Herptiles.net - Understanding Ultraviolet and Calcium

Understanding Ultraviolet and Calcium
Christina Miller

Last update: Wednesday, May 21, 2008.

Calcium (symbol Ca) is an element in our diets that is important for the health of bones, teeth and muscles. But even if a herp is consuming the recommended daily value of calcium, if they do not have any means of acquiring vitamin D3, they can develop serious health issues like metabolic bone diseases. Many organisms require ultraviolet radiation in order to produce their own vitamin D3. We will discuss the physics behind ultraviolet light and why special fluorescent bulbs are necessary for many species to flourish, as well as the physiology behind calcium metabolism and vitamin D3.

Catch the wave

The sun produces radiant energy, more specifically, electromagnetic radiation. As humans, we predominantly notice heat and visible light, but a whole range of wavelengths from the electromagnetic spectrum (EM spectrum) of energy are actually produced. "Visible light" is a form of electromagnetic radiation that humans can see, and radiated heat (infrared) is a form of electromagnetic radiation that we detect with specialized nerves. (Benson 1996)

Electromagnetic radiation occurs both naturally and can be manufactured artificially (like by your television or computer monitor), and consists of time-varying, oscillating electric and magnetic fields. They are measured by either their wavelength (literally, the physical "length" of the wave, measured between two corresponding points) or their frequency (the number of wave repetitions during a given time period, which is inversely proportional to the wavelength). The spectrum includes (from shortest to longest wavelength) gamma rays, x-rays, ultraviolet radiation, visible radiation (light), infrared radiation (heat), microves, UHF ("ultra high frequency," used in old televisions, and in cell phones), television and FM radio, short wave AM radio, and long wave radio. (Benson 1996)

Note that many reptile bulbs are touted as "full spectrum." This is not very accurate- we have seen that the "full" electromagnetic spectrum encompasses many different waves with different applications. For this reason, bulbs that produce a wide range of visible and invisible light, specifically ultraviolet light, will be referred to as broad spectrum bulbs from here on.

Ultraviolet radiation ranges from 400-10 nanometers in wavelength, and is nestled between x-rays and the "blue" end of visible light on the EM spectrum. There are three classes of ultraviolet radiation, be aware that there exists an alternate classification, but the following is more appropriate for our concern. The actual number ranges varies between countries (North America versus Europe), but in any case, they are approximate. The limits between classificaitons are not clear-cut and exact like diagrams depict! (Angelo 2002)

Ultraviolet A (UVA): Wavelength range from 400-315 nm. Also called long wave or "black light." This class of UV is important in regulating natural cycles in some reptiles, such as brumation ("hibernation") and reproduction. It is part of the visible spectrum for many species of reptiles (Fleishman et al. 1993), insects (Salcedo et al. 2003) and birds, (Cuthill et al 2000) it is beneficial in recognizing conspecifics or even prey in some of these visually-oriented species. It has been demonstrated that reptiles provided with UVA light generally do better in captivity, with increased levels of natural activities, greater reproductive success, and better appetites. (Klaphake et al. 2004)

UVA can cause skin damage with overexposure, and potentially skin cancer with chronic overexposure (although it does not cause sunburn). UVA is produced by the sun, by broad spectrum fluorescent bulbs and black lights. (Gehrmann 2006)
Ultraviolet B (UVB): Wavelength range from 315-280 nm. Also called medium wave. UVB is critical for calcium absorption in many animals, because it converts provitamin D3 in the skin to the active form of vitamin D3. This will be discussed more in detail, later. It is uncertain whether reptiles can see or detect UVB. Ultraviolet B may also serve to disinfect the skin of external parasites, (Pritchard and Greenhood 1968), possibly in semiaquatic animals who may be constantly exposed to aquatic pathogens but also spend significant time basking.

Ultraviolet B can cause skin damage (best recorded in humans) if overexposed (moreso than UVA), resulting in sunburn, or potentially skin cancer with chronic overexposure. UVB may also damage the eyes with overexposure. It is produced by the sun, and by broad spectrum fluorescent bulbs. (Gehrmann 2006)
Ultraviolet C (UVC): Wavelength range from 280-100 nm. Also called short wave, or germicidal. UVC is filtered by our atmosphere, but artificially produced UVC is used in many air and water sterilization systems. It is very hazardous to living tissues, and is not necessary in any captive situation. (Gehrmann 2006)

A few more pertinent comments about these waves, is that they do not have an infinite distance (they "dissipate" over distance), and all UV is filtered by regular (soda-lime) glass.

