Human Haircoats and Evolution

[popsy]

Most land mammals have a haircoat that gives them some protection from the elements and helps them maintain a constant body temperature. Humans don't have much, even those of us that are considered hirsute have a laughably sparse haircoat. Why don't we have more hair? I assume that at some point in our evolution, our distant ancestors didn't have highly vulnerable naked skin. What is the reason that we have become a virtually hairless species?

Is there an accepted school of thought on the subject? I am admittedly ignorant on this, but am very curious, as it seems as if we gave up a lot when we gave up hair, so I'm assuming there must have been an evolutionarily compelling reason.

[Gwyn ap Nudd]

Many animals that have adapted to living in water (e.g. whales, dolphins, and seals) or mud (e.g. hippos and pigs) have lost most of their fur and use body fat for insulation, instead. There is a theory out there that when we first separated from the apes, we were forced into a swamp. We didn't stay long enough to fully adapt, but it was long enough to lose the heavy coat.

Another theory is that we were selectively bred for less fur. In this theory, bare skin would make it harder for fleas, lice and other parasites to cling, and impossible for them to hide. A bare-skinned mate is easier to groom, and it is easy to see how strong and healthy he is. So bare skin became a desirable trait in a prospective mate.

http://www.freerepublic.com/focus/f-news/966532/posts

[Art Vandelay]

I guess there's also the question of whether we wear clothing because we don't have fur, or whether we lost our fur because we started wearing clothing (and did pigs lose their hair because of being in mud, or did they start being in mud because they lost their hair?)

Or we perhaps we just evolved in a really hot environment?

[athon]

The most commonly accepted reason is actually quite simple.

At some point it became more of an advantage to move from trees to open grassland. We probably adapted to bipedal movement as it was less of an advantage to have quadrapedal movement in an open area, and bipedal movement uses less energy.

Out in the open, heat loss through perspiration has more of an advantage than when in an enclosed area, such as a wood or forest. We have more sweat glands than any other land-based mammal. It also means we can use our mouths to communicate, rather than pant to lose heat. Those humans with less body hair could lose body heat faster and more efficiently. Retaining it on the scalp assisted in greater control (most heat is lost through the head, hence being bald would mean losing heat too quickly. This has homeostatic repurcussions).

The whole aquatic ape theory is nice, but doesn't work for a whole number of reasons. The first is that a number of semi-aquatic mammals never lost their hair. If it is of such an advantage in the water (which I doubt) we'd surely see more of it. The perspiration theory works because there are numerous selective pressures that are unique to human development.

What I want to know is why we can grow our hair to ridiculous lengths? Having this condition (lout's disease) myself, I figure having long hair can only be of disadvantage. Could it be a form of sexual selection?

Heelloooo ladies. ;) :D

Athon

[Kiless]

Originally posted by athon
What I want to know is why we can grow our hair to ridiculous lengths? Having this condition (lout's disease) myself, I figure having long hair can only be of disadvantage. Could it be a form of sexual selection?

Heelloooo ladies. ;) :D


Bipedal lout. :p

[Rolfe]

Originally posted by athon
Out in the open, heat loss through perspiration has more of an advantage than when in an enclosed area, such as a wood or forest. We have more sweat glands than any other land-based mammal. It also means we can use our mouths to communicate, rather than pant to lose heat. Those humans with less body hair could lose body heat faster and more efficiently. Retaining it on the scalp assisted in greater control (most heat is lost through the head, hence being bald would mean losing heat too quickly. This has homeostatic repurcussions).This, as I understand it, is the currently favoured theory. Or it was the last time I looked, which was about 20 years ago!

See "The Energetic Paradox of Human Running and Hominid Evolution (http://www.publicanthropology.org/Archive/Ca1984.htm), by D. R. Carrier. In this paper, Carrier argues that the hairless condition of man is particularly important in improving the efficiency of cutaneous evaporative heat loss, and advances the hypothesis that this is an important evolutionary reason for the loss of the hair coat, in that it improved man's performance as a persistence hunter.

Interestingly, the only other species which uses profuse quantities of aqueous sweat as its main thermoregulatory mechanism, the horse, adopts an entirely different strategy. The horse is able to pant as well as sweat, it has not lost its hair coat, its sweat glands have a different embryological origin from human sweat glands, and it chooses to secrete thermoregulatory fluid which is isotonic to the ECF and so prioritise maintenance of ECF tonicity over elecrolyte conservation (while man prioritises electrolyte conservation over maintenance of ECF tonicity).

