In apocrine sweating there is efficient control not only of the amount of fluid loss, but also the loss of salt (sodium chloride). In many animals living on continental grasslands where sodium is in short supply in the environment, any salt in the sweat is largely reabsorbed. In sheep sweat, the sodium component can be replaced by potassium; in areas where sodium is particularly scarce the ratio of potassium to sodium may be as high as twenty to one.
On the supposition that man’s ancestors moved out from the trees to open ground and needed sweat-cooling, they might be expected to have followed the example of the wild ass and the camel in adapting their apocrine glands for that purpose. They too would have needed a system not too profligate of water and salt – both scarce resources on the savannah.
But our ancestors did not make use of this thrifty and efficient method – and for a surprising reason. Humans cannot use their apocrine glands for sweat-cooling because, except for a few restricted areas of the body, they have lost them. Apocrines are found all over the body in most primates, up to and including the gorilla and the chimpanzee. Almost all over the human body they are missing. They begin to develop in the human embryo and are present all over the body of a foetus in the fifth month of gestation – but then they disappear.
The few remaining apocrines we possess are clustered in special areas and have never been adapted for temperature control. Their secretions are not diluted enough to be of any use for sweat-cooling. They are found in the armpits, the pubic area, the navel, the ears and the nipples, and they appear to have retained the original purpose of scent-production. For the young of most mammals, and perhaps too for human babies, the scent of the nipple is an aid in locating it.
Many primate species – and we are among them – have acquired specially evolved scent organs for communication, either to warn off rivals or to attract sexual partners. For example, a woolly monkey has scent glands on its chest, which it rubs against branches as a trade mark, laying claim to its territory. Lemurs have a cluster of apocrines forming an arm-pad near the inside of the elbow. They wipe off the scent with the opposite wrist and rub it into their long tails, which they then wave in the air to send the scent message far and wide to attract possible mates.
We (and the gorillas and chimpanzees) have a pad of apocrines structurally resembling the lemur’s, but known to scientists as the ‘axillary organ’ (meaning ‘situated in the armpit’). The apocrine glands there secrete a thick, greyish substance onto the surface of the skin; they are accompanied by other glands which supply a fluid to dilute the secretion, and pubic hair which helps to diffuse the aroma into the surrounding air.
These glands only become active at puberty, and their rate of secretion increases rapidly in response to emotional stimulation such as fear or sexual excitement. By analogy with other primates possessing such pads, it would seem that a primary function of the axillary organ was, originally, to act as an erotic attractant.
Something has clearly gone wrong. The resulting odour is more likely to keep a potential mate at arm’s length than to lure one closer. This reaction is not the aesthetic affectation of a hyper-civilised population hooked on hygiene, and the proof of that is that it has gone wrong for the apes also.
Special scent glands are normally accompanied, as in the lemur and woolly monkey, by special behaviour patterns exploiting their potential. Some deer, for example, have scent glands just below their eyes and they perform the rather risky manoeuvre of depositing the scent onto twigs and thorns at a level where other deer will detect it.
Apes display no such behaviour in connexion with the axillary organ, nor is the armpit among the various parts of one another which they take delight in sniffing. It seems that at some point in their evolution they must have forgotten the axillary organ was there, or else lost confidence in its power to charm. Among our many inconvenient or embarrassing physical malfunctions this is clearly one that descended on us prior to the ape/man ‘split.
When the axillary secretion is produced inside the gland it is virtually odourless. If our sense of smell were more acute we might even find it subtle and irresistible. But we are never able to encounter it in its pristine state, because the site has been invaded by a population of bacteria, which has been with us even longer than our fleas. The bacteria have settled permanently into the warm, moist, nutritious habitat of our armpits, feeding on its secretions and causing them to decompose instantly and emit noisome odours.
These local clusters of apocrine glands at armpits, pubic area, and so on, are all that remain in Homo of apocrines that in our ancestors appeared all over the body; all the others disappear before we are born. There has been remarkably little speculation as to why they disappeared. It is unlikely to be because we lost our hair. Even in areas of skin where the hairs are invisible, the hair follicles remain, and the sebaceous glands still make use of the follicles for channelling their secretions out to the surface of the skin. There is no physiological reason why the apocrine glands could not have continued to do the same thing.
When an organ atrophies in this way it is often because it has ceased to perform a useful function. If the original function, as is generally believed, was scent production, then the glands would not work so efficiently for an aquatic mammal because the pheromones – the scent-bearing secretions – would tend to be washed away in the water. In Egypt the water buffalo is a recent import, descended from an ancient swamp-dwelling species; it has only one-tenth as many apocrine glands as domestic cattle although they both belong to the same family, the Bovidae. It is often suggested that many aquatic species have dispensed with their sweat glands because in water ‘they have no use for them’, but there is great reluctance to believe that the same thing may have applied to Homo.
When the need arose for sweat-cooling in the hominids, it could not be effected by adapting the apocrine glands for that purpose because they were no longer there. Humans have adapted for this function another kind of skin gland – eccrine glands – which seem to have evolved originally to prevent an animal’s feet from slipping.
