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The Science of Reptile Husbandry: UVB: Ferguson’s Zones

In the last issue of The Reptile Academy Magazine we discussed the importance of UV-B light in the prevention of metabolic bone disease (MBD) in reptiles and amphibians. Be sure to check out this article to learn what UV-B is and how a lack of this high energy light contributes to disease. We also explored the full recovery of our rescue Giant Argentine Tegu, Salvator merianae, Salvador, from MBD. However, there is widespread misinformation about which animals should be provided with UV-B as part of an environmentally enriched husbandry approach.

How do we actually know?

Just to recap

UV-B from sunlight is a high energy wavelength of light that catalyses vitamin D3 synthesis in the skin, which is converted into calcitriol in the liver. Calcitriol promotes efficient absorption of dietary calcium from the gut as well as being an important factor for 2000 genes and their respective processes across the body. Correct calcitriol levels inhibit parathyroid hormone that drives a process called osteoclast-mediated bone resorption; the digestion and release of calcium from the bones into the bloodstream that is a natural yet emergency seasonal undersupply homeostatic mechanism. Lack of UV-B and dietary calcium will result in impaired neuronal processes, muscle contraction, heart health and normal organ function throughout the body; processes for which calcium is essential. Perpetual under-exposure to artificial UV-B and calcium undernourishment in captivity forces this mechanism in overdrive, causing the animal to develop MBD where the bones are so depleted of calcium. Eventually, a combination of a damaged skeletal structure and severe organ failure will be fatal. To our current knowledge there is no evidence of MBD in wild animals. This disease is a product of poor husbandry in captivity and is completely preventable.


The Ferguson’s Zones & UV-B Lighting

A ground-breaking investigation into the thermoregulatory behaviour and microhabitat preferences in 15 wild reptiles across southern USA and Jamaica (Ferguson et al., 2010) led to the categorisation of 4 main zones of ultra-violet light index (UVI) exposure from the sun, using a solarimeter 6.5 UV Index Meter calibrated to the UV-B wavelength range. Essentially this measured how much UVB animals were exposed to in their natural habitats, according to whether they bask in direct or indirect sunlight. Reptiles are ectotherms; animals that do not produce their own body heat but instead absorb heat (or cool down) according to the temperature of the environment. However, reptile species are highly diverse and have very different thermoregulatory behaviours and microhabitat preferences to each other, which affects the intensity and duration of direct sunlight exposure.

Zone 1 species are crepuscular (active at dawn and dusk, during twilight hours) or shade-dwelling, so are only exposed to low intensity sunlight and therefore have a low UVI. They are also thermal conformers, meaning that their body temperature is proportional to the surrounding ambient or environmental temperature without actively basking in the sun to absorb extra heat. Therefore, these species have a background range of 0.0 - 0.7UVI and a maximum possible exposure of 0.6 – 1.4 UVI. Animals characterised in this zone should be provided with a UV-B light of around 2.0%.

The amount of background range UVI and maximum exposure increases with each zone. Zones 2, 3 and 4 include species that are not thermal conformers, but behavioural thermoregulators, meaning that the species will actively move into warmer or cooler areas to adjust its body temperature. As the zones increase through 2, 3 and 4, reptiles will more frequently bask in sunlight that is more intense, less shaded and towards the middle of the day, being exposed to a greater UVI and therefore more UV-B. A table is summarised below.

Table. 1. Thermoregulatory characteristics of reptiles and amphibians in each Ferguson Zone 1 – 4, and their respective ambient UVI ranges and maximum UVI exposures.[1]

Note: the background UVI range is the suitable ambient UV-B for the species in question whereas maximum UVI reflects the highest UVI that the animal could be exposed to at the time of activity in that habitat ; for example, sun breaking through the tree cover.