This may be understood by realising that the horse is not a hunter, it is the hunted, and therefore its strategies have a different aim. The difference is mainly noteworthy in that exercising humans drink frequently during exercise while horses can go for a very long time without needing - or even apparently wanting - to drink.

Sorry I will now shut up about the subject of my PhD thesis....

Rolfe.

[Gwyn ap Nudd]

Originally posted by Art Vandelay
I guess there's also the question of whether we wear clothing because we don't have fur, or whether we lost our fur because we started wearing clothing (and did pigs lose their hair because of being in mud, or did they start being in mud because they lost their hair?)

Or we perhaps we just evolved in a really hot environment?

Since we lost our fur somewhere between 1.2 and 1.7 million years ago, and didn't start wearing clothes until from 42 to 72 thousand years ago....

From 1.6 million years ago the world was in the grip of the Pleistocene ice age, which ended only 10,000 years ago. Even in Africa, nights could have been cold for fur-less primates. But Dr. Ropers noted that people lived without clothes until recently in chilly places like Tasmania and Tierra del Fuego.

[H3LL]

I understood it to be sex.

An instinctive predisposition for a specific characteristic in choosing a mating partner can and does produce rapid changes in a species (the evolutionary term I forget), as indicated by some highly impractical mating related adornments such as peacock feathers and bright red inflatable throat pouches.

There is no doubt that human mating habits are highly prolonged and tactile compared to other mammals. Fur would inhibit much of this.

It is also unusual in mammals (if not unique...maybe someone knows. I know bonobos are very active but unsure if fertility is indicated) for humans to be sexually receptive all year and for females not to indicate when they are fertile.

The fullness and roundness of human females, I understand, is only exhibited when fertile and receptive in other primates. The female chimpanzee being an obvious example.

I personally find the "aquatic ape" idea a little flawed as other obvious adaptations to an aquatic lifestyle that could conceivably have occurred throughout the same time-frame are conspicuous by their absence. The great difficulty we have looking ahead while swimming, being one and the swimming instinct disappearing after only a few months (evolutionary suicide for offspring born into an aquatic environment) while many non-aquatic mammals retain the ability although they may never use it.

[Deetee]

Who wants to see a skepchick that looks/feels like a gorilla? (apologies here to tragic monkey and BSM).
All I can say is thank God for evolution ;)

[Asolepius]

Originally posted by Deetee
Who wants to see a skepchick that looks/feels like a gorilla? (apologies here to tragic monkey and BSM).
All I can say is thank God for evolution ;)
Other gorillas do. However you are genetically programmed not to fancy gorillas.;) Presumably....

[athon]

Originally posted by Rolfe

Interestingly, the only other species which uses profuse quantities of aqueous sweat as its main thermoregulatory mechanism, the horse, adopts an entirely different strategy. The horse is able to pant as well as sweat, it has not lost its hair coat, its sweat glands have a different embryological origin from human sweat glands, and it chooses to secrete thermoregulatory fluid which is isotonic to the ECF and so prioritise maintenance of ECF tonicity over elecrolyte conservation (while man prioritises electrolyte conservation over maintenance of ECF tonicity).

Sorry I will now shut up about the subject of my PhD thesis....

Rolfe.

Rolfe, my day was going to be worthless until now. I have learned something very cool. So, even if I'm the only one reading it, post away about your thesis.

Thanks for teaching me!

Athon

[athon]

Originally posted by H3LL
I understood it to be sex.

Well, that's pretty much all we men think about, so I guess...

An instinctive predisposition for a specific characteristic in choosing a mating partner can and does produce rapid changes in a species (the evolutionary term I forget), as indicated by some highly impractical mating related adornments such as peacock feathers and bright red inflatable throat pouches.

Sexual selection plays a large role in evolution (hence why I thought maybe long cranial hair might be linked with it), but doesn't adequately explain our loss of hair.

One area we have retained hair is in the pubic region. Surely this is where 'tactile' sensation would be best without hair (anybody care to comment?). Or observation of sexual sensitivity or fertility in females, would be impeded by this hair.

Athon

[Rolfe]

And what about the fact that many humans, especially the female of the species, seem to think that hair (especially fur) is seriously sexy?

Rolfe.

[Asolepius]

Originally posted by athon


Well, that's pretty much all we men think about, so I guess...

[/b]

Sexual selection plays a large role in evolution (hence why I thought maybe long cranial hair might be linked with it), but doesn't adequately explain our loss of hair.