Eccrine glands in all non-primate species such as wolves, lions, bears, mice, badgers, cats and dogs, are confined to the pads of their feet. They also differ from apocrines in other ways: they are never attached to hair follicles, but open directly onto the surface of the skin; they are active from birth and not only from puberty; they emit a clear, colourless fluid which is virtually odourless, with no oily component; its main constituent other than water is salt.
In arboreal primates eccrines are found on the soles of the feet and the palms of the hands where they perform the same function of protecting against slipping. Clearly they are more important to primates than to ground dwellers. If a dog’s foot slipped when it was climbing up a rocky slope, the worst consequence would be an undignified slither down to the bottom, but if a monkey’s hand slipped when it was hanging onto a branch, it could crash to the ground.
The hands and feet of monkeys and apes and humans are doubly guarded against slipping. They have ridges, like the whorls on our fingers, which improve the grip just as the treads on a tyre do. And they have the eccrine glands which dampen the skin, and damp skin provides a better purchase. We demonstrate our awareness of this when, for example, we moisten a finger-tip when turning over a page, or counting bank notes, or when a gambler licks his thumb before dealing cards.
Some primates therefore developed eccrines in additional places. Some monkeys in South America (though not in Africa or Asia) use their prehensile tails as an additional anchor for hanging on to the branches, so that the underside of their tails is naked, and the skin is ridged and well supplied with eccrine glands. Chimpanzees and gorillas walk on their knuckles and their knuckle pads bear ‘finger-prints’ and eccrine glands.
None of these examples has anything to do with sweat-cooling. A monkey’s palm sweats, not in response to a rise in temperature, but in response to a consciousness of danger. When the monkey takes a decision to launch itself into spac
e to leap from one branch to another, the brain sends out a signal which quickens its heartbeat, sharpens its perceptions, and at the same time dampens the palms to ensure a good grip on the branch it is aiming for.
Our own palms sweat in exactly the same way. They do not respond to changes in temperature. Instead, the wetness breaks out when we are tense or apprehensive – standing in the wings with stagefright, being introduced to someone we are in awe of, or contemplating a crucial shot in a snooker final so that the hands have to be dried on a cloth before cueing. Sometimes the moistening is overdone. In one person in a thousand sweating palms are troublesome enough to require medical treatment – but normally it is only a minor inconvenience.
Unless, perhaps, you are being interrogated. Even a slight dampness of the palms increases the conductivity of your skin, and it may be registered and recorded by a polygraph – the so-called ‘lie detector’. If you think: ‘I am about to tell a lie and perhaps they will find me out,’ the machine will detect your alarm at the preconceived risk. But the machine does not detect lies – it only detects anxiety. You may feel a stab of apprehension before embarking on a perfectly true story if you know it sounds too improbable to be believed. On the other hand, if you are a hard-boiled type with a hundred per cent certainty of never being found out, you may lie your head off and still pass the test.
In the course of primate evolution some eccrine glands began to appear scattered at random over the body surface. In small species they are comparatively few and apparently ‘ectopic’ – simply occurring by chance in the ‘wrong’ place. In larger monkeys there are more of them. By the time we get to the African apes they are as common as apocrine glands, or slightly more so, in a proportion of about 52 to 48 per cent. In humans the eccrines have made a quantum leap, to something nearer 99 to 1 per cent.
As far as anyone can discover, the apes do not use them for anything at all. Humans use them for sweat-cooling.
Compared to our naked skin, this is not a phenomenon that arouses popular speculation, because it is invisible to the naked eye. But it has always been a source of amazement and bafflement to specialists who spend their lives studying mammal skin – from P. Schiefferdecker who said in the ’20s: ‘Homo sapiens may accurately be described as the eccrine gland mammal’, to V. E. Sokolov, who declared in the ’80s: ‘Eccrine sweating is a human characteristic as unique as speech or bipedalism.’
It is the specialists, on the whole, who are least likely to proclaim eccrine sweating as a great step forward. Everything points to the fact that it is a fairly recent development, certainly arising later than the ape/man split. Deficiencies in the system may thus be due to the fact that it is still in the running-in period. As William Montagna has observed: ‘The several million (eccrine) glands on the human body act principally as heat regulators, but the function is perhaps too recent to be totally effective.’
It is inefficient in four ways: (1) it is slow to start; (2) it is wasteful of water; (3) it is wasteful of salt; (4) it is slow to respond to danger signals when the body’s resources of water and salt are getting low.
The slow start is the cause of the phenomenon known as ‘sunstroke’. Unlike the palm sweat of embarrassment which can respond within seconds to the perception of a tricky situation, the heat regulating eccrines take around twenty minutes or even longer before they respond to a rise in temperature. Within that period the body’s internal temperature, including the brain temperature, may have climbed high enough to impair brain function and cause a sudden collapse. A typical example would be that of a cricketer walking out of a cool pavilion on a very hot day, or soldiers marching onto a parade ground to stand to attention in blazing sunshine. However strict the discipline, there is always the possibility that two or three of them will keel over in a dead faint.
