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Videos links Links Contact | Sweating and thermoregulation Ways of keeping cool Many animals do not sweat, or at least, it is not the primary means of cooling themselves down. It's excessive and biologically expensive as it depletes the body of water and salt. Small mammals generally reduce heat by panting, which allows moisture to evaporate from the lining of their mouths, tongue and lungs. Some animals in hot countries spread saliva over their bodies to moisten the skin, which cools the body surface as it evaporates. However, Homo sapiens is the only terrestrial mammal which does not pant in order to cool down, although we do pant after exerting ourselves in order to repay our oxygen debt more quickly. Some larger mammals such as grazing animals that may spend many hours under a hot sun away from the shade of trees, sheep, cattle, horses, etc., have acquired the ability to sweat, and they do this by using pores in their skin which may have originally evolved for another purpose: scent signalling.  Apocrine glands
Apocrines are modified glands that cover the bodies of all sweating animals, with the exception of Homo sapiens. They are located at the base of hair follicles along with sebaceous glands which produce sebum for lubricating the hairs. Apocrines produce an oily or waxy substance, which is often odorous, and which finds its way to the skin via the pores, or holes, made by the skin follicle. In sweating animals, they respond to a rise in temperature by exuding a thin, watery emulsion, which helps to lower the body temperature as it evaporates. Apocrine glands are highly efficient for this purpose in that they appear to release just enough water to cool an animal down, but no more. A camel for instance, will only exude a tiny film of moisture, but it's enough to do the job, and is usually not detectable to the naked eye. Horses do sweat quite profusely but this appears to be as an emotional response to extreme exertion, adrenaline or stress, rather than a raise in body temperature; when they are grazing under a hot sun, they do not sweat noticeably. Apocrine glands also limit the amount of salt that is excreted and much is reabsorped back through the skin. Eccrine glands Unlike the majority of sweating animals including the great apes, human skin does not have any apocrine glands, except in a few specific areas (eg: armpits, pubic area, nipples) and instead is characterised by a large number of eccrine sweat glands that produce copius amounts of sweat in hot and humid conditions. While this sweat can help in body cooling, it is typically produced in much greater excess than needed, leading to a risk of dehydration. As we have seen, most animals have apocrine glands to generate sweat in the appropriate amounts necessary for cooling. [1] Eccrine glands on other animals tends to be confined to the pads of their feet and are not attached to hair follicles but open directly onto the skin. They produce a clear, colourless fluid which has almost no odour, and is made up mostly of water and salt. In arboreal primates, eccrine glands are found on the hands and feet, and the moisture they produce enables the animals to grasp hold of a branch without slipping. Like humans, a monkey's hands, feet and finges are covered in whorls which improve their grip. Chimpanzees and gorillas which walk on their knuckles also have eccrine glands and ridges there. However, none of these animals uses their eccrine glands for thermoregulation.  Apocrine glands or eccrine glands?
Over the course of primate evolution, some eccrine glands have spread from the palms of the hands and souls of the feet, to other places on the body. The African apes have by far the most eccrine glands, slightly more in fact than apocrine glands (52% to 48%). In humans, however, there is a huge difference. We have 99% eccrine glands as opposed to 1% of apocrine glands, and unlike the other animals, we do use them for sweat cooling. In the 1980s, V. E. Sokolov remarked: "Eccrine sweating is a human characteristic as unique as speech or bipedalism." But using eccrine glands for sweat cooling does not appear to be a very effective method for a number of reasons: - It is slow to start: it can take up to 20 minutes for the sweat cooling system to kick in, unlike with apocrine glands which respond instantly. This means that a person is more likely to suffer from heatstroke, or keel over in hot weather, because their bodies are not able to respond to the rise in temperature fast enough.
- It is wasteful of water: a person engaged in strenuous physical activity in a hot climate may lose 10-15 litres of water a day, which has to be replaced quickly, or it will lead to dehydration and death. Unfortunately for humans, we also suffer from another disadvantage that many animals do not have - we cannot store much water in our bodies and we need to drink small amounts frequently.
- It is wasteful of salt: salt deficiency causes cramps and can lead to death. At maximum sweating capacity, humans can lose all their vital sodium in just 3 hours.
- it is dangerous when we overheat quickly: one of the prime causes of death during a heatwave is thrombosis. This is because the fluid lost by sweating is usually not replaced fast enough, causing the platelet count and blood cholesterol to rise, leading to stroke or heart attacks, especially in the elderly.
It seems clear therefore that using the eccrine glands for sweat cooling is extremely inefficient in areas which are far from water and sources of sodium such as the African savannah. So why is the human body covered in eccrine glands, and what happened to our apocrine glands? The mystery of the human cooling system  Fully aquatic mammals such as whales and dolphins do not sweat, because they do not need to. Their primary need is for keeping warm and they have plenty of blubber to do this. A number of other animals that do not have sweat glands include: hippos, rhinos and pigs. These animals are either semi-aquatic animals that rely on entering a body of water to cool down, or have possibly descended from a semi-aquatic ancestor. Terrestrial mammals, as we have seen, utilise modified apocrine glands (which may have evolved primarily for scent marking) as a way of releasing enough moisture to cool themselves down, or by panting through their mouths. They tend to use the relatively few eccrine glands on their paws or hands/feet to enable a better grip. The arboreal primates, who need these more than fully terrestrial mammals, have more eccrine glands, but human beings appear to have lost all of the apocrine glands on their bodies with the exception of those under our arms and pubic areas, and instead, our bodies are covered in eccrine glands that we utilise - totally inefficiently for a terrestrial animal - to produce copious amounts of liquid and salts as a means of reducing our body temperature, even when we risk dehydration and death. Why?
The only other mammal that appears to sweat as abundantly as humans is the northern fur seal (Callorhinus) when they are on land. The most likely reason for this is that they have a very thick layer of blubber that keeps them warm in the sea, but can cause them to overheat when they come ashore. Almost any activity on land causes them to pant and raise their hind flippers, which are abundantly supplied with sweat glands, and wave them about. [WN McFarland cs 1979 "Vertebrate life" Collier p.773]
Another reason why evolution may have reassigned the function of our eccrine glands and distributed them all over our body could be for maintaining water/salt homeostasis while in a marine environment by the purpose of 'reverse osmosis'. If we were able to absorb water from the sea, but not the salt, this would have enabled our ancestors to survive in the sea, with no access to fresh water.
Why did we lose most of our apocrine glands? As we have seen, apocrine glands probably evolved for scent communication among terrestrial mammals and adapted to provide a farily efficient means of sweat cooling in hotter climates. It is generally accepted that the reason most aquatic mammals have lost their apocrine glands is because they have no use for them, for either of these reasons. This would also explain why hippos and pigs have lost their apocrine glands, and why water buffalo have only 10% as many as domestic cattle. Apocrine glands 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. When the need for sweat cooling arose in the hominids, they could not resort to using their apocrine glands, because they were no longer there, so they modified their eccrine glands to do the same job instead. It's clearly a fairly recent adaptation and it is by no means perfect, as we have seen, but in an environment where both water and salt were in plentiful supply, it would do quite nicely, thank you very much.
Genetic evolution The AQP7 gene shows a human lineage-specific (HLS) copy number increase in humans compared to great apes, monkeys, and prosimians. This copy number increase (at least 5 copies in humans vs. 1-3 in chimpanzees and other apes) was detected by cDNA array-based comparative genomic hybridization (Fortna et al. 2004). Aquaporins are thought to play a key role in water transport across membranes (Preston et al. 1992), and of the eight aquaporin family members assayed, the only one that showed an HLS copy number increase was AQP7 (Dumas et al. 2007; Fortna et al. 2004). AQP7 is also an aquaglyceroporin, and in addition to water, AQP7 is also capable of transporting glycerol, an energy storage molecule. Most of the AQP7 copies map to the pericentromeric region of chr 9, one of the most evolutionarily dynamic regions of the human genome and the location of the greatest concentration of HLS gene copy number increases (Dumas et al. 2007; Fortna et al. 2004). The HLS increase in copy number of AQP7 is a candidate to play a role in thermoregulation (sweating) and energy mobilization (via glycerol transport) from fat to muscle. [2] Further references: [3] [4]
Morphology and distribution of sweat glands in the Cape fur seal, Arctocephalus pusillus pusillus (Carnivora: Otariidae)
Lia Rotherham, M van der Merwe, Marthán N Bester & WH Oosthuizen 2005
Austr J Zool 53 doi 10.1071/ZO04075
...
Sweat glands have an important role in thermo-regulation. We investigated the presence/absence of sweat glands in the fur-covered & naked skin areas of the Cape fur-seal, using standard histological procedures & light & scanning electron microscopy. Sweat glands were present in both fur-covered & naked skin areas.
The skin layers in the naked skin areas were thicker than those in the fur-covered areas, presumably to protect them against abrasions in the absence of hair.
The density of apocrine sweat glands did not differ among the body regions, but both apocrine & eccrine sweat glands were larger in naked skin areas than in fur-covered areas.
This increased size of the glands suggests:
- a more active role for the glands in the naked skin areas &
- a higher heat-loss capability through evaporative cooling in these body regions.
Coping with Heat:
Function of the Natal Coat of Cape Fur Seal (Arctocephalus Pusillus Pusillus) Pups in Maintaining Core Body Temperature
Nicola Erdsack, Guido Dehnhardt & Wolf Hanke 2013 PLoS ONE 8(8):e72081 doi 10.1371/journal.pone.0072081
Cape fur seal pups spend the first weeks of life exclusively or mainly ashore.
They are exposed to intense solar radiation & high Tps for long time periods, which results in Tps up to at least 80°C on their black natal coat.
Does the natal coat have a crucial function in coping with these extreme conditions?
We investigated the insulating properties of the natal coat in 6 captive newborn Cape fur-seals during the first 50 days after birth.
The natal fur differs from the adult fur not only in colour, but also in density, structure & water repellence.
We measured Tp on the fur surface & within the fur, & skin & rectal Tp under varying environmental conditions, comparable to the species' habitat.
Experiments were designed to not influence the spontaneous behaviour of the pups.
- Rectal Tp was constant as long as the pups stayed dry, even during long-lasting intense solar radiation for up to 3 h.
- Skin Tp remained close to rectal Tp as long as the fur was dry, while with wet fur, skin Tp was significantly reduced as well.
Our results show:
the natal coat provides an effective insulation against overheating.
The severely reduced insulation of wet natal fur against cold supports the assumption that the natal fur is an adaptation to the pups' terrestrial phase of life.
Cetacea (whales & dolphins) & Sirenia (manatees & dugongs) don't have sweat glands.
Pinnipedia (seals, sea-lions & walruses), Lutrinae (otters), Castoridae (beavers), the kinkajou (honey bear) & Homo all have both extensive sweat glands & squalene-rich sebum for waterproofing.
Furseals (which are otariids = sealions) sweat abundantly thermoregulatorily on land.
Bartholomew & Wilke 1956 J.Mammal.37:327-333 "Body temperature in the northern fur seal, Callorhinius ursinus"
Sweat & SQ-rich sebum combined would appear to be the semi-aquatic signature.
The Skin of Non Human Primates, William Montagna [pdf]
Sweating is an essential and uniquely human function [website link]
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