One area we have retained hair is in the pubic region. Surely this is where 'tactile' sensation would be best without hair (anybody care to comment?). Or observation of sexual sensitivity or fertility in females, would be impeded by this hair.

Athon [/B]
But this is sexual selection again - very important to have a clear sign of maturity (a sort of badge). I could never figure out armpits though. Also I don't know if this is retention of hair or secondary development after we became completely bald all over. Will we ever know?

Mind you, I remain baffled by male pattern baldness. Why did it arise, and why did it not die out? Maybe it's another `badge'.

[athon]

Originally posted by Asolepius
But this is sexual selection again - very important to have a clear sign of maturity (a sort of badge). I could never figure out armpits though. Also I don't know if this is retention of hair or secondary development after we became completely bald all over. Will we ever know?

For some reason, I don't think this works. It might well explain why we remain relatively hairless until puberty, but there are other physical clues that convey maturity. For instance, if maturity-display were the sole reason, a female with breasts but no pubic hair would surely be fitter ('fit' in a non chav way...lol) than females with both pubic hair and breasts.

I won't argue that it plays no role, but I wouldn't limit it to that reason.

Mind you, I remain baffled by male pattern baldness. Why did it arise, and why did it not die out? Maybe it's another `badge'.

As I understand, it's passed on via the x chromosome. Hence it is maternal. Add to that the fact that it doesn't express until later in breeding age, it becomes easier to see how maybe it was never lost.

Athon

[Asolepius]

Originally posted by athon

As I understand, it's passed on via the x chromosome. Hence it is maternal. Add to that the fact that it doesn't express until later in breeding age, it becomes easier to see how maybe it was never lost.

Athon
You are right. It's on the X chromosome but only turned on by testosterone. You are also probably right about breeding age, as our ancestors must have bred as soon as they became fertile. All this explains why it didn't get selected out, but not why it became so widespread in the first place. BTW is it true that native Americans don't go bald? Neither do native Africans as far as I can tell. Sorry, this is traying off-topic a bit - and I hope I am not splitting hairs ;) .

[popsy]

Originally posted by athon
The most commonly accepted reason is actually quite simple.

At some point it became more of an advantage to move from trees to open grassland. We probably adapted to bipedal movement as it was less of an advantage to have quadrapedal movement in an open area, and bipedal movement uses less energy.

Out in the open, heat loss through perspiration has more of an advantage than when in an enclosed area, such as a wood or forest. We have more sweat glands than any other land-based mammal. It also means we can use our mouths to communicate, rather than pant to lose heat. Those humans with less body hair could lose body heat faster and more efficiently. Retaining it on the scalp assisted in greater control (most heat is lost through the head, hence being bald would mean losing heat too quickly. This has homeostatic repurcussions).

The whole aquatic ape theory is nice, but doesn't work for a whole number of reasons. The first is that a number of semi-aquatic mammals never lost their hair. If it is of such an advantage in the water (which I doubt) we'd surely see more of it. The perspiration theory works because there are numerous selective pressures that are unique to human development.

What I want to know is why we can grow our hair to ridiculous lengths? Having this condition (lout's disease) myself, I figure having long hair can only be of disadvantage. Could it be a form of sexual selection?

Heelloooo ladies. ;) :D

Athon


Thanks for the explanation. Could the long-growing head hair be a compensation for a naked body? The long hair falling over the head and shoulders could help direct rain away from the body. As for pubic hair and armpit hair seems like I read somewhere that it helped to hold scent which was helpful in finding a mate.

[Soapy Sam]

Here's a guess. It may seem daft. I've never seen it in print.

A naked man, with a typical distribution of patchy hair, is rather well camouflaged in several different environments, including open sand desert, semi arid limestone terrain, scrub and grassland.

It works best for tanned white men. Really dark skin destroys the effect.

Try it sometime. You know you want to.

[popsy]

Originally posted by Soapy Sam
Here's a guess. It may seem daft. I've never seen it in print.

A naked man, with a typical distribution of patchy hair, is rather well camouflaged in several different environments, including open sand desert, semi arid limestone terrain, scrub and grassland.

It works best for tanned white men. Really dark skin destroys the effect.

Try it sometime. You know you want to.

Does Scotland have open sand deserts? :-D

[aerosolben]

Originally posted by popsy
Does Scotland have open sand deserts? :-D
The image that appeared in my mind upon reading this was a horde of naked Scotsman stealthily sneaking up on an unsuspecting caravan in the desert.

