The Transmission of Ultraviolet Light

through Reptile Skin Shed.

Simple "kitchen experiments" to assess the relative amounts of UVB light reaching the deeper layers of the skin in different species of lizard and snake.

Reptile Skin - The Investigation - Results: UVB readings - Spectrometer recordings

Discussion on results from different species

Introduction

When studies were made on vitamin D synthesis in human skin, it was soon discovered that the colour of skin you have determines how sensitive it is to ultraviolet light. This is because the brown pigment, melanin, which is found in the skin, protects it from sun damage (eg. sunburn) by blocking UV light. So, if you are dark-skinned, you are less likely to get sunburnt; but your skin is not as efficient as that of a fair-skinned human at producing vitamin D3 in sunlight. If you are dark-skinned, and only receive a little UV light, you are more likely than a fair-skinned person to suffer from vitamin D3 deficiency.22

Now, different species of reptiles have very different skin types. Some sun-worshipping species, like bearded dragons and uromastyx, have thick armour; some nocturnal geckos have delicate skin like pink tissue paper. We wondered whether we could find out how much UV is being absorbed by different types of reptile skin.

This has practical application. If, for example, a UVB lamp is emitting 100uW/cm² at 12", and two lizards sit under it at that distance, will they both be able to make the same amount of vitamin D3? Obviously, the answer is no, if one has skin which blocks more UVB than the other. To get the required daily amount of vitamin D3, reptiles which have less sensitive skin may well need either higher UVB levels... or more time under the lamp.

We decided to find out more about this, and first, we took a closer look at reptile skin....

About Reptile Skin

Reptile skin, like that of many vertebrates, has two principal layers: the dermis, which is the deeper layer of connective tissue with a rich supply of blood vessels and nerves, and the epidermis which in reptiles consists of up to seven sub-layers or "strata" of closely packed cells, forming the body's outer protective coating. 46

The epidermis has no blood supply, but its innermost living cells obtain their nourishment by the diffusion of substances to and from the capillaries at the surface of the dermis directly beneath them. It is on the cell membranes of these cells that pre-vitamin D3 is synthesised from the cholesterol precursor, under ultraviolet light. Once formed there, the pre-D3 is ejected from the cell membrane to the intercellular fluid where it is isomerised to vitamin D3. This diffuses into the capillaries of the dermis, is picked up by the D3 plasma binding protein and carried into the body.23

The ultraviolet light reaching these cells has to come through the outermost layers of the epidermis first. The epidermis of different reptiles varies widely in its thickness and pigmentation. (Melanin, responsible for brown colouration, is found in the epidermal cells and absorbs UV light. In reptiles the other colours are derived from cells in the dermis beneath, which may contain pigments producing, for example, reds and yellows; or reflect certain wavelengths of light, giving, for example, iridescent blues).47

The seven epidermal layers are

1. The stratum germinativum, the deepest layer of living cells which have the capacity for rapid cell division;

and the six layers which form each "epidermal generation" - the old and new skin layers - which are:

2 - 3. The clear layer and lacunar layer, which mature in the old skin layer as the new skin is growing beneath.

4 - 6. The alpha layer, the mesos layer and the beta layer; these layers consist of cells which are becoming keratinised, with production of two types of keratin (alpha and beta keratin). These cells are thus being transformed into a hard protective layer.

7. The Oberhautchen layer, which forms the toughest outermost layer of keratinised, dead skin cells.

In most mammals, the structure of the epidermis is less complex and the outermost, dead skin cells are constantly flaking off (as "scurf"); this protective layer is constantly replaced from below. The deepest layer of cells, the stratum germinativum, is constantly dividing and multiplying, and so all the layers are "on the move" outwards.

In reptiles, however, this cell division in the stratum germinativum only occurs periodically, and when it does, all the layers above it, in the area where the cell division occurs, are replaced in their entirety - the reptile literally grows a second skin, underneath the old skin, and then "sheds" the old one.

Some lizards (such as the leopard gecko) and snakes, shed the skin from the entire body all at once. Many other lizards shed the skin in large patches; bearded dragons, for example, may only shed the skin across the back, or the tail, or the legs at any one time. (Figs. 2 and 3.)

About 2 weeks before the reptile sheds its skin, the cells in the stratum germinativum begin active growth and a second set of layers form slowly underneath the old ones. At the end of this time the reptile effectively has a double skin, and this can be seen as a darkening in colour, in many species.

Then, the cells in the lowest layers of the old skin, (the clear and lacunar layers) and the Oberhautchen layer of the new skin below, undergo a final maturation and a so-called shedding complex forms. Fluid is exuded and forms a thin liquid layer between them. This gap between the two "skins" gives the familiar milky appearance to a shedding reptile. Enzymes in this fluid break down the connections between the two layers; the old skin lifts; and the reptile actively removes it. 46,48,49,50.

A useful diagram which shows the structure of the epidermis of a reptile, just before the skin is shed, may be viewed from the University of Michigan Museum of Zoology's The Animal Diversity Web, by clicking on this link:- Reptile epidermis (opens in a new window)51. (We cannot show it on this site for copyright reasons)

The Investigation

Two related studies by Carman et al 9 and Ferguson et al 16 compared the vitamin D3 production of the skins of four species of lizard - a crepuscular house gecko, a shade-dwelling anole, an anole which basks in sunshine, and the sun-dwelling Texas spiny lizard. They found that the amount of vitamin D3 production was inversely related to the amount of exposure to UVB light the reptiles would normally receive in the wild. The Texas spiny lizard skin produced the least; in a high UVB environment, this thick skin might well be resistant to UV damage and yet still produce sufficient vitamin D3. The house gecko skin was the most sensitive; presumably this gecko would be able to make the most of the smallest amounts of ultraviolet light that came its way.

We wondered whether the reason for the variation in sensitivity of the skin to UV light was simply due to reduced UV penetration of the skin in the reptiles which are regularly exposed to higher levels of UV light. Casual observation suggests that species of reptile which regularly expose themselves to full sunshine, in hot dry environments (eg. uromastyx, chuckwallas, bearded dragons) seem to have thicker, tougher skins than crepuscular species from similar environments (eg. leopard geckos) or diurnal reptiles which inhabit shadier areas (eg. rainforest chameleons). No doubt, the heavier body armour of the former species protects the rest of the body from water loss and from intense solar radiation. However, these species presumably have similar vitamin D3 requirements to their cousins which live under lower levels of light. To make the required amount of vitamin D3 each day, would sun dwellers actually need full sun, for sufficient UV to penetrate the armour? And in the case of crepuscular reptiles, would the skin allow even the smallest amounts of UV through?

To test this theory fully would presumably require the sampling of living skin removed from reptiles, and comparison of the UV transmission through the cell layers. We reasoned that although shed skin is not structurally identical to unshed skin, and any comparison would therefore be crude at best, there were sufficient similarities between the two to make this very non-invasive skin sampling technique worthwhile.

We have therefore begun collecting fresh shed skin samples from a range of lizard species. When the shed skin first lifts, and is pulled off by the shedding lizard, it is still hydrated, soft and flexible. If pieces of this are rescued before they dry out or become damaged, they can be used to gain a very crude approximation of the UVB transmission of that reptile's skin.

All that is required is to place the sample between a UVB meter and a known UVB source, and record the observed reduction in UVB output.

By placing the skin sample between the sensor of a spectrometer and a known light source, it is also possible to observe the absorption of light (across all wavelengths) by the piece of skin.

Method

(1) UVB Meter Readings

A Solarmeter 6.2 meter, recording UVB output in uW/cm², was used for all tests. The meter was fixed in position below various known sources of UV light, including the sun and a range of UVB fluorescent tubes and mercury vapour lamps from the 2005 Survey. Readings were taken before and after the fresh shed skin samples were placed directly over the sensor of the meter in such a way that they acted as a filter for the UVB light. Care was taken to ensure undamaged parts of the sample completely covered the sensor on each occasion.

One set of recordings are shown in Fig.5. To the left is the unobstructed Solarmeter under the UV lamp, at a distance at which the output is 100uW/cm². In the centre, a sample of shed skin from a leopard gecko (Eublepharis macularius) is over the sensor, and a transmission of 38% is recorded. To the right, this has been replaced by a sample of bearded dragon (Pogona vitticeps) shed skin. Transmission is now only 2%.

A further set of readings, for a small number of samples, was taken using a Solarmeter 6.4, which has a narrower band of sensitivity and records UVB output in terms of International Units of D3 per minute (as produced by "typical" human skin - see the manufacturer's details for an explanation of this). (http://www.solarmeter.com/model64.html)

(2) Spectrometer Recordings

The spectrometer used for all tests was an Ocean Optics Inc. USB 2000 Fibre Optic Spectrometer. (http://www.oceanoptics.com) The light source used was a ZooMed Reptisun 10.0 fluorescent tube with known output (ref. BZ9). The spectrometer sensor was clamped in place at a distance of 10cm from the light source and recordings taken before and after the skin samples were placed immediately in front of the sensor.

Results

(1) UVB Meter Readings

Tables 1a (lizard skin) and 1b (snake skin) show the results obtained from the Solarmeter 6.2 meter, when sampling skin shed from more than 60 animals from 23 different species. Where more than one sample was tested within a species / group, the number of samples is indicated in brackets and the average of the results is given. We are grateful to members of UVB_Meter_Owners group who have expanded our original study, contributing samples and readings for this section. (Please see Acknowledgements at foot of page)

Table 2 shows the results obtained using a Solarmeter 6.4 meter to compare the transmission of UVB light in the wavelengths which are optimal in producing vitamin D3. (290 - 315nm).

Species	Origin of skin shed sample	Percentage transmission
Bearded dragon	back	4%
Leopard gecko	head	27%
Leopard gecko	back (2)	35%
Leopard gecko	tail (2)	34%
Leopard gecko	axilla / belly (2)	39%

(2) Spectrometer Recordings

The spectrometer is able to process the data in several ways. Firstly we acquired a simple set of spectrograms which show the spectral power distribution of the lamp with and without the skin samples in place. Although the spectrometer provides data across all wavelengths from UVC to infra-red, Fig.6 shows the results in the UVB wavelengths only.

The scale of this graph does not permit close inspection of the data in the range of wavelengths responsible for D3 synthesis (290 - 315nm; peak production being at 297nm). This can be seen a little more clearly in Fig. 7, below.

Discussion

The results clearly demonstrate a huge variation between species, in terms of the transmission of UV light through shed skin. Although we have no proof that the UV transmission in any given species is directly related to the levels of UVB in the natural environment of each species, it is interesting to ponder the fact that there does appear to be a surprising correlation...

Bearded dragons (Pogona vitticeps), a species representing the "sun worshippers", live in arid Australian scrubland and bask in the open sun. The shed skin from their backs blocks all but 3-6% of the total UVB according to the Solarmeter 6.2; these figures are confirmed by the spectrograms which suggest an even greater absorption of UVB at the wavelengths which are responsible for D3 synthesis. Presumably, they receive enough high-UV direct sunlight every day to make sufficient vitamin D3 from just the small percentage of it which reaches the deeper cell layers of the epidermis. Skin from their undersurfaces (belly and throat) seems less protective; the samples allowed up to 16% total UVB through; we might speculate that these areas will receive less direct sunlight and so this optimises the D3 production from these more sheltered parts of the body.

In captivity, we might expect this species to have a very high UVB requirement, since so little of what is provided actually penetrates the skin.

Green iguanas (Iguana iguana), live in tropical rainforest, where they normally bask for long periods in full sunlight early and late in the day. During the heat of the day, they move in and out of leafy shade; here they are exposed to reflected and diffused UVB in similar amounts to that of the morning and evening sun in which they bask 27,32.  The shed skin from the tail of an iguana is strongly banded, matching the black melanin bands in the dermis. (Melanophores in the dermis produce cellular extensions which infiltrate the epidermal layers; these leave small amounts of melanin in the shed skin.11) The shed skin from a mature adult iguana (which had lived in a naturalistic, high UVB environment for years) was very thick and heavily keratinised; even the unpigmented areas on the dorsal surface blocked all but 4-6% of the total UVB; areas with melanin only transmitted 0-1%. Shed skin from the chest area of two young adults under treatment for metabolic bone disorder (MBD) was slightly more permeable to UV light, transmitting 7% and 11% respectively. Whether their age, or history of inadequate exposure to UV light has any bearing upon the result is not known. Even so, 7%-11% transmission is low. In captivity, this species might, like the bearded dragon, be expected to have a very high UVB requirement, since such a small proportion penetrates the iguana's armoured hide.

The Argentinian Black-and-white Tegu (Tupinambis merianae), a diurnal basking lizard, appears to have skin which protects the lizard from most of the sun's UVB, somewhat similar to that of the bearded dragon. The skin on the back is slightly more transparent to UVB than bearded dragon skin, the skin on the flanks and belly slightly less so. It is tempting to speculate that this is the result of the wild tegu's less harsh solar environment; they live in a temperate climate in rough grassland and mountain valleys.

Ornate Uromastyx (Uromastyx ornata) are lizards which live in extreme desert conditions in their native Egypt, Sinai and northern Arabia, in steep, rocky ravines that are dry for most of the year. Unlike the previously-mentioned species (which apparently spend much of their time in the daylight) ornate uromastyx, when not basking or feeding, shelter from the extremes of the desert environment in crevices in rocks. Other uromastyx species inhabit burrows which they dig in the ground.53 The sample of shed skin tested was more transparent to UVB than bearded dragon skin but whether this is related to the difference in behaviour is not known. In captivity, they might be expected to have a high requirement for UVB light - but also the need to be able to shelter from it in hides or burrows, as they would in the wild at certain times during the day.

Pigmented areas of the shed skin, containing melanin, blocked UVB more strongly than unpigmented areas in a similar way to that seen with iguana skin.

Leopard Geckos (Eublepharis macularius) are crepuscular although they have often been observed basking in captivity, and it is also likely that small amounts of UV light penetrate their daytime shelters in the wild. All the samples taken from the backs of four different geckos proved remarkably similar, transmitting 37 - 44% total UVB (measured with the Solarmeter 6.2, confirmed by the spectrograms) and 34-35% in the D3 range (measured with the Solarmeter 6.4, also confirmed by the spectrograms). The skin shed from a leopard gecko's back transmits up to 14 times more UVB than the shed from a bearded dragon's back. It seems likely that this skin would respond efficiently to low levels of UV light, since even small amounts of light would reach the deeper layers where D3 synthesis can take place.

The skin shed sample from the head of the leopard gecko was less permeable to UV light. It looks darker to the naked eye, as well, and the pigmented spots are more clearly seen. Why the skin of the head should provide better UV shielding than skin on the rest of the body is unknown. Although skin from the head was only sampled from one gecko, it is interesting to note that the skin from the back of this animal was the most transparent to UV of all the gecko skin tested, making the contrast with the head more remarkable.

In captivity, we might expect this species to benefit from low levels of UVB, but high levels might be harmful, since such a large proportion of it penetrates the skin.

Chameleons appear to fall between the two extremes.

Panther Chameleons (Furcifer pardalis) and Oustalet’s Chameleons (Furcifer oustaleti) are found in open forest as well as trees and bushes alongside Madagascan roads. Both species are often found in the same trees.
Their environment is bright with high levels of UV, however neither species basks to the extent that a bearded dragon does. In addition they are arboreal, and as vegetation absorbs UVB, parts of their day are spent within trees and bushes in sheltered UV areas.

Oustalet’s are larger than panthers with thicker, tougher skin. Surprisingly, testing various skin samples for males of both species using the Solarmeter 6.2 showed Oustalet’s skin has a higher UVB transmission rate. This would suggest the Oustalet’s chameleon may have a higher UVB requirement than the panther chameleon.

Adult Panther Chameleons showed profound differences depending upon their sex and breeding condition. A gravid female had a transmission rate of 34%; an old, non-breeding female and an adult male both had skin transmitting only 19%. It is possible the gravid female's increased requirement for vitamin D3 and calcium is, in some unexplained way, altering the skin's absorption of UVB.

Juvenile Panther Chameleons
Skin from several baby panthers were tested and UVB transmission was found to be around 50%, and a sample from a sub-adult male transmitted 42%. These readings are significantly higher than for the adults tested. A number of reasons for this difference are suggested below:

1. Body size / overall skin thickness.
   A panther’s skin is thinner when a baby than as an adult.
   However when comparing skin thickness and UV transmission across species, both Oustalet’s and Parson's skin are thicker than panthers and yet both allow more UV through. Therefore it seems likely this extra transmission is for reasons other than, or at least not limited to, body size.
2. Requirements for Growth.
   Baby chameleons are fast growing and the demands of laying enough calcium down for strong bones means their D3 requirements and therefore their UVB requirements are higher than those of adults.
3. Basking Preferences. At times of higher D3 requirements (e.g. gravid female) adults increase the time they spend basking, and the resulting additional UVB exposure facilitates higher vitamin D3 photobiosynthesis. Unfortunately basking in the wild carries a cost, namely, exposure to predators. (It’s also worth mentioning that basking reptiles tend to bask longer and have slightly higher body temperatures in captivity than in the wild, possibly because the threat of predation is removed.)
   The dangers of predation for a baby chameleon would be higher than for an adult. In fact it would seem so much more dangerous that baby chameleons inhabit a different area from the adults. The wild Jackson’s chameleons from the back garden of Mary Lovin in Hawaii 52 illustrate this point well. Adult Jackson’s tend to inhabit the middle to top of the trees and bushes. The babies spend their time closer to the ground in more sheltered low-level bushes.
   The babies by habit and habitat are not exposed to the same levels of direct sunlight as the adults. It seems possible that the higher UVB transmission in the babies’ skin would compensate for the more sheltered environment, where there is less UVB light, and also for the babies’ higher demand for vitamin D3.
   In captivity, we should consider the extra UVB requirements that a baby may have compared to the adult. However, sufficient UV shelter (generally provided by live plants) and a UV gradient must be provided to allow for proper self-regulation of exposure.

Parson’s Chameleons (Calumma parsonii parsonii) and Globifer Chameleons (Calumma globifer) are two closely related species inhabiting Madagascan rainforests well within the canopy. They are shade dwelling, slow growing chameleons, which are not considered basking animals.
Skin samples tested allowed moderate levels of UVB through, with readings close to the Oustalet’s. But because Parson's and Globifers come from a lower UVB environment and do not bask (at least to any great extent), their UVB requirements in captivity should not be considered similar to Oustalet’s. In the wild they would be receiving fairly even ambient levels of UVB all day, rather than moving in and out of the light, as a basking species would do.

Yemen or Veiled Chameleons (Chamaeleo calyptratus) are native to Yemen and southern Saudi Arabia, and are found in a wide range of habitats including dry plateaus, mountains, and river valleys. They are diurnal, usually found in bushes and shrubs in the daytime, retreating into shade during the middle of the day. Skin samples allowed moderate amounts of UVB through. These chameleons live in an environment with high levels of UVB in the sunlight, but because they are predominently shade dwellers their UVB requirements in captivity may be considered similar to those of Parson's and Globifer chameleons.

Unlike the bearded dragons tested, chameleon skin gives more uniform results, regardless of where on the body the samples come from. This is probably because, being an arboreal and lateral basker, the chameleon’s whole body would be exposed to UVB.

Crested geckos (Rhacodactylus ciliatus) originate from New Caledonia where they can be found on the southern half of the main island Grande Terre as well as on the neighbouring islands of the Isle of Pines and Isle of Komoto.
Their natural habitat consists mainly of humid, lowland forests, where they spend their days hiding in leaves on trees, bushes and the forest floor. As arboreal geckos, they can easily situate themselves in positions of direct sunlight if required. Crested geckos have been observed outdoors moving throughout the day, keeping their body exposed to rays of natural sunlight.
They change colour at times, when moved from indoors to an outdoor position in full sun and temperatures within their normal range. Although the exact purpose or cause of colour change is at present unknown, there are theories linking levels of light with changes in skin colouration. The shades and degree of colour can be very marked; one day changing to much darker or vivid colours, but the next day changing to very pale, almost off-white /beige even in the same sunlight conditions.
In captivity, crested geckos can be observed following their usual nocturnal behaviour, usually selecting a favourite sleeping spot, within leaves. Out of a group of 2 males and 3 females, over the winter period (with slightly lower temperatures and a break from breeding) only one female slept partially exposed to UV lighting. When temperatures started to rise and mating began, all 3 females were observed to sleep at least part of the day partially or fully exposed to the UV lighting. In the previous summer, only 1 male chose not to expose itself to the UV lighting on a daily basis, within its vivarium. But when taken outside on a sunny day, this gecko relished climbing to the top of its tree and basking in full sunlight. All the group are proven breeders.

Snakes
More research is needed as to the relationship between UV light and vitamin D3 metabolism in the snake. Traditionally, nocturnal and crepuscular snakes are not thought to require UV light. However, there would not seem to be any reason why they should not be able to utilise it for vitamin D3 synthesis. Is there any difference in transmission of UV between the skin of nocturnal and diurnal species of snake?

Snakes such as the forest-dwelling kingsnakes, which live in shaded habitats and are often active at night, might be expected to have skin which allows a high degree of UVB penetration; and indeed, this does seem to be the case; the skin from several species transmitted 45% or more of the UVB.

However, the inverse relationship between skin permeability and the amount of sun to which the reptile exposes itself does not seem as straightforward in snakes as it does in lizards.

Garter snakes (Thamnophis sirtalis), for example, are diurnal and often seen basking in morning sun, in a wide range of habitats across Canada and the USA from Alaska to Texas. Surprisingly, skin from the back of the snake would appear to transmit a similar proportion of UV light as that of the back of a leopard gecko. The skin on the belly was even more translucent, allowing 56% of the UVB through.

The skin of boas and pythons (with normal pigmentation) is much more protective. The skin of the Argentine Boas (Boa constrictor occidentalis), for example, permitted only 18% transmission. This is a nocturnal snake of the forest, grassland and scrub of Paraguay and Argentina; one might have expected a more "transparent" skin. It is tempting to speculate that these snakes, although active at night, do not hide away during daylight but rather, rest in well-lit tree branches where they are exposed to significant amounts of scattered UV light, if not direct sunlight.

Not surprisingly, the skin of 'albino' animals, and patches of unpigmented skin in piebald morphs, permitted a high percentage of UVB penetration compared to normal, pigmented skin. Such animals might be expected to be abnormally sensitive to UV light; melanin is one of the skin's main defences against UV light.

Further food for thought/research needed

• It would be interesting to be able to compare shed skin from wild reptiles, exposed to natural sunlight, with the shed skin from captive ones housed indoors with access to varying amounts of UV light.
  
  
• “Tanning” is a well-documented response in humans to an increase in UV exposure. Does a similar process occur in reptile skin?
  
  
• Do animals that have been bred in captivity for many generations have more transparent skin than their wild relatives?
  
  
• Is there any difference in UVB transmission between sexes, in species other than the Panther chameleon? Generally females have a higher UVB requirement when gravid. Does their skin transmit more UVB; or do they merely increase the time spent basking?
  
  
• Chameleons flatten their body when basking to create a larger skin area, which traps heat quickly. This also creates a wide skin surface for D3 photobiosynthesis. The same chameleon walking around has a more tubular shape and therefore a smaller surface area exposed to the sunlight. Does this change in surface area have a direct effect on D3 photobiosynthesis?
  

Acknowledgements

We are grateful to the following members of the UVB_Meter_Owners Group for submitting the additional samples and test results which appear above:

Green iguanas (Iguana iguana) - Robert MacCargar

Ornate uromastyx (Uromastyx ornata) - Torey Lehman

Crested gecko (Rhacodactylus ciliatus) - Angi

Grey-banded kingsnake (Lampropeltis alterna) - Brian Stearns

Variable (50/50 Californian) kingsnake (Lampropeltis getula x) - Brian Stearns

Green Basilisk (Basiliscus plumifrons); Gila Monster (Heloderma suspectum); Beaded Lizard (Heloderma horridum); Sinaloan Milksnake (Lampropeltis triangulum sinaloae); Albino Nelson's Milksnake (Lampropeltis triangulum nelsoni); Grey-banded kingsnake (Lampropeltis alterna); Ball Pythons (Python regius); Dumeril's Boa (Boa dumerili); Sarong Green Tree Python (Morelia viridis); Argentine Boa (Boa constrictor occidentalis) - Jon Coote

We welcome further contributions from members of the group.