39°C [97–102°F].) The camel’s apparent “inability to regulate body temperature,” like a camel’s slow running, was long thought to be a deficiency. Instead, both are elegant adaptations for ultraendurance on long treks through the desert. Whether their solutions to endurance and water economy are the result of previous exposure to heat and thirst, of genetic preprogramming only, or of both is not known. Our own body temperature, like the camel’s, dips 2–3°F below “normal” at night, when heart rate drops as well. It takes most of us a while to warm up and get up to speed.
In summary, camels are able to cover long distances in desert heat because they are masters of water economy. They do so by starting slowly with relatively low body temperature, and tolerating dehydration and a high body temperature. They reduce their body temperature wherever possible by shielding themselves from the sun, and they have a remarkable series of physiological adaptations that minimize the use of water for waste excretion. Their blood chemistry favors tolerating dehydration when it does occur. Given the camel’s example, an ultramarathoner who remains vertical in the heat should have long head hair or wear a hat, and shield his body with looseclothing. Frequent small drinks are better than tanking up, because we lack the camel’s water-balance adaptations, being instead evolved for more speed at the cost of greater water loss. Beyond that, an ultrarunner would be ill advised to follow the camel’s example and drink salt water, ingest huge amounts of freshwater at one time, or eat thornbushes.