One group distracts the target by waving their bits and pieces about in a comical manner, while another group sneaks up behind them and knocks them down with an overstuffed sheeps' bladder. The datardly Scots then proceed to kick their victims while they're down, and then make off with their whisky and potatoes.

Also, I haven't been sleeping enough lately.

[AWPrime]

Originally posted by Rolfe
And what about the fact that many humans, especially the female of the species, seem to think that hair (especially fur) is seriously sexy?

Rolfe.

That is the reason that humans have fur-envy.


Also a factor for the lack of a furcoat (and a tail) is that they need to be supported and that takes food and water.
So when there is a lack of food/water in a warm climate, evolution would favor the tail/furless.

In a very cold climate the situation would be reversed, but humans evolved in a warm climate.

Now it would likely take genetic manipulation to give fur to those in colder climates.

[Soapy Sam]

Nothing comical about Scotsmens' bits and pieces, I'll have you know!

Scotland is lacking in deserts. Deserts are not lacking in Scotsmen though. We get around.

Kidding apart, the effect really can be quite significant.
I've been surprised on several occasions by how hard it can be to spot a man working in just a pair of shorts in clear desert in bright sunshine. You tend to see glare from tools first , then the brain suddenly puts together the image of the man holding the tools.

I don't know if anyone has ever considered disruptive camouflage as an explanation of male hair distribution or hairlessness.

Mind you, the glare from a bald bonce rather works against the theory. I speak from experience.

[athon]

Interesting idea, Soapy. Possibly a selection pressure, although I wouldn't imagine a very strong one.

Popsy mentioned that pubic hair helps to trap and hold scents. This is a theory that has been touted quite a bit, and I like it. It makes sense as to why we have hair in places that, well, are rather fragrant.

I asked a colleague before, and he offered that the beard and long hair might have something to do with appearing more fierce, like a lion's mane. Interesting speculation, I thought.

Athon

[Rolfe]

I've had another look through my PhD thesis, because I remember dealing with this point.Man is a bipedal runner and so there is no obligatory coupling of respiration to locomotion, which suggests that it should be possible for him to pant even while running, if necessary. It may be, however, that the particularly short, flat face of the human species (possibly a result of the necessity of balancing a large-brained head above an upright body) and the consequent small area of oral and nasal mucous membrane in comparison to body size, has rendered panting so inefficient that it was abandoned. Carrier (1984) points to the hairless condition of man as being particularly important in improving the efficiency of cutaneous evaporative heat loss, and advances the theory that this is one evolutionary reason for the loss of the hair coat, as it improved man's performance as a persistence hunter. Considering this from the viewpoint of man as a non-panting animal, however, it may be that the improved efficiency of sweating provided by the absence of a hair coat is a necessary substitute for panting in man as a means of thermoregulation while at rest. Indeed, while the horses were beginning to pant as they entered the environmental chamber, the humans were removing all the outer clothing that decency permitted!It's an interesting paradox. A horse seems to rely on panting to cope with high temperatures at rest - sweating alone doesn't seem to cut it. However, horses cannot pant while they gallop, because the respiratory rate is irrevocably tied to the stride rate (the muscles of the forelimb are involved in respiration). A human runner could pant while running, but doesn't. In fact, humans don't and can't pant, even at rest.

Lacking the panting mechanism, have we lost the hair so that sweating becomes a more efficient way of dealing with high environmental temperatures (rather than coping with metabolic heat load)? Just my tuppenceworth, but they did give me a PhD for it.

Rolfe.

[Badger]

Rolfe, I read somewhere (J. Bronowski, maybe? I don't know) that man can run down almost any land animal. The gist was that we can keep up a rapid enough pace for a long enough time to allow us to eventually catch up to an exhausted animal. I can't remember for sure, but it seems to me that the reference timeperiod was over days.

Did you run across this in your studies? Or has it been disproven?

[Rolfe]

Originally posted by Badger
Rolfe, I read somewhere (J. Bronowski, maybe? I don't know) that man can run down almost any land animal. The gist was that we can keep up a rapid enough pace for a long enough time to allow us to eventually catch up to an exhausted animal. I can't remember for sure, but it seems to me that the reference timeperiod was over days.

Did you run across this in your studies? Or has it been disproven? I think this is correct. Most land mammals can't make any sort of endurance because of the lack of any mechanism for heat dissipation during exercise. The fast ones just go for deferred heat dissipation - they let the body temperature rise, and just quit running to cool off when they hit about 40<SUP>o</SUP>. Trials with cheetahs trained to run on treadmills showed that when they reached the crucial temperature (after about a mile at the most) they simply lay down and let the treadmill belt slide under them.

Dogs can only hack endurance running at extremely low environmental temperatures.

I'm a bit hazy about the camel, because it's touted as being another evaporative sweater, but I couldn't find any actual publications involving that species. I was intrigued by how similar the horses' strategy was to that generally recognised for the camel though - don't drink at all until you reach base camp, then suck up water for what seems like half an hour without drawing breath.

The species to beat is the horse, and human runners definitely can, over a sufficiently long course. Recently, someone walked (or rather ran) off with a longstanding prize for beating a horse over a comparatively short course - though I have to say I would consider this rigged, as the medical restrictions on how hard you are allowed to push a horse are far tighter than those on what humans are allowed to do to themselves.

My only caveat is that if you tried to chase a species with a particularly fast sprint, could they put enough distance between you to be able to recover and manage another sprint when caught up with? I'm not absolutely sure, but I rather think the hunter would win in the end.

But look at the body man is issued with. The sheer proportion of leg muscle to total body mass is impressive in anyone's book. The only reason we don't perform to potential is that most of us are unfit lazy slobs.

Rolfe.

[popsy]

Rolfe,

I've really enjoyed your PhD excerpts. Some new information for me to think about.

About horses: An acquaintance of mine swore that her horse liked cantering (galloping) I asked her how she knew that and she said because of the way it breathed. One breath per stride somehow meant that it was 'happy' to be cantering. I don't remember her reasoning behind her supposition. Of course she also was the one that asked me not to laugh in front of her horse because the horse thought it was being laughed at and got hurt feelings.:rolleyes:

[Rolfe]

Originally posted by popsy
An acquaintance of mine swore that her horse liked cantering (galloping) I asked her how she knew that and she said because of the way it breathed. One breath per stride somehow meant that it was 'happy' to be cantering. I don't remember her reasoning behind her supposition. Of course she also was the one that asked me not to laugh in front of her horse because the horse thought it was being laughed at and got hurt feelings.:rolleyes: In the horse the respiration rate is tied to the stride at the canter and gallop, but not at the walk or trot. If you listen to walking or trotting horses breathe, it's often quite irregular. So when they go into a canter, it will sound as if their breathing is steadying and becoming regular.

That's a new interpretation on it, though!

Rolfe.

[Bearguin]

Originally posted by popsy
Rolfe,

I've really enjoyed your PhD excerpts. Some new information for me to think about.


Ditto.

[Soapy Sam]

In fact, humans don't and can't pant, even at rest.-Rolfe.


I thought I could pant.
Many's the time I've stood in Strathclyde Park, bent double, head between knees, purple faced and gasping to breathe after a lap or two of the pond.

If I wasn't panting, what was I doing? Or to put it another way- what is the technical definition of panting?

[Kiless]

Originally posted by Soapy Sam
If I wasn't panting, what was I doing? Or to put it another way- what is the technical definition of panting?

Is this of help? "Panting does not involve full alveolar inflation and O2/CO2 exchange. Most of the air movement in panting is in the larger bronchioles, bronchi and trachea and this is called their "dead space" , where there is no air exchange. Dogs breathe this column of dead space air up and down when they pant." http://www.newton.dep.anl.gov/askasci/vet00/vet00018.htm

So, the act of panting is not the same as humans breathing or hyperventilating?

[Kiless]

Originally posted by AWPrime
That is the reason that humans have fur-envy.

Fur envy? Is that why I have all these female students who are horse crazy?? :eek:

[Kiless]

Originally posted by athon
One area we have retained hair is in the pubic region. Surely this is where 'tactile' sensation would be best without hair (anybody care to comment?). Or observation of sexual sensitivity or fertility in females, would be impeded by this hair.

I thought it had lubricating qualities. Or at least, that's what was said on a late night radio show about dating / sex.

[Rolfe]

Originally posted by Kiless
Is this of help? "Panting does not involve full alveolar inflation and O2/CO2 exchange. Most of the air movement in panting is in the larger bronchioles, bronchi and trachea and this is called their "dead space" , where there is no air exchange. Dogs breathe this column of dead space air up and down when they pant." http://www.newton.dep.anl.gov/askasci/vet00/vet00018.htm

So, the act of panting is not the same as humans breathing or hyperventilating? Exactly. Soapy Sam was hyperventilating. Tachypnoea and hyperpnoea for the purpose of getting more oxygen in.

