How Do We Tan?

How does our skin "see" UV light?  How does your skin know to get darker?   To flush with blood during a sun-burn?

If I understand correctly...

• UV radiation around the 300 nm wavelength hits DNA (chromosome telomeres are particularly vulnerable because of their repeat series of thymine)
• The photon elevates an electron to an outer shell on part of the molecule
• This change in atom shape changes the molecules shape
• The change in molecule shape makes a hickey on the DNA spiral called a pyrimidine dimer
• The dimer stops proper DNA replication creating incomplete single stranded DNA sequences
• These sequences combine with an enzyme (Lex A ) which then causes the expression of genes that produce enzymes that repair UV damaged DNA (nucleotide excision repair)
• The presence of the repair enzymes (somehow) causes...
• Flushing of blood, aka a sun burn
• This results in
• More repair
• Removal of dead cells (apoptosis)
• Increased melanin production (by melanocytes) thereby "shading" the cells from further damage
• We know the DNA repair enzymes communicate to our skin to make more melanin because we can artificially add them to skin cell sample or cause their numbers to rise by chemically damaging DNA and it produce more melanin without UV exposure.  More here.

Real guy, real sunburn

Melanin

• Melanin dissipates 99.9% of UV radiation as heat
• Melanin is also in parts of the fluid in your inner ear, medula, zona reticularis, adrenal gland and other neurons in the form of neuromelanin

Who knew there was more than one kind of melanin?

• Phaeomelanin that's yellow or red
• More common in whites/asians
• Eumelanin that's brown

Vitamin D3

• So why aren't all humans black?  If melanin is so great for protecting us from DNA mistakes and cancer why don't we all have it in abundance?
• We need vitamin D3 to make healthy bones
• UV light turns 7-dehydrocholesterol deep within the skin into vitamin D3
• I speculate that the heavy use of clothing in colder climates, thus covering more skin, would cause a greater need for lighter skin that is better at producing vitamin D3.  Meaning, that perhaps it wasn't less UV light towards the poles that made some skin colors so light as much as it was heavy use of clothing over evolutionary time.  

What about freckles?

• Why are freckles patchy?  Why not uniform hue across the skin?  Why do they tend to be on extremely fair skinned people?  Do they use more phaeomelanin over eumelanin?  Are they an adaptation to deal with a loss of function mutation?  Please let me know if you find any answers...
• Freckles apparently aren't a concentration of melanin producing cells (that is what a mole is), but only a patch of normal density melanocytes that has produced melanin.

Pictures from here, here, here, here, here.

Works sighted [sic]:
https://mail-attachment.googleusercontent.com/attachment?ui=2&ik=456a53bd10&view=att&th=1320e998bf429a6b&attid=0.1&disp=inline&realattid=f_grvgy34w0&safe=1&zw&saduie=AG9B_P8GDCc0dwBqoQc8G36dp0NA&sadet=1314556509504&sads=XcarW9THBtclWSIb4mapkRCLbw4

http://en.wikipedia.org/wiki/Melanin
http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.1982.tb04362.x/abstract;jsessionid=042D2310D61189980612F01997231EDD.d01t04
http://www.sciencedirect.com/science/article/pii/S0027510704004907
http://en.wikipedia.org/wiki/Vitamin_D
http://en.wikipedia.org/wiki/Xeroderma_pigmentosum

Evolution, Why So Blue?--An Idle Speculation on Coloration

Seed question that propagated this blog:  Why are some skink tails blue?

Then, the questions kept rolling in:

• Why is blue a relatively rare color in nature?  Is it actually rare?  What percentage of different clades like flowers, reptiles, birds, etc. are blue?
• Is blue the anti-red/orange?
• Could it actually be to attract predators to its tail (so as to distract from its body/head) much like false eye spots/false heads?
• Does it serve a mating purpose?
• Why do some of the same species of skink have red or pink tails?
• Does the skink's tail detach?  Would a predator attacking its tail as opposed to its body actually make it more likely to survive?
• Is sight an important sense for the important predators of the skink?  What are the main predators?  Is there variation in coloration in areas that have more of a certain kind of predator?
• Are the pigments that create blue difficult to evolve or metabolically costly to make?

Hypothesis Number One: Blue is the anti-red/orange.  Meaning it functions as the opposite of a warning color.

You may be well aware that many species use the bright red/orange coloration to advertise themselves as poisonous, venomous or foully flavored.  It's literally a bold move, but for many, many organisms it's a great way to avoid getting eaten.

So, why on earth would you do the opposite of that warning coloration and essentially advertise that you're edible?  It seems suicidal, but there may be strategy in it yet.  First, consider some other animals that do something sort of similar:

False Eye Spots (so a predator goes toward the end that you're best at getting away from and that is the least vital to survival):

False head Tails:

I've even seen video where this guy (or perhaps a similar species) will move their wings up and down to create a motion with the false antennae that is picture perfect of the real deal. 

How we could test to see what the purpose of the tail really is:

• Paint the tails other colors such as red or the normal color of the skinks body and observe
• Differentiation of mating success
• Differentiation of death by predation
• Find out predator distribution and abundance in areas that have color variations such as red or pink.
• Test the break-ability of the tail during predator attacks to see if attracting predators toward tail and away from body/head is actually effective or not.
• Count the number of broken/attacked tails in a normal population in comparison to comparable lizard species to in comparison to skinks with variant tail colorations to determine if the tails does in fact serve as a escape mechanism.

Second hypothesis: Blue is the anti-red/orange for mating displays.  Blue is dangerous since it is the opposite of a warning coloration and therefore red would more likely be used first, but once red/orange is taken subsequent species will want to contrast themselves to avoid inbreeding and watering down their evolved adaptations.

Is it just coincidence that blue footed boobies and the red decorated frigate bird nest on the same islands?

How we could test it:

• See if there areas that have blue species but no red species
• Count and establish the coloration break down as percentages
• In areas that there is variation or even reversal of coloration establish population sizes of both contrast species as well as the significance of predation.
• Genetically trace the age of species and see if blue species or red species split first from a common ancestor.

Their hypothesis:  Blue is the anti-red/orange for displays in flowers.

• Same as above.

Hypothesis Meltdowns?

Images are from here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.