When the sweat finally breaks out, the body temperature comes down very rapidly. It can descend farther and faster in a short time than in any other animal, and this is sometimes cited as proof of the superiority of our own peculiar system. It could equally well be a sign that the temperature had climbed to heights it should never have been allowed to reach in the first place.
Human sweating is also praised because it breaks all records in respect of its profusion, as if there could never be too much of a good thing. This is a fallacy. All that is needed to cool the skin is a thin film of moisture such as that produced in the sweat glands of the camel. A more copious supply does not do the job any better. The truth of this is brought home to anyone who has been called out of a bath to answer the telephone in a draughty hall. A perfunctory towelling of the limbs does little or nothing to reduce the chill factor, which only wears off once the skin is completely dry – proof that a very thin layer of moisture cools the skin quite as effectively as a more copious one.
The sight of an athlete, or a traveller in a hot climate, with sweat streaming down the skin and dripping off onto the ground is not the sign of an efficient system, but a wasteful one. Sweat surplus to requirement serves no useful purpose. Among New Yorkers the commonest gripe about their summer climate is: ‘It’s not the heat – it’s the humidity.’ When the surrounding air is too moisture-laden to allow evaporation, sweating affords no relief. The continuing pointless flow of perspiration only adds to their discomfort and is no better than a nuisance.
In some circumstances it is much worse than a nuisance. Desert fighting in the Second World War stimulated research into the question, and it was found that a soldier engaged in strenuous activity in a hot climate may lose up to ten or fifteen litres of water per day through his skin, and he cannot survive for long unless all that is replaced. For anyone cut off or left behind with limited supplies of water, profuse eccrine sweating would greatly accelerate the onset of dehydration and death.
It seems extremely unlikely that eccrine sweat-cooling would have evolved in an ape living on the savannah, where water is precious. Having to return frequently to one of the widely scattered water holes would have limited the range of its foraging and increased its vulnerability to the predators that frequent such places. And, unlike the camel which can drink 30 per cent of its body weight without stopping, humans are unable to drink enough to supply their needs for several days. For an animal of our size we are near the bottom of the league in the amount we can drink at a time. One scientist, R. W. Newman, summed up our predicament in a memorable sentence: ‘Man suffers from a unique trio of conditions: hypotrichosis corpus, hyperhydrosis and polydipsia’ – which is to say we have a subnormal supply of body hair, a supernormal output of sweat, and are under the necessity of drinking much and often.
The other precious commodity which is squandered during profuse sweating is salt. In the interior of continents, salt in the soil and vegetation is often in poor supply. Over millions of years rainfall and erosion and rivers have been transferring more and more of the world’s resources away from the continental masses and into the sea. Some is returned in the rain, but the salt in rain water decreases rapidly with distance from the ocean. Over much of Africa’s grassland the kind of sweating which causes salt to crystallise out onto the surface of the skin would be a heavy liability to a hominid.
Salt deficiency causes debility, impairment of functions and heat cramps. It was J. B. S. Haldane in the 1920s who discovered that the violent attacks of abdominal cramp which affected miners, stokers and steelworkers were due to prolonged sweating. He advised ship’s stokers in hot countries to add ten per cent sea water to the water they drank, and that cured the problem. At maximum sweating capacity the body can lose its entire sodium pool in just three hours, ultimately leading to death.
It is strange, therefore, that man does not diminish sweating in the heat in response to signals of dehydration or salt deficiency until these have reached dangerous levels. This failure does not merely affect the athlete at the end of a marathon, who may be in a state of mental confusion and physical collapse.
Mortality tables show that even in temperate countries like Britain a
heat-wave is invariably followed a day or two later by a peak of deaths from thrombosis. During one heat-wave in Greater London the daily thrombosis death rate doubled, and the same phenomenon occurs every year in the United States. It happens because even when water is freely available, the fluid lost by sweating is not replaced. The blood volume goes down, the platelet count goes up and the plasma cholesterol rises. These are precisely the conditions liable to cause blood clotting, with the consequent danger of stroke or heart attacks in elderly people or others with high blood pressure. Deaths from this cause are much commoner than the more dramatic collapse from heat stroke.
In view of these facts it is hard to understand the savannah theorists’ assertion that the hominid’s unique system of thermoregulation achieved new ‘high levels of efficiency’. In the severe heat-wave in 1987 in Greece it was not the goats on the hillsides that fell victim to it. With their protective coats and low-output apocrine sweat glands, they were survivors. But thirteen hundred of the people died.
8
Sweat and Tears
‘The more you cry, the less you pee.’
Folk saying
A sweat system prodigal of water and salt and leisurely in its response to rising temperature suggests an environment with an ample supply of water, an ample supply of salt and not much threat of over-heating – that is, somewhere wet and cool and briny. But none of the research on the subject addressed itself to the question of ‘Where would this development have been at a premium?’
The Scars of Evolution Page 9