Panting is tachypnoea for the sole purpose of losing excess heat by evaporation from the oral and nasal passages, and as Kiless says, mostly involves movement in the upper airway passages to avoid problems with hypocapnia (excess loss of CO<SUB>2</SUB> due to overbreathing). Dogs use the mouth, mainly, but horses can't mouth-breathe in any case, so the panting is all carried out via the nose.

It was really interesting, leading the horses into the hot, humid, environmental chamber, and just leaving them standing there. They started to puff out their nostrils and pant almost at once, well before any trace of sweating started. A couple of them maintained their body temperatures for considerable lengths of time without sweating at all, which is a bit of a pain when the point of the experiment is largely to collect sweat samples.

At the same time, I just had to strip off, down to the thinnest aertex shirt I had. (This was a nuisance, because this was during a cold winter in Ayrshire, and the lab outside the environmental chamber didn't have much heating, and every time I came out, sweating like a pig, it was like a cold plunge. In the end I just had a pile of clothes at the door, and suited up to go outside.) I watched the horses, and tried to see if I felt any impulse to do what they were doing, but didn't. I tried to breathe in synch with them, but couldn't keep it up for more than a very short time, nose or mouth breathing, and I felt myself becoming dizzy from hypocapnia. That's when I realised I just didn't have the upper airway capacity to do what they were doing. Tiny mouth and nose compared to horses and dogs. Just not anatomically equipped for any meaningful evaporative heat loss there.

Why? Well, like I said, if you walk upright, a long snout could be very difficult to organise. It's all connected back to the bipedal running and the use of the hands, I suppose.

Rolfe.

[athon]

Rolfe, again thanks for the info. I am learning so much here.

It's led me to wonder, however...

Is sweating a more or less efficient way of losing heat? Do you lose more water through sweating? Would early man have needed to be kept hydrated better?

The nasal theory works, however I am curious to know whether the loss of nasal space is still linked with greater communication. We could support some form of snout for cooling, surely, regardless of bipedal movement. Neandertals developed it (albeit to reclaim exhaled heat rather than lose it through evaporation) to a limited extent.

Athon

[Soapy Sam]

I have to know.


Do elephants pant?

[Rolfe]

Originally posted by athon
Is sweating a more or less efficient way of losing heat? Do you lose more water through sweating? Would early man have needed to be kept hydrated better?Sweating is much more efficient than panting. Or mud wallowing or saliva spreading or whatever else critturs have come up with. However it has a much bigger cost as far as fluid/electrolyte conservation are concerned.

Man absolutely has to drink during exercise. This is because human sweat is very dilute indeed and if you don't drink, your plasma electrolyte concentrations will go through the roof, which is seriously not good. (You do lose some salt of course, which has to be replaced, but as you acclimatise, the sweat concentration ajusts to minimise this.) Ever seen a marathon course? Drinking cups all over the place.

In contrast, you can take a horse to water, but you can't make it drink. Horses secrete thermoregulatory fluid which is isotonic to ECF (combining hypertonic sweat with respitatory water loss). Thus, during endurane exercise, they simply shrink like prunes. They lose a lot, but the concentration of what remains doesn't change. So they don't need to drink during exercise, and often prefer not to. Get them back to base camp and they simply siphon it up, like camels. They then use the salt content of their capacious large intestines to make sure their plasma electrolytes have a soft landing, and just normalise rather than plummeting. Neatest thing I ever saw.

I hypothesise that the difference is due to man being a hunter, and being able to stop to drink or even carry water. While the horse is prey, and daren't stop. Also, man lacks the large intestine of the horse, while the horse lacks the ability to carry water.

Human and equine sweat glands are actually quite different structures, with different embryological origins - clearly an example of parallel evolution, two single and unique examples because as I said, no other species use their sweat glands for thermoregulation (grip and scent marking are more popular). And although the end purpose is the same, the mechanisms chosen are elegantly different, reflecting the differences in the requirements of the two species.

(Actually, just an idea - the equine large intestine is often ridiculed as bad design, in the wrong place and subject to catastrophic failure. I wonder if the whole salt regulation is the reason? You couldn't work that one with a rumen instead of a caecum. But on the other hand the horse evolved from a thing like a rabbit, which has the equine intestine but doesn't sweat. But then rabbits double-digest their food, getting round the problem of the fermentation vat being after the absorption tube by eating their first-time-round faeces. Horses don't. Any connection? Don't mind me, wild speculations which may be entirely moonshine.)