Animals in these subsequent zones should typically be provided with UV-B 5.0% - 6.0% (zone 2), 8.0% - 10.0% (zone 3) and 12% – 14% zone 4. To complicate things, different proprietary brands will provide slightly different percentage bulbs, and the UV-B your animal actually receives will depend on how far away the UV-B bulb or tube is. For a UV-B tube positioned inside the roof of the vivarium, the UVI output will diminish with distance. The optimum distance was generally 6-8 inches for the T8 variety of UV-B tubes although emerging better-quality products now yield T5 bulbs with more effective ranges of 12-24 inches. Therefore, the provision of décor such as a raised cork bark or petrified log that allows your animal to climb and perch closer to the UV-B will support healthy exposure. This efficiency loss can also be minimised with the addition of a reflective foil between the UV-B tube and the roof of the vivaria. Caution should also be taken with compact-top UV-B bulbs placed on the mesh roofs of some glass vivaria. The mesh grating will eliminate 50% of the UV-B before even reaching the inside of the vivarium, with foliage coverage eliminating more. Consider whether your animal is terrestrial or arboreal. Terrestrial vivaria are typically no taller than 2ft so UV-B will be efficient to ground level, whilst arboreal vivaria may be 4ft tall. Does your animal spend most of its time well exposed?

What’s more, these tubes will misleadingly continue to put out light but decrease their UV-B output over time. Very rarely do these tubes blow like an incandescent heat bulb, so people forget to replace them. At the Reptile Academy we have stickers on the side of the vivaria with the dates of the bulbs, so we know when they need changing.

You should replace your UV-B tube every 6 months for most brands, whilst some brands maintain efficient output for 9 – 12 months.

Depending on your species, you can also use the ‘shade method’ or ‘sunbeam method.’ The former is suitable for FZ 1 – 2 by providing lower level UV-B irradiation across the entire enclosure without an intense spot. This can be achieved with T8 (1 inch) fluorescent tubes, T5 (16mm), Mercury Vapour lamps (provide basking heath and UV-B) as well as metal halide lamps, of an appropriate percentage and distance. The latter is suitable for FZ 3 – 4 by providing an intense UV-B basking zone. Whilst T8 tubes have limitations for this intensity, it can be achieved with T5 tubes (with a reflector), mercury vapour and metal halide lamps of higher percentages.


So, what does my animal need?

Whilst the initial study did make us think about UV-B in a different light, the reptile keeping community is custodian to hundreds of species that are evolutionarily diverse and are from geographically very different places. Simply, 15 species across 2 regions is not representative (but it was a great start). These findings were followed up later by The British and Irish Association of Zoos & Aquaria (Baines et al., 2016) who allocated 254 species to these Ferguson’s zones based on our understanding of their wild ecology and natural behaviours. This is a thorough, reliable, trustworthy and peer-reviewed publication that serves as a great tool for providing the correct UV-B to our animals in captivity. The full original article is freely accessible online and provides a wealth of information including photoperiodicity (day-night light hours), both ambient air and basking surface temperatures, seasonal temperature changes, day-night temperatures as well as microhabitat details. We’ve summarised the UV-B requirements for some of the animals here at The Reptile Academy;

What are the limitations?

How do we overcome these? However, it is not perfect. Several species that are commonly kept in captivity, such as Superbia our Brazilian Rainbow boa, Epicrates cenchria, are omitted from this list. From discussions with fellow Epicrates cenchria keepers, those who choose to provide UV-B as part of a naturalistic husbandry approach generally provide 5-6% UV-B bulbs, corresponding to a FZ of 2. These keepers also acknowledged that whilst their animals are nocturnal and hide away during the day, the animals may benefit from low level ambient UV-B and regulation of a daynight cycle. Some keepers reported a short period of UV-B exposure when the light comes on in the morning and just before it turns off in the evening. Many keepers choose to provide a UVB tube because it enhances the stunning iridescence of the BRB. However, other keepers claimed that due to the nocturnal behaviour of Brazilian Rainbow boas, they get very little exposure in the wild and from their experiences have shown no illnesses that may be consistent with a need for UV-B in captivity. Of these 254 species categorized, the most closely related is the Cuban boa, Epicrates angulifer, that offers the best suggestion of UV for the Brazilian Rainbow boa. Similarly nocturnal, this animal was surprisingly allocated to FZ 1-2, demonstrating that even nocturnal species still have some UVI exposure in the wild.

Some insight has been provided on why the notion that nocturnal animals not requiring daytime UV-B is simply untrue. For example, nocturnal and crepuscular species, that would be expected to have no or minimal UV-B exposure, actually have increased UVB transmission efficiency. Therefore, whilst their thermoregulatory and basking behaviours dictate less frequent and intense exposure to UVB, their transmission efficiency may be greater than midday baskers due to the pigmentation and thickness of their skin. In addition, ‘accidental’ exposure by means of choosing poor daytