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Why
does human hair on the head grow so long while hair on the rest
of the body will grow to a specific length and stop ?
The
answer is probably rooted in the evolutionary origins of humans.
Our ancestors were considerably hairier than ourselves and certainly
our nearest primate ancestors the orangutans have a good crop
of hair all over their bodies. There is a disease of that involves
total body hypertrichosis called “congenital hypertrichosis
lanuginosa” where hair follicles seem to revert to their
ancestral terminal hair state. Individuals with this condition
have inherited and genetic mutation and this disease can be passed
from parent to child. It is very rare and only occurs in a few
families around the world. However, it would seem that we are
only one genetic step away from our ancestors in terms of having
extensive body hair growth.
Clearly there are advantages to having hair on some regions
of our body. Eyebrows protect the eyes from sweat and sun glare.
Eyelashes protect the eyes by sweeping away dust and debris.
Nasal
hairs trap dust before reaching the lungs. These sites of terminal
hair growth have probably been maintained to keep these advantages.
Scalp hair does give us one advantage of trapping heat. One third
of our heat loss is through our scalps so terminal head hair does
have a practical action. However, this does not explain why it should
grow so long. There are two hypotheses based on evolution to explain
long head hair. 1) Many animals have "manes" as sexual
attractant signals or to increase their apparent size during sexual
contests. Some feel this was a factor in human evolution. Cotton
top tamarins may be an example with their extensive, white, billowing
hair that frames their small face. Mature male gorillas develop
a gray "silver back" appearance that reflects their status
in the group. 2) It has also been suggested that through human evolution
infants needed something to hold onto while being carried by the
mother. Long head hair would serve such a function. Note there is
no real evidence in support of any hypothesis.
What
are the adaptive advantages of a lack of hair in animals?
The primary function of hair is for thermoregulation. Elephants
and rhinoceroses are large animals which have very low surface area
to volume ratios, and live in warm climates, and have no need of
hair to keep warm. In contrast the woolly mammoth, had lots of long
hair and was adapted for cold climates. Other mammals with little
hair might include hippos, whales, the naked mole rat, and manatees/dugongs.
The whales have low surface to volume ratios, insulating blubber,
and presumably have eliminated most hair to cut down on drag in
the water. Technically they do still have hair. The filters they
use to collect food are keratin based and grow from highly modified
hair follicle-like structures. Manatees live in warm coastal tropical
waters and have no need for hair. The mole rat lives in a hot climate
and stays underground pretty much all the time.
Some domestic animals such as pigs or pets such as the sphinx
breed of cats have very little hair. It is either short or sparse.
These strains have been selectively bred and their wild ancestors
clearly have plenty of hair. Once domesticated these animals can
look to humans to provide artificial heat sources in compensation
for their lack of hair.
In hot climates the ability to easily dissipate heat is important
for survival. A lack of hair allows reduction in drag for aquatic
animals. Professional swimmers sometimes shave exposed skin including
their scalps to give them reduced drag when swimming. It does not
make that much difference in their speed but it may be just enough
to make the distinction between a gold and silver medal in a competition.
Would
it be possible to alter my follicles, by use of some genetic
or ingested means, to very curly and flattened?
Straight hair fiber comes from straight hair follicles with a
circular cross section. Curly hairs come from curly hair follicles
that have a flattened cross section. There are some cases of people
with one hair follicle type having a change to the other hair follicle
type and so changing from straight hair to curly hair or vice versa.
Sometimes these changes come about after an illness or temporary
hair loss from telogen effluvium or alopecia areata. A few people
have developed kinked hair while using the drug etretinate (a vitamin
A derivative). However, what actually causes this change in hair
fiber production from straight to curly is not known and to my knowledge
has not been researched in any way.
The short answer is there is no known treatment to alter straight
hair follicles to curly ones or vice versa. The only option is a
cosmetic salon procedure to chemically perm the hair fiber into
the required shape. Of course the perm procedure would have to be
repeated every two months or so to maintain the curly hair.
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