Re: Why is Homo sapiens hairless?
Rohinton Collins (firstname.lastname@example.org)
4 Nov 1996 11:29:48 GMT
John Waters <email@example.com> wrote in article
> JW: In the case of Lions and Zebras the hair is very short
> and has little thermoregulation value.
However short it is, it is still capable of holding a layer of air within
it which acts as an insulatory buffer. This would still contrast quite
sharply with no hair at all.
> Its main purpose
> seems to be related to camouflage. Lion infants are
> spotted, to enable them to merge in with the bush country
> where they are born. Zebras are striped to create confusion
> in the minds of potential predators.
I was expecting this. I do not dispute the purpose of camouflage, but it is
a secondary characteristic of fur, not its raison d'tre. After all, there
is nothing stopping a bald animal adopting camouflage by vari-pigmentation
of the skin.
> Generally, hair is adopted by species which need a variable
> insulator. Extra hair can be grown in winter, then moulted
> in the summer. In marine species of mammals, such as seals
> and whales, there is a contant temperature differential
> between that of the marine environment and the mammalian
> core body temperature. So a fixed insulator like blubber is
> perfectly appropriate. Seal infants are born on the land,
> where there is a large differential between day and night
> temperatures, so they have a thick coat of fur.
Exactly. As well as the reasons given above marine species of mammals do
not have hair as it would cause drag in the water, and (for seals etc..)
would retain water when on land which could dangerously lower the body
> Human females have a fixed layer of subcutanous fat
> concentrated in certain parts of their body.
Exactly, only certain parts. This is for storage, not insulation (although
it doubles up on this score)
> This is a
> permanent form of insulation which means they require less
> body hair than primates without such layers of fat.
This is too simplistic. You are equating the present human condition (who
mostly wear clothes, by the way) with that of the first hairless hominid.
> If clothes were used as a means of camouflage, and the
> maintenance of the camouflage was essential for survival,
> then there would be an advantage accruing to hominid
> individuals with less hair, or more sweat glands etc.
Again, camouflage in this instance would also have been a secondary
consideration. Surely clothes would first have been used for insulation and
protection, and then for camouflage?
> However, this form of evolutionary selection would only
> occur if the hominid thermoregulatory mechanisms were
> insufficient to combat overheating problems. This could
> well be the case in daytime savannah conditions in Africa.
Not necessarily. To begin with, early hominids would have sought shelter
during the heat of mid-day, as do other savannah animals (notably, the
baboon). Now they may have lost body hair as a result of wearing clothes
(over many generations - I won't be accused of Lamarckism again ;-) ),
which would be accompanied by a concomitant increase in sweat glands due to
increased efficiency of heat loss due to a more naked body.
OR - an early hominid which sought to take advantage of the time when most
other savannah animals had taken shelter, for foraging, scavenging, or
whatever; would have increased the number and efficiency of sweat glands,
which would have required a concomitant reduction in body hair, in order to
increase heat loss efficiency.