Broad spectrum light sources

Not all light bulbs are created equal! All fluorescent lights contain a small amount of mercury, that becomes vapour and produces ultraviolet radiation when agitated with an electric discharge. In order to produce visible light, the inside of the tube is coated with a powdered phosphor, that produces light in our visible range by absorbing the ultraviolet radiation and "converts" it to visible light. The glass tubing filters all ultraviolet radiation, so only visible light is emitted by the fluorescent bulb.

Special fluorescent bulbs manufactured for reptile or bird use are constructed with quartz glass instead of regular glass. This substitute allows ultraviolet radiation to pass the bulb wall. This glass is more expensive than the typical, soda-lime glass, which is why these bulbs are more expensive than plain old hardware store fluorescents. Manufacturers have measurements for these bulbs in percentage of UV output compared to the output of visible light. For example, the Exo Terra Repti Glo 5.0 bulb produces 30% UVA and 5% UVB (as stated on the packaging), meaning the remaining 65% of light produced is visible light. These readings are measured at very close proximity to the actual tube, and do not describe the actual intensity of ultraviolet radiation produced.

Compact fluorescent bulbs are simply linear fluorescents in a coiled up design, minus some electrical parts. The main practical difference between these two bulbs is that the compacts will produce a more localized, comparatively more intense region of UV radiation compared to the long tubes.

Mercury vapour bulbs work in a similar fashion to fluorescent bulbs. A small quartz tube filled with an inert gas and a small amount of mercury at high pressure is found in the centre of the bulb. Electrodes are found at each end of the tube; when electricity is discharged, the mercury vapourizes and emits radiation. A much larger amount of UV, as well as heat and visible light, are produced compared to the fluorescent bulbs. The UV is permitted to pass through the outer bulb casing, as it is also made of a UV-permeable material. These lamps are suitable for large enclosures only, since they produce so much heat they provide a risk of overheating smaller enclosures. Because of the very high output of UV, they are not recommended for species with low UV needs, and must be used at a minimum distance for safe basking. (Baines et al. 2006)

UVB and calcium metabolism

Skin normally contains cholesterol, a compound that is produced by the body, and obtained from the animal portion of the diet diet in carnivores and many omnivores. Cholesterol is transformed by the skin cells through a series of chemical reactions into 7-dehydrocholesterol (7DHC, also called provitamin D).

When the skin is exposed to ultraviolet B radiation, more specifically between 290-315 nm, a carbon ring in the molecule is opened, converting the compound into previatmin D3. Previtamin D3 is slowly transformed into vitamin D3 through isomerization (a process where the components of the molecule remain the same, but their structure is altered). This reaction needs warmth in order to occur at a regular rate. Homeothermic mammals and birds produce their own body heat, but reptiles need warmth from basking in the sun to allow this reaction to occur over several hours. Vitamin D3 in this form is also called cholecalciferol. (Holum 1998)


When 7-dehydrocholesterol is exposed to UV-B, the ultraviolet waves open one of the carbon rings to produce vitamin D3.

The new compound proceeds from the skin cells to the bloodstream, where it is picked up by a protein and transported to the liver. It is transformed into calcidiol (25-hydroxy-vitamin D3) through hydroxylization, and it is stored in the liver until needed. (Holum 1998) Calcidol will eventually be sent to the kidneys via the bloodstream, where it may be converted to calcitriol. Calcitriol acts like a hormone, and it is considered the active form of vitamin D3. This hormone increases the levels of blood calcium in three ways: (Colville and Bassert 2002)

It increases absorption of calcium and phosphate (two major bone compounds) from the digestive tract.
It increases calcium and phosphate reabsorption in the kidneys (excess would noramlly be passed in the urine or urates).
It inhibits the release of parathyroid hormone (PTH). See Metabolic Bone Diseases in Reptiles and Amphibians for more information on calcitriol/PTH interactions.

Suffice it to say, ultraviolet B radiation is necessary for adequate dietary calcium usage in many species of reptile, and possibly some amphibians.

Dietary D3

Hypervitaminosis D, an overdose of vitamin D3, can occur when a dietary source of vitamin D3 is overused. This vitamin is fat-soluble, so excesses obtained from the diet are stored in the body instead of being excreted. Hypervitaminosis D is a serious condition that results in kidney damage and soft tissue mineralization (including major veins and arteries), and can be fatal as a result of complications.

When an animal relies only on UV exposure for vitamin D3 production, overdoses cannot occur. (Holick 2004) The body has multiple "failsafe" mechanisms to prevent this from occurring. Considering this we can state that for basking species, it is safer to rely on providing UV lighting in captivity than to attempt to replace UV with dietary supplements.

How much UV does my reptile need?

There is no simple answer to this question. Ultraviolet light needs will vary greatly between species according to their habitat and geographical location. The sun, the "ideal" naturally controlled source of UV, does not provide equal amounts of UV everywhere at every time. Think back to grade school geography: The Earth's axis presents it at an angle to the sun, so different places will receive different amounts of sunlight at different times of the day. Additionally, due to the Earth's elliptical orbit around the sun, it receives different amounts of sunlight depending on the time of year.

Also consider that species that live in open grasslands or deserts will be exposed to much more sunlight when they are not hiding, compared to forest or shoreline species that will be exposed to sunlight filtered through vegetation. Green Iguanas, a forest-dwelling species, will deliberately spend time high up in the forest canopy where sun exposure is maximal, to bask for heat and UV. It has been suggested that some nocturnal or crepuscular animals, will spend some time basking at dawn and dusk to obtain some UV exposure.

Looking back at our commercial UVB-producing bulbs, they are rated in percentages, which does not truly represent the potential UV output of these bulbs (a percentage is only a proportion, we could be looking at proportions of very small amounts of actual light output). A more accurate measurement of UV provided is irradiance ("intensity"), the amount of watts per unit of area on the surface. A common unit of measurement seen in the literature is microwatts per square centimetre (µW/cm^2).

The sun probably has a much greater irradiance in most places at most times compared to a UV-producing bulb for home use. Where a wild Green Iguana may only need a few hours of basking in the sun daily to produce enough vitamin D3, a captive iguana may need close to 12 hours of exposure under a broad spectrum bulb. Species that are constantly exposed to the sun have skin that appears to have adapted to this constant exposure, by letting through comparatively little of the necessarily UV rays, possibly to prevent skin damage, but the amount of time spent exposed to sunlight compensates for this. Species that are exposed to little sun may have adapted skin that is very sensitive to solar radiation, making the most of the little sunlight exposure they encounter (these species are also more likely to have adapted to obtain more D3 from dietary sources). (Baines et al. 2006)

Remember high school geography? Because of the Earth's tilt, the angle of incidence of solar radiation varies with latitudinal location...

...and light received also varies with the season, considering Earth's elliptical orbit around the sun.

It is also possible that many herps can "sense" UVB irradiance, and will select it according to their needs. Panther Chameleons (Furcifer pardalis) have been demonstrated to choose UVB-intense light according to physiologic demand for calcium. Reproductively active females and growing juveniles increased their exposure to UVB (as a result of increased bodily need for available calcium) compared to males and older females. (Ferguson et al. 2003) It is certainly possible that other species will demonstrate this kind of behaviour, if tested.

Some herp species still thrive without UV lighting. Most snakes, many nocturnal and crepuscular lizards, some diurnal forest-dwelling or otherwise extremely secretive lizards, and some chelonians have been kept very successfully without broad spectrum lighting. These species are probably adapted to diets containing lots of whole prey (with significant amounts of D3) and minimal exposure to sunlight due to their habits. Herbivorous species obtain little to no vitamin D3 from the diet, as plants only contain vitamin D2 (ergocalciferol) that cannot be transformed into D3. It is safe to assume that these species require a source of UV.

Herps have ultraviolet light needs that vary with the species, varies between individuals of the same species, and may even vary seasonally. It is expensive to buy a UV meter for home use, and data on UV preferences is lacking for most species. So what can we do for our captive herps that live in terrariums?

Examples of UVB requirements for some popular species
UVB is definitely required to maintain healthy captive animals	Uncertain if UVB is required, probably beneficial	UVB not required to maintain healthy captive animals
Green Iguana, Iguana iguana (Bernard et al. 1991; Allen and Oftedal 2003)	Pond Turtles (Emydidae) (Tortoise Trust 2008)	Most snakes
Old World Chameleons (Chamaeleonidae)	Mountain Horned Dragons (Acanthosaura sp.)	Nocturnal gekkonids (Gekko sp.)
Bearded Dragons (Pogona vitticeps)	Bluetongued Skinks (Tiliqua sp.)	True eyelid geckos (Eublepharidae), ex: Leopard Geckos (Eublepharis macularius), African Fat-tailed Geckos (Hemitheconyx caudicinctus)
Water Dragons (Physignathus sp.)	Diamond Pythons (Moreila spilota spilota) (O'Brien 2001)
Tortoises (Testudinidae) (Tortoise Trust 2008)	Green Snakes (Opheodrys sp.) (Jordan 2005)
Spiny-tailed Agamids (Uromastyx sp.)
Day Geckos (Phelsuma sp.) and diurnal gekkonids (Lygodactylus sp. and Gonatodes sp.)
Anoles (Polychrotidae)

UV in the terrarium

The ideal situation for terrarium animals would be to duplicate what they encounter in nature. Miniature suns for terrarium use do not exist, and there is a wide variety of products available. Keep the following points in consideration when selecting lighting for your reptile or amphibian terrarium: