For a long time it was common practice to describe the apocrines used by other animals as the ‘pseudo sweat glands’, and eccrines as the ‘true sweat glands’. The implication seemed to be that in this respect all other animals are out of step except Homo.
Since our eccrines functioned as temperature regulators, it was baffling that the same glands in other animals refused to perform the same service. Many experiments were conducted with animals to try to induce eccrine sweating by raising the ambient temperature. Favourite subjects were cats since they were readily available, and chimpanzees because of their close relationship to ourselves.
The results were negative. Neither the cat’s paws nor the chimpanzee’s body could be induced to sweat within the range of temperatures they would ever encounter in the wild. At higher temperatures a few drops of moisture were observed around the eyes, lips and scrotum of a chimpanzee. But that is not necessarily evidence of eccrine activity. A shot of adrenalin produced similar results at the same sites in a horse, and a horse has no eccrine glands at all.
Since the 1970s emphasis has moved to studying cooling strategies in savannah primates. All use panting as a means of relief and some – for example, baboons and patas monkeys – supplement this by sweating.
The patas, reputedly the fastest-running primate, under laboratory conditions in high temperatures with drinking water freely available, has been observed to exude sweat at a rate of up to 50 per cent of the human maximum per square centimetre of skin. In the wild they are better able to afford sweat cooling than other savannah species because they feed largely on fruit instead of almost exclusively on grass and roots.
The question is whether this demolishes the long-held belief that human thermoregulation is as unique to our species ‘as speech or bipedalism’. Like the apes, savannah primates do have some eccrine glands on body and limbs as well as on hands and feet. Hence there is a disposition to assume that their sweat-cooling, like ours, must be eccrine.
But this remains an area of contention. The difficulty is that in these animals the two types of glands are closely interspersed. This makes it very difficult to design an experiment which both measures the sweat and ascertains which type of gland it comes out of. A series of experiments with the patas monkey confined itself to measurement alone. In one experiment with a rhesus monkey the sweat was described as eccrine; in another experiment with baboons the verdict was apocrine.
It is perhaps unnecessary to point out that in these monkeys the sweating is in no case ‘facilitated’ by the shedding of body hair. The patas, in fact, has a thicker coat than some arboreal members of the Cebidae.
The strength of the aquatic case lies in the fact that those features of the human skin that are undeniably unique among primates – the nakedness, the underlying fat, greater elasticity, dearth of apocrines, proliferation of sebaceous glands – can all be paralleled in aquatic species, and none in grassland species.
A study of sweat and sebaceous glands in seals by J. K. Ling reports that ‘… with progressive hair loss, the need for waterproofing the skin is met by enlarged lipid-secreting sebaceous glands.’ Waterproofing seems to be the only function of sebum. In the apes, sebaceous glands are consistently found in those parts of the body which are always, or frequently, wet and at the same time in contact with the air – around the mouth, along the edges of eyelids, around the anus. In a wading ape finding some of its food on the sea bed, waterproofing would be most essential in those parts of the skin which would be most frequently wetted and then exposed again to the sun and wind, namely, the head and face and upper parts of the chest and back. These are precisely the locations where our sebaceous glands are found.
There is even one clear aquatic instance of eccrine sweat-cooling. When the fur seal goes ashore to breed it suffers from the heat because of its double insulation of fat and fur. It is believed to be descended from a possibly dog-like land-dwelling ancestor with foot-pads of the standard mammalian type: the flippers are covered with eccrine glands which sweat profusely as it waves them in the air to cool itself.
If, then, we provisionally adopt the aquatic hypothesis, we are confronted with some entirely new questions. Several dermatologists have voiced the conviction that our sweat glands must have originally evolved for some other purpose, without putting forward any suggestion about what that purpose could have been.
What purpose could they have served in the sea? All they are structurally designed to do is to emit a saline solution – water with salt dissolved in it. There are trace elements of urea, and of ammonia (probably due to bacterial decomposition), but the main constituent – 90 per cent of everything except the water – is salt.
Birds and mammals which get their food from the sea sometimes take in more salt than their bodies need, and too much is just as harmful as too little. Many species have had to evolve special ways of getting rid of the surplus. The most familiar example of this is found in the salt glands of seagulls.
From time to time dermatologists have referred to the possibility that the non-volar eccrine glands (those not found on the palms and soles) originally served the purpose of excreting salt. The names first given to the skin glands by Schiefferdecker reflect his conviction that the eccrine glands were excretory in function. ‘Apocrine’ indicates that the secretions, such as oil or proteins, were made by or out of the gland itself; ‘eccrine’ means that the sweat was being passed out through the gland without having been manufactured in it.
(In recent papers the terms apocrine and eccrine are often replaced by ‘epitrichial’ and ‘atrichial’ respectively, meaning ‘attached to the hair’ and ‘not attached to the hair’, but as a general rule changes in established terminology are better avoided. For example, it would be easy to invent an apter name for the chimpanzee than Pan troglodytes – ‘cave-dwelling satyr’ – but the only result would be confusion.)
Whenever anyone suggests that the eccrine glands evolved to excrete salt, the idea has been discounted for one simple reason. Human sweat is hypotonic – that is, it is a saline solution more dilute than the blood. To serve any useful purpose in eliminating salt it would have to be, like urine, saltier than the blood.
It is quite possible, however, that at an earlier stage in our evolutionary history the excreted fluid was more concentrated than it is now. Even today, when sweating is prolonged, it becomes saltier. In scientific terminology the sweat gland becomes ‘fatigued’, or ‘progressively suffers a loss in its capacity to produce a hypotonic fluid’.
This capacity to keep the fluid dilute may have been a comparatively late evolutionary development. The energy for keeping the fluid hypotonic is obtained by converting glycogen in the glands into lactic acid. But this only happens in the more recently evolved eccrines on our body and limbs. In the more primitive eccrine glands of our hands and feet this conversion does not take place.
All that is highly speculative. But there is one piece of persuasive evidence that our ancestors did at some point in our evolution encounter a salt crisis and needed some extra mechanism, in addition to the action of the kidneys, for eliminating salt.
Marine birds which catch and swallow salt-water fish inevitably swallow some sea water in the process. Some sea food such as squid is intrinsically salty. Drops of clear fluid can sometimes be seen dripping from the birds’ beaks. On investigation this has proved to be a concentrated salt solution coming from specially evolved nasal salt glands, and the largest salt glands belong to those species which spend most time at sea.
Marine reptiles, and some marine mammals, encountering the same problem, deal with it not by nasal dripping but by weeping salt tears, for example, marine iguanas and turtles, marine crocodiles, sea snakes, seals and sea otters.
“I weep for you,” the Walrus said, “I deeply sympathise,” holding his pocket handkerchief before his streaming eyes.’ Lewis Carroll, author of Alice’s Adventures in Wonderland, wrote fantasy, but he got his biology right. The only weeping creatures in his books are t
he Walrus, the Mock Turtle – and Alice. (His crocodile was not a marine one but lived in the Nile, so it remained dry eyed and ‘gently smiling’.)
Man is the only weeping primate. With our copious sweat and our copious tears, we are by far the leakiest of all the apes. Because the phenomenon is so hard to explain it is often dismissed (as nakedness used to be) as non-existent or ‘merely quantitative’, a slightly increased activity of the lacrimal glands which nearly all land mammals possess.
That is not true. Emotional (or ‘psychic’) tears are quite unlike the tears which coat the eyes with a film of moisture and flow more freely in response to irritating vapour or a foreign body in the eye. Psychic tears respond to different stimuli and are controlled by different nerves. If the root of the trigeminal nerve which leads from the brain to the eye is severed, ordinary reflex weeping in response to irritants is prevented, but tears of grief or joy will still flow. The same phenomenon is observed if cocaine is applied to the surface of the eye.
