Wikipedia and the scientific community
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.
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.
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.  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 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 did humans evolve a different method for sweat cooling to almost all the other terrestrial animals, 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]
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.
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. 
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