But the bit before the bracketed section is kosher, and I discovered it! My one contribution to real original knowledge. Except I saw someone had speculated that's what might be going on, in an aside in an obscure textbook, after my thesis was submitted. Nothing new under the sun!

Rolfe.

Soapy, I don't know. They have to lose heat somehow, but they could mud wallow, or spray themselves with water from an external source.

[AWPrime]

Wouldn't radiating heat be even more efficient that sweating (not that I know of any species that does this).

[Soapy Sam]

Elephant ears are radiators for elephants- lots of blood vessels, huge surface and flappable. I have to assume though, that a nose that long is involved in air cooling.

I don't recall much evidence of elephant sweat. But they certainly love getting wet. And they like shade.

Horses, like dogs have a long history of human tampering in their make-up. I wonder how different their internal design is from zebras for instance? Are they, like cows, becoming animals so symbiotic with humans that they are being pushed away from selective viability without humans?

Do they, for instance, drink like camels because we have selected them that way?

Fascinating stuff. And I thought they were just udderless cows.

[Rolfe]

Originally posted by AWPrime
Wouldn't radiating heat be even more efficient that sweating (not that I know of any species that does this). Everything does it to some extent, and as Soapy Sam has just pointed out, that is a function of the elephant ears. However, it's far less efficient than evaporative cooling. And it doesn't work at all if the environmental temperature is higher than the body temperature - as it often is, in the tropics. Or - am I really meaning conducted heat here? Well, radiating IR doesn't get rid of a lot, in comparison with the heat load involved.

Rolfe.

[Soapy Sam]

I think in air, the distinction between heat radiation and conduction / convection becomes pretty vague.

[davefoc]

Not sure I agree with that last comment Soapy Sam.

It seems that they are two different mechnamisms with not much grey area between the two.

Clearly we can be warmed by a fire when the air between the fire and us is quite cold. We are being warmed by radiant heat and losing heat through conduction/convection to the air.

Conversely, if one walked into cold room and abrubtly raised the temperature of the air in the room I think one would still continue to lose heat to the walls through radiation.

[davefoc]

a few comments on different subjects.

Rolfe
I enjoyed your comments about heat loss in horses thanks. Also, I had no idea that horses can't breath through their mouths. Interesting. Is that typical of most mammals?

Advantage of bipedalism
I have read a variety of ideas about this over the years. My personal favorite is my own idea. The driving force behind bipeadalism was the club. A club gives a human a huge advantage over a similar sized animal or or over a similar sized male human when we are fighting for the charms of fair maidens. So, my thought, is that once that first guy figured out he could pick up a club and whack somebody his genepool and the genepool of others that adapted to the needs of being a mobile club swinging platform were on the fast track to sticking around.

[Soapy Sam]

Davefoc-
In the case of high temperature infrared sources like fires, then yes, the distinction becomes clear, but I only mean in the context Rolfe described- homeotherms in hot, probably tropical climates. I think it would be hard to distinguish between heat loss through conduction in probably moist and moving air and heat loss through radiation and convection.

ETA-Incidentally, have you ever seen "2001 A Space Odyssey"? Clarke and Kubrick were also members of the club Club.

[popsy]

Originally posted by Soapy Sam

>snip<
ETA-Incidentally, have you ever seen "2001 A Space Odyssey"? Clarke and Kubrick were also members of the club Club.

Isn't the point of "2001", that is if it has a point, that the club led to technology (HAL) taking over and the downfall of humans? I can't remember the guy's name - was it Adam? - that HAL cast out into the wilderness(space)?

[davefoc]

Originally posted by Soapy Sam
Davefoc-
In the case of high temperature infrared sources like fires, then yes, the distinction becomes clear, but I only mean in the context Rolfe described- homeotherms in hot, probably tropical climates. I think it would be hard to distinguish between heat loss through conduction in probably moist and moving air and heat loss through radiation and convection.

Hmm, I still don't think I agree but I am on shakier ground here. Assuming one is in a jungle surrounded by plants, dirt rocks and air that are all approximately at the same temperature as one's body I would expect the radiation heat loss to be limited to a small amount that one might lose to the sky. And of course conduction/convection cooling would be small also because the air and the body are at nearly the same temperature. So most of the heat loss needs to come through evaporative cooling. Hmm, maybe that's what you're saying.


ETA-Incidentally, have you ever seen "2001 A Space Odyssey"? Clarke and Kubrick were also members of the club Club.

