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First written 1992
Last Revision 13.1.2003
Israel Fried
The Tower Hypothesis
Abstract
The following was taken out of the article on the "eye" of the peacock's train.
We suggest the Tower hypothesis that may explain the direct mechanism connecting the outside physical conditions with
the inside formation of specific colors and patterns in feathers. This mechanism may lead to the beautiful "eye"
of a peacock's train.
Colors and Physical functions
From [Durrer 1965] we see there is a correlation between the colored zones in the peacock's "eye" and the quantity
and density of melanin layers imbedded within the barbules of those zones. These contribute to the mechanical properties
of the barbules. One may learn it from e.g. p. 393 of [Lucas 1972]:
"Melanin also has the effect of making a feather denser and more resistance to wear and photochemical changes. This
property of melanin has long been known from gross observations and has recently been supported in studies with an electron
microscope. From his observations, Carr (1957: 161) gained the impression ' that the particles arranged in lengthwise rows,
contributed quite notably to the bulk and strength of the feather .
Therefore, we may explain connections between colors and mechanical functions. There is yet an interesting question:
how Nature knows what is the exact amount of melanin to introduce into the barbule in order that this barbule may sustain
specific physical conditions. It is known that the blackness of a feather, or an hair, occurs because granules of melanin,
accumulating near by the base of the feather, are introduced into, and between, keratinised cells which build the feather
or hair. This mechanism dictates the color of the feather or hair. An overview reference on this subject may be found in Ch.
7 of [Lucas, 1972]. In p. 394 of this reference there is a description of a deposition of melanin in natal downs. We may
quote some relevant sentences to our subject:
"Two sorts of evidence suggest that the feather cells play an active role in the uptake of melanin, that they are
not simply injected by the melanocytes. First, the barbule cells actually ingest the initial blobs of pigment, and second,
they apparently can receive pigment only when they have reached a definite stage of differentiation, characterized by elongation
and the onset of keratinization."
The important question for us in this article is if and how this injection of melanin into the feather or hair depends
on outside physical conditions. Since there is no answer known to me in the literature I suggest here a simple hypothesis,
which should be checked by experiments during the next years.
The Tower Hypothesis
The idea was occurred to me while I was trying to understand the mechanism of creating one black hair instead of a white
(or more exact - transparent) hair. I call it "The Tower Hypothesis". Imagine a commencement of a growing of an
hair. We may look upon the hair as a cylindrical tower built of transparent bricks. (Those bricks resemble the keratenised
cells composing the hair). Suppose our tower is growing up by adding bricks at its base, (not at the top as done in real buildings)
at a constant rate. Suppose also the base is immersed in a sea of water where there are also small transparent needles (melanin)
accumulated in groups (granules). As long as the tower is built without disturbance the transparent bricks fit each other
so there is no spaces between them. If this situation continues the tower will be transparent. This is the undisturbed case.
Now, let us imagine that while the tower is built there are horizontal wind blows, of constant direction northward, just above
the surface of the sea. While the top of the tower is coming out of the sea the wind bend it northward by an amount which
depend on the strength of the wind, the height of the tower and its stiffness. This stiffness depends on the stiffness of
the bricks (cells) as well as on the glue connecting those bricks in the spaces between them. As the tower is bent northward
due to the wind, the bricks along side south may be departed slightly and the spaces become larger. There is created vacuum
areas between some bricks on the southern surface of the cylindrical tower, including the parts under sea. Water with groups
of needles are sunk into the vacuum regions between the bricks. The amount of water sunk in each region, and the number of
groups goes with those water depend on many parameters, including the amount of vacancy, the mass and shape of groups, etc.
The more strongly the wind, the more water and groups go in between the bricks. The needles in a group may each be transparent.
But the transparency of the whole group may depend on the number of needles, their disorder, etc. If they are too many without
any order it is expected there is no transparency at all and the group may be seen as black. If the wind blows in the same
direction for long period there may be created a black line along the south side of the tower. If the wind changes directions
alternately, from north to south and vice versa, then one may expect two black lines along the tower. One in the north side,
the other in the south side. If the wind blows in many directions in some specific order we may expect black patterns as
dotes, lines, etc. all along around the tower. If the wind is turbulently we may get all the tower black. The amount of blackness
as well as the number of dotes or lines depend on the strength of the winds, length of tower build and on the stiffness of
the tower. Those last two parameters are changed gradually. Thus, even if the wind is constant, in strength and direction,
we still expect different amount of blackness along the tower. It is also expected that while water and groups of needles
get in between the bricks the tower become more flexible and may sustain the strengthens of winds which it encounter while
growing. It is also reasonable that the shape of the tower may be different from that of the undisturbed case. For instance,
it is reasonable to expect that it will be larger, since there is more substance between the bricks. It may also be thinner
since the bending may also enlarge the size of some bricks. If the wind was blowing from one direction only the tower may
not be grown into a cylindrical shape but of a wide and flat shape. To emphasize this possibility I may quote from p. 31
of [Voitkevich, 1966]:
"In iridescent feathers the second-order barbs are wide and flat and lie at right angles to the first-order barbs."
If we transform this entire story, of the tower, onto the growing mechanism of a hair or a feather we may have a way to
explain the connections between their colors and the mechanical functions they should sustain due to environmental physical
conditions.
Some other sentences from that p. 31 of Voitkevich may strengthen my new idea:
"Wear and tear, and mechanical erasure of the structural elements on the surface and at the feather tips, lead in
time to changes in the colour of vestiture, even between moults. This applies equally to colouring produced by pigments and
to structural effects."
We have shown above a possible mechanism for making black color and patterns due to mechanical conditions during the growth
of the tower, which resemble here an hair or a feather. Is it possible that such a mechanism may explain the coloring of the
tower by white patterns. The answer is positive. Recall the undisturbed growth of the tower which lead to a transparent tower.
Suppose now that when the tower start to go out of the sea we put some weight on its head, say an heavy hat. While the new
bricks (cells) are added to the base, the tower grow out through the sea. But now the heavy hat pushes the bricks downward,
due to gravity, and they are shrunk. In addition to the shrinking the spaces between the bricks are neglected and granules
of melanin can not go into these spaces. The shrinking of the transparent bricks (cells) make them to become white. Transparency
means specific order of the structure which allow the light to pass without any reflection. If the structure is changed, as
may happen while the bricks are shrunk, then there might be a situation where light is totally reflected of translucent so
that we get the color white. See e.g. pp. 410-411 of [Lucas 1972]. In addition, Lucas stresses that "white feathers
show a pale-blue flourescence under ultra-violet light. This comes from the keratin itself, and it is masked by the flourescence
of light-brown melanin or porphyrins in feathers where these pigments occure (Fox and Veres, 1969: 196). "
So far we have seen a potential mechanisms for the acceptance of black and white. What about the other colors. There are
two options. First, structural colors, i.e. colors which are due to specific orders of the layers of granules melanin, as
pointed out above. The reception of such specific orders may be done by specific frequencies of blows of winds, or other mechanical
influences, while the tower is growing. For instance, suppose a series of low frequency blows leave a specific space between
lines of melanin granules introduced into the tower so that the color obtained is green. If in the next series of blows the
frequency is higher. Then we may get a color of shorter wavelength, say violet. Thus, the green part of the tower may have
less stiffness than the violet part. Thus, the mechanical properties and the color may have a one to one correlation. The
experiences we describe above regarding the green and violet parts of the "eye" of a peacock's train may support
this line of thought.
Second, it is optional that in addition to melanin granules there will be colored granules as well. It is expected that
each color have it unique kinds of granules with specific dimension and other physical properties. Therefore, due to specific
outside mechanical conditions, the spaces between the bricks may accept specific colored granules, which nay lead a specific
part of the tower to get a specific color. Here again, changes in the outside mechanical influences may lead to a specific
patterns with different colors along the whole tower. Each color is a hint to the specific mechanical influence at a specific
time of growing. It is clear that the specific mechanical influence impose a specific physical property of the part which
was imposed by that influence. Now we see the connection between the colors and the physical properties of biological parts
such as a feather, hair and any other.
So far we discussed only mechanical influences on the growth and coloration of the tower (hair). The same idea may explain
also the influences of temperature and other surrounding conditions on the coloration. For instance, if the tower is growing
on while the temperature of the sea is very high, one may expect e.g. an elongation of the bricks as well as larger spaces
between the bricks. Thus, more granules of melanin enter in-between and the tower will be black at those places. On the contrary,
if the temperature of the sea is very low, i.e. cold conditions, the tower may shrink and the spaces between them become narrow.
Thus, in the extreme situation, there will be no melanin getting into the tower. The transparent bricks may become white
due to the shrinking. Thus, all the tower is colored white. If the temperature of the sea is changed alternately during the
growth of the tower one may expect alternate lines of black and white colors along the tower. Other patterns may be accepted
due to other surrounding conditions as explained above.
The Tower hypothesis may also explain why black surface may insulate better against high temperatures while white surface
may insulate better against low temperatures. A black tower due to heat during growing means that the bricks (cells) are already
expended to a certain level. As with a spring, the more you stretch it the more difficult is to farther stretch it. Biological
cells have their own flexibility due to the fact that they are build of molecules. In fact, all the biology depends on the
ability of the envelope of the cell to be stretched or shrunk. This is true for keratin cells as well. Therefore, when blackness
occur due to expansion of keratin cells and flowing of melanin into and between the cells, the whole black surface may expand
less while being under heat condition, than in the non-disturbed situation, i.e. when the surface is transparent.
In the case of the white color the story is opposite. The white bricks, in that example, is already shrunk. While cold
wind impinge on this white surface there is less spaces for the cold air (i.e. turbulent air winds) to transfer through the
surface. Therefore, the body under this surface is protected from cold. One may regard it as a mechanism for immunization.
The Tower hypothesis gives us a way to describe the process of immunization against heat, cold or mechanical influences.
The same procedure may be useful also for other outside physical influences on the growing hair, feather, or any other kind
of biological material.
Conclusion
The Tower hypothesis allow also the possibility to explain all colors patterns in animals and plants and their connections
to physical proprieties of the colored parts. One may find a support to this hypothesis from the fact that in the dermis of
the skin of birds there is found blood vessels besides the granules of pigments, as one may see in Figures 252 and 253 of
[Lucas, 1972]. From those Figures and from the Tower hypothesis it is reasonable to deduce that mechanical, thermal or any
other outside influences may impose different amounts of deposition of melanin and colored granules into the feathers. The
blood vessels in those Figures may serve as the sea in the Tower hypothesis.
References
Lucas, A.M. & Stettenheim, P.R. (1972). Avian Anatomy Integument, Part II, U.S. Government Printing Office, Washington,
D.C.
Voitkevich, A. A. (1966). The Feathers and Plumage of Birds, University of Washington Press, Seattle.
Durrer, H. (1965). Bau und Bildung der Augfeder des Pfaus (Pavo cristatus L.). Rev. Suisse de Zool. 72. pp. 263-411.
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