The connection between weeping to excrete salt and weeping from emotion is not easy to understand, but it is an ancient one. There are accounts of copious nasal dripping in seagulls in situations of aggressive confrontation, and weeping in sea otters when distressed or frustrated. The stimulating hormone prolactin, which appears to be involved in human weeping, is released in response to emotional stress.
Like human sweat, human tears are hypotonic, but may not always have been. Tear glands, too, can get ‘fatigued’ if weeping goes on long enough; the salt may become more concentrated and can sting the eyes. Shakespeare in King Lear described a depth of grief in which ‘mine own tears did scald like molten lead’. It seems quite possible that eccrine sweating and psychic weeping evolved at the same time. When malfunctioning occurs, it is liable to affect both equally. In cases of cystic fibrosis, for example, one of the diagnostic signs is extra saltiness both of sweat and tears.
Not much is known about psychic weeping because it is very difficult to research. The phenomenon does not occur in any of the normal laboratory animals; Homo sapiens is the only available subject. Having obtained a willing volunteer, the scientist is still confronted with the problem of how to induce the weeping. Reflex tears are easily stimulated: onion vapour, which turns to sulphuric acid on contact with a moist eyeball, can be administered in controlled doses. But not all volunteers can weep emotional tears to order.
Darwin’s account of weeping in The Expression of the Emotions in Man and Animals drew heavily on observations of young children. He had a large family and when the children were young and tearful he studied them closely in the short interval before the paternal instinct to comfort them triumphed over scientific curiosity. He was among the first to note and record that in the first few weeks of life a baby’s vocal cries are unaccompanied by tears, although reflex tears are operative from birth.
The theory he finally advanced was that when a child has a screaming fit, this leads to engorgement of the blood vessels of the eyes (as does, for example, vomiting). That in turn leads to spasmodic contraction of the muscles around the eyes in order to protect the blood vessels against the danger of bursting, and he believed the resulting pressure could activate the lacrimal glands. He conceded that adults often shed tears unaccompanied by screaming or screwing up the eyes, but suggested that by then an association between heightened emotion and weeping has become automatic.
A leading present-day authority on the subject, William Frey, failed in his first attempts to collect psychic tears. Volunteers who claimed to be able to weep whenever they wished – including an actress who had repeatedly demonstrated this ability on stage – were inhibited by the clinical atmosphere of the laboratory.
The problem was only solved when Frey invited an audience of volunteers to watch ‘tear-jerker’ movies and collect their tears in test tubes. In ancient times similar vessels called lachrymals were specially made for the purpose, and Nero was said to have shed a copious supply while he watched Rome burning.
Some interesting discoveries have been made. Frey found that the chemical composition of the two kinds of tears is different, with a protein content at least 20 per cent higher in emotional tears than reflexive ones. He established that, contrary to the traditional belief, lacrimal glands are capable of excretion. For example, the element manganese – deleterious in high levels – is found in tears at a concentration 30 times higher than its concentration in blood. He believes that one modern function of tears may be to eliminate excess stress-related chemicals, and that that is why weeping can bring emotional relief. He also found that the tear-generating emotions aroused by sad movies were more containable when the viewers were in physical contact with each other.
As in many other instances, scientific investigation confirms what most people instinctively know. Confronted by children or friends in a state of distress, they feel the impulse to establish physical contact by embracing them, or else to utter the familiar piece of folk wisdom: ‘Go on, have a good cry – you’ll feel better.’ Frey has shown that the instinct is sound. As far as I know, no one has tested the validity of the piece of folk wisdom at the head of this chapter. But it is known that the excretion of urine diminishes during copious sweating, so there could conceivably be an element of truth in the old adage.
There is one final indication that psychic weeping may have had a primitive connection with something undesirable being swallowed – for instance, too much sea water. That is the lump in the throat which is part of the weeping experience and often precedes or accompanies bursting into tears. It has been described as being like ‘a football in the throat’, and early physicians concurred with this description by naming it the ‘globus hystericus’. In a survey of 331 people, females reported having a lump in the throat in 50 per cent of crying episodes, and males in 29 per cent. It is caused by a cricopharyngeal spasm – an involuntary muscular contraction which blocks off the entrance to the gullet, temporarily preventing anything from entering the stomach. No other explanation of this phenomenon has ever been attempted.
A salt crisis in our evolutionary history would go far to explain one other specifically human characteristic – the fact that we have no instinctive awareness of the state of the sodium balance in our bodies. When we are suffering from a deficiency of water we feel thirsty, and take active measures to find a source of water and set the balance right. If the thirst remains unslaked it intensifies until all other considerations become subordinate to the need to drink.
Most mammals respond just as urgently to the need for salt if they are deprived of it. Derek Denton, in his classic study The Hunger for Salt, describes his researches into salt appetite in non-human mammals such as sheep, rats and rabbits. In all these species there is a precise correlation between the amount of salt their bodies need and the amount they will take in.
Species living in habitats far from the sea go to great lengths to satisfy their salt hunger. Grazing animals will make long treks to locations where volcanic action, or the relic of an ancient sea bed, have left deposits of salt in the earth. At Mount Elgon in the African Rift Valley there are caves which harbour salt-rich soil. Herds of elephants journey to these caves and venture deep into the darkness, as many as ten at a time, to dig out and eat lumps of the earth.
Some areas are so naturally deficient that man himself may be an important source of salt. Climbers in mountain areas have found that wild sheep will be lured to lick their clothes where they have been saturated with perspiration. In the wooden houses of parts of British Montana, men had to abandon their habit of urinating against their verandah posts because porcupines came out of the woods at night and were liable to chew right through the posts for the sake of the salt in the urine-soaked timber. Gorillas and chimpanzees are among the animals which have been observed to dig and eat earth for the sake of its mineral content.
On the other hand, when an animal has had enough salt it will take no more. In humans neither the compulsory search nor the abrupt cut-off point can be r
elied on. Their intake bears no relation to salt deficit or surplus. This is surely not a characteristic that would have been acquired on the savannah. As Denton points out, in savannah conditions ‘… selective pressures would favour retention and perhaps elaboration of salt mechanisms’.
J. B. S. Haldane was one of the first to suspect that this was a specifically human defect. In a paper based on his researches into heat cramps, he reported:
We may, if we please, regard the symptoms due to shortage of sodium chloride as due to failure on the part of the nervous system, since miners, stokers, etc., can obtain as much salt as they ‘like’.
The problem was solved by adding salt to the workers’ food or drinking water. Much more recently a similar ‘miracle cure’ was found which saved millions of lives in the Third World: oral rehydration.
It is an extraordinary fact that what The Lancet described as ‘… probably the most important medical break-through of the century’ was one of the simplest and cheapest. By this time it has saved millions of lives in the Third World. As the UNICEF report pointed out in 1984: ‘These children do not die from exotic causes requiring sophisticated cures. Five million of them die annually in a stupor of dehydration caused by simple diarrhoea.’ Salt eliminated in the faeces left their bodies short of sodium. Oral rehydration therapy – a simple cheap solution of sugar and salt – instantly halved the diarrhoea death rate in the villages where it was administered. No other animal needs a professor or a travelling nurse to make it aware that it is suffering from a salt deficiency, or to issue advice on what to do about it.
At the other extreme, people in the First World are liable to consume up to fifteen times as much salt as they need, and the same ‘failure on the part of the nervous system’ leaves them unaware that they are overdoing it. Many doctors have told us that statistics from many countries seem to suggest that a high intake of salt correlates with a high incidence of hypertension. There are tribes living in low-salt areas where blood pressure actually goes down in old age. Among white races, high salt intake has been identified as a factor in hypertension for some people, though not for everyone. But the ill-effects of over-consumption of salt are less dramatic and immediate than the results of deficiency, so their warnings to date have made little impact on our eating habits.
The Scars of Evolution Page 10