I never made the connection between my theory and 2001. What a maroon. I thought it was something of an original idea although I was sure I wasn't the only person to have ever thought it. I am afraid I saw that movie many years before I had my "original" thought. The idea came to me, I thought, when I was in the woods and I realized just how much more willing I was to go investigate the unexplained shaking in the bush with a club in my hand than I was without it. It was also based on my observations of how much more easily four legged animals travel through rough or steep terrain than us humans with our high centers of gravity so it looked like we needed some really good advantages to compensate for being so much slower and awkward when we traveled than a four legged animal is.

[Soapy Sam]

I find I'm fairly stable when I crawl. No matter how drunk I am, I don't fall over!.
And yes, I do agree about clubs. There are few things more comforting on a dark night than a three cell Maglight! :D

Popsy- It was Frank Poole that Hal jettisoned. Clarke brought him back to life in "3001 The Final Odyssey".

[Pangloss]

Three comments:

Sexual selection theory
Any characteristic that evolves because it is attractive to the opposite sex takes energy from other potential functions, so in an environment of resource scarcity needs to be functionally valuable on a regular basis as well as an occasional turn-on. I don't know that hairlessness qualifies - for example, my wife won't let me shave my beard.

Savannah Life
I remember watching a documentary once that advanced the thesis that an upright posture might have evolved at least in part, because it reduces the exposure of the body to direct sunlight at the hottest part of the day. Combined with the advantages to a hunter/nomad of true bipedalism, the adaptation of hairlessness/sweating can be understood as part of a suite of developments suited to exploiting an ecological niche in mid-lattitude savannah country.

Australian Aborigines are tougher
Someone mentioned these folk in a previous post. A recent worldwide analysis of DNA suggests the Australian aboriginal peoples' immediate ancestors were part of a migration movement by modern humans 'out of Africa' that took place during a relatively short time period around 60-70,000 years ago. Whilst aborigines living in Tasmania and the cool southern mainland coast regions and uplands are known to have regularly worn animal skins, they also possess a physiological tolerance of cold greater than that of Europeans, whose ancestors split from mid-latitude wanderers only around 40,000 years ago. Unfortunately, upon the arrival of the latter in the territory of the former, the adaptations that Europeans underwent in the intervening years brought problems for the aborigines that went deeper than 'guns, germs and steel', and to the more direct product of an agricultural lifestyle, the high-carbohydrate diet:
Colagiuri, S., Miller, J.C.B. (1999).
The Metabolic Syndrome: from inherited survival trait to a health care problem.
Experimental and Clinical Endocrinology and Diabetes, 105(sup2), 54-60.

A critical role is proposed for the quantity and quality of dietary carbohydrate in the pathogenesis of the insulin resistance and hyperinsulinaemia which characterise the Metabolic Syndrome. We propose that an insulin-resistant genotype evolved to provide survival and reproductive advantages for the cold-climate, large- game hunters of the last Ice Age who consumed a low carbohydrate, high protein diet with periodic starvation. Insulin resistance would have minimised glucose utilisation by muscles thereby facilitating the preferential utilisation of glucose by the brain, foetus and mammary gland. Beginning about 10,000 years ago, following the end of the last Ice Age and the development of agriculture, dietary carbohydrate increased and the selection pressure for insulin resistance decreased in some groups. Agriculture began in the Middle East and spread throughout Europe long before it was developed elsewhere. Hence the prevalence of the insulin-resistant genotype decreased in Europeans and other groups exposed to a high carbohydrate intake for sufficiently long. Some geographically isolated groups such as the Pima Indians and Nauruans experienced conditions which further diminished the gene pool diversity and resulted in particularly insulin resistant populations. Traditional carbohydrate foods have a low glycaemic index and produce only modest increases in plasma insulin. However, the constant supply of highly refined high glycaemic index carbohydrate in modern diets results in postprandial hyperinsulinaemia. The insulin-resistant genotype is now disadvantageous and predisposes to the development of the Metabolic Syndrome.
Experimental and Clinical Endocrinology and Diabetes abstract

Searching for the reference above, I came across the following charming item:

The Book of Mormon is unique in American literature for its explanation of American Indian origins It says that American Indians were cursed with a dark skin because they were 'Lamanites.' Their way out was conversion (to Mormonism).

[popsy]

quote:

The Book of Mormon is unique in American literature for its explanation of American Indian origins It says that American Indians were cursed with a dark skin because they were 'Lamanites.' Their way out was conversion (to Mormonism).



Is that what happened to Michael Jackson, Mormons 'cured' him of the dark skin? Oh wait, he's not Mormon, I guess he's still cursed.:con2: