The results of our measurements of solar UVB from across the world show clearly that in the course of a single day, reptiles in the wild would be exposed to huge variations in the amount of UVB they receive, depending on the time of day and amount of cloud cover; whether the reptile is in sun or shade, and whether sunlit objects nearby are reflecting more or less UVB in the reptile's direction.
This does not mean that all reptiles need exposure to UVB light, only that it is normally present in their environment. When reptiles choose to bask in full sun, they receive large amounts: even in the UK this may be 20-30 times the level they receive in the shade. Many reptiles never choose to do so, however. Some species live almost entirely in leafy shade or in burrows; nocturnal reptiles and crepuscular (dawn and dusk) species actively avoid bright daylight although some may bask in late evening sun. Even sun-worshippers retreat from the sun at certain times during the day, and many species are "shuttle heliotherms", moving in and out of sunlight many times as they thermoregulate during the course of the day.

Variation in skin sensitivity to UVB light.
All living things on the surface of the planet are exposed to ultraviolet light to some extent. Diffused and reflected UVB will penetrate into deep shade and even nocturnal creatures may be exposed to small amounts whilst hidden in their daytime retreats, particularly in locations where daytime UVB levels are high. 

It is likely that all reptiles can synthesise vitamin D in their skin when exposed to UVB light. Very few vertebrates do not synthesise its precursor, provitaminD, in the skin.23 

Two related studies compared the sensitivity 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. The amount of vitamin D3 synthesised by the skin of each of the species was compared. They found that the skin sensitivity was related to the amount of exposure to UVB light the reptiles would normally receive in the wild. The Texas spiny lizard skin was the most insensitive; 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.9,16

We have conducted a new study on the way the skin of reptiles from different habitats varies in its transmission of UVB light, by examining the shed skin from a range of species from very different environments. We have found that in general, our findings tie in well with those described above. Lizards which are normally exposed to high levels of UV light have shed skin which lets only a small percentage through to its deeper layers; this barrier would thus presumably have a protective function. Species that would receive lower levels of UVB in their environment have shed skin which lets a higher percentage of UVB light through.
However, we had the opportunity to test samples from males, females and juveniles from one species (the panther chameleon) and we found that juveniles and a gravid female had skin that was more transparent to UVB than males and an older female, which presumably have lower vitamin D3 requirements. It is thus possible that there is a dynamic balance between a reptile's need for protection from excessive UV, and for its need to allow UV to reach deeper skin layers for the purpose of D3 synthesis.

Our results are described fully on the separate page The Transmission of Ultraviolet Light through Reptile Skin Shed. (click to view in a new window. Also accessible from the side navigation bar in the normal way.)

How much UVB do reptiles need?
There is no easy answer to this. For millions of years, our reptiles have lived in a world in which UVB, UVA and visible light are all around them, and different species have evolved in every ecological niche, with behaviours (such as basking preferences) and body characteristics (such as thick or thin skin, heavy or light pigmentation) to equip them to use what UVB is available to them, in the most efficient way possible. 

It would seem logical that keepers cannot go far wrong if they seek to emulate the natural UVB environment inhabited by the species they keep. 

There is hardly any scientific data to back the recommendation of any particular level of UVB for any particular species. Most suggestions are still based on keepers' personal experiences although slowly, new data is being collected and advances in our understanding are being made. Studies on UVB light, basking preferences, and vitamin D3 production are appearing steadily, and work has been done on species as diverse as chuckwallas, iguanas and chameleons. 

Researchers have charted the response of D3 deficient green iguanas and Komodo dragons to ultraviolet light and measured the serum levels of calcediol in wild lizards including green iguanas, crocodile monitors, and Komodo dragons.1,8,20,27 

In the USA it is becoming fairly common for iguana owners to include blood tests for serum calcediol levels in their regular veterinary health checks.27

Self-determination
We all know that reptiles need a "heat gradient" and part of their normal behaviour involves moving in and out of the warmth so that they maintain a healthy body temperature. Since some reptiles are also aware of ultraviolet light "gradients" and will actually move in and out of them according to their bodily vitamin D3 status and requirements, it would thus seem very appropriate to provide our animals with UV light in the same way we provide heat, ensuring they can choose how much, or how little UV they absorb by establishing a UVB gradient in our vivaria. 

We have observed the behaviour of some of our lizards, and their reactions to UVB light supplied in different ways.
Bearded dragons (Pogona vitticeps), both adults and hatchlings, kept under fluorescent lights providing UVB chose mainly to bask directly under the basking lamp used as a heat source, rather than the UV light.  Therefore if using fluorescent tubes as a UVB source, these should be placed as close as possible to the basking zone.

Collared lizards (Crotaphytus collaris) supplied with the same fluorescent lamps were observed to sit much closer to the UV source for several hours in the morning and afternoon, moving away throughout the middle of the day. This might be indicative of their behaviour in the wild, where the natural heat, light and UVB levels reach a high at midday. (Figure 3)

When a mercury vapour lamp with a higher UVB output was introduced to the bearded dragons, however, a change in behaviour was seen. Their activity levels increased and they chose to bask directly under the mercury vapour bulb, rather than their incandescent basking lamp, for much of the day. In particular, one adult bearded dragon recovering from metabolic bone disorder (the result of a lack of UVB in the past) basked directly underneath the UV source for most of the day, sitting as close as she could safely get (which with this particular lamp, was 12"). This dragon was never seen basking under the fluorescent tube that was provided for her before the mercury vapour lamp was installed. (Figure 4)
Controlled experiments would be needed to ensure that the reptiles were not merely attracted to the mercury vapour lamps because of their bright visible light; however, these observations do tie in well with the published findings cited earlier, that lizards are not only attracted to ultraviolet light, but appear to know when they need to absorb more UVB, and increase their basking time accordingly.15 

There are also reports of the same behaviour in green iguanas (Iguana iguana) under UVB treatment for metabolic bone disorder. Early in the course of their therapy, they choose to lie right under high-intensity UVB lamps for most of the day, at levels of around 175uW/cm² (the maximum offered). As recovery progresses, they move towards the edges of the high-UVB zone.33

Such observations further highlight the need to provide a "light gradient", whereby reptiles can move in and out of the UV range at their own preference, as they would do in the wild. It is also important to ensure that the ultraviolet light is placed close enough to the warm basking spot that a reptile can obtain both heat and ultraviolet light together. If a reptile has the choice between two equally bright lamps, one of which emits UVB, it will prefer that one 15; however, if the UVB source is not as bright, reptiles may select the brightest or warmest part of the vivarium for basking, and thus fail to benefit from the ultraviolet light. 12

UVB Recommendations for different species.

"Sun-worshippers"
Some reptiles are described as "sun-worshippers" including uromastyx, bearded dragons, chuckwallas, and many species of tortoise. In the wild, these spend many hours a day exposed to hot sunshine and high UVB levels, in places such as the Mediterranean, Arizona and the Australian outback. Peak readings of 350 - 450uW/cm² are commonplace for two or three hours a day, around noon, in the summertime in these areas; however, even "sun-worshippers" avoid the midday sun. Observers usually report that most basking is done in full sun before 10.30am, when readings are considerably lower.
Frequently, lizards are seen basking in full sun (or under basking lamps) with their mouths wide open. This gaping is a mechanism for heat loss; the saliva evaporating from mouth and tongue cools the body. In an arid environment, animals do not deliberately lose water  un-necessarily.
Other species of basking lizards lie on rocks so hot they lift their feet to avoid burning them. Why would a wild animal continue to bask in the open, risking been seen by a predator, when it is already too hot? Is it possible that the reptile is choosing to stay out for longer, in order to absorb more ultraviolet light?

When kept in captivity these species need - and seek out - high levels of ultraviolet light to enable them to produce enough vitamin D3 to remain healthy. However, there are no published guidelines regarding the optimum UVB levels for these species. We can only offer observations.

Chuckwallas (Sauromalus obesus) appear to thrive under strong lighting in the vivarium, choosing to bask in areas of high UVB exposure for part of the day when this is available to them. In one study 3 these were offered gradients of up to 92uW/cm² (estimated reading if measured with a Solarmeter 6.2, according to conversion factors published by Gehrmann et al 19) and they preferred the zones with highest UVB, moving in and out of these areas freely, however, just as they move in and out of hot basking spots.

A thriving group of Uromastyx aegyptius in a zoo in the UK have been given access to high UVB supplied by a commercial tanning lamp for several years. The lizards are reported to bask frequently right under this lamp.39 

One of the current authors keeps bearded dragons (Pogona vitticeps) and chuckwallas (Sauromalus ater) with access to a gradient of up to 140uW/cm² under mercury vapour and metal halide lamps. These bask right under their lamps in the zone of maximum UVB, and seem to spend most of their time, when not basking, in the regions where UVB levels are between 30 and 75 uW/cm².

Rainforest Species
Rainforest species such as some types of chameleon naturally avoid any such high exposure. They need UVB, but at much lower levels. Their more sensitive skins manufacture all the vitamin D3 which they need from brief periods of basking early and late in the day, and the diffused and reflected ultraviolet light permeating the rainforest shade. All species of chameleon have different requirements, but authors vary, too, in their recommendations. The one scientific study we are aware of addresses egg hatchability in Panther Chameleons (Furcifer pardalis) and the authors suggest that a low level of UVB (a gradient between 15-33uW/cm² as measured with a Solarmeter 6.2) supplied for 12 hours a day is optimal. High levels are seen to be harmful.14,19 Our experiences are similar to these findings; one of the current authors maintains gradients of up to 30uW/cm² in all his chameleon vivaria.

Iguanas appear to have requirements - and behaviour patterns - somewhere between the two. Green iguanas (Iguana iguana) have been observed to bask for long periods in full sunlight early and late in the day, when UVB levels are lower, then move to leafy shade during the heat of the day. Here they avoid the high UVB of the tropical noonday sun, which can be as high as 450 uW/cm², but they continue to be exposed to considerable amounts of reflected and diffused UVB which may reach values little different from those of the morning and evening sun in which they bask freely; levels of 200uW/cm² have been recorded.27,32 

One author recommends keepers of green iguanas to make levels of at least 75-150 uW/cm² available to their animals for at least 6 hours a day, and reports that iguanas, given the opportunity to do so, choose to bask at these levels. 32,33 Another study indicates that an absolute minimum of 10uW/cm² is required for maintaining adequate vitamin D3 levels, but the author recommends that keepers aim for a gradient of 20 - 40 uW/cm² in the iguana's basking spot. 27 However, these levels would all be around the very minimum level a wild iguana would experience during the day, whilst in deep shade; iguanas are not shade dwellers. The recommendation first described (around 100 uW/cm² in the basking zone itself, with a gradient into shade) would seem much more appropriate.

Creatures of Dusk and Dawn
Crepuscular lizards (active at dusk and dawn) such as leopard geckos and house geckos are traditionally considered to require no ultraviolet lighting in the vivarium. In the wild, however, there is no doubt that they do receive small amounts of ultraviolet radiation from the dusk and dawn skies and possibly also a little diffused UV penetrating between rocks and bark above their daytime retreats. A recent report described a wild Texas Banded gecko (Coleonyx variegatus) thought to be basking in evening light when the ambient UVB level was at least 33uW/cm².11 Leopard geckos in captivity may be observed basking occasionally (Fig. 10) and one of the authors of UV Guide UK provides UVB fluorescent tubes as daytime background lighting for all her geckos, with observed improvements in fertility and health.

Recent research has discovered that the skin of house geckos (Hemidactylus turcicus) is extra-ordinarily sensitive to UVB light; very low levels can enable large quantities of vitamin D3 to be produced in their skin.9 In captivity, this would suggest that although their diet can normally provide all the D3 they need, very brief daily exposure to low levels of UVB might be beneficial, especially if the diet was lacking in vitamin D3. If they were already receiving maximum safe levels of vitamin D3 in dietary supplements, however, there might be a risk of vitamin D overdosage from the supplements.

Snakes
Few snakes are thought to require ultraviolet lighting, at present, since vitamin D3 from the diet is almost certainly adequate for most species. However, some authors believe that Diamond Pythons (Morelia spilota spilota) 37, Indigo Snakes (Drymarchon corais) 39, some aquatic species 11, the insectivorous Rough and Smooth Green Snakes (Opheodrys aestivus and O. vernalis) and other arboreal, diurnal snakes 26,38 may benefit from low levels of UVB light in captivity. There is also new research showing that even crepuscular corn snakes can synthesise vitamin D3 if exposed to suitable levels of UVB, so many species probably would benefit from the ability to experience at least some gentle UVB during daylight hours. Snakes are sensitive to excessively bright artificial light, however, and this must be avoided.

This feature continues with an overview of artificial lighting for reptiles in:
UV in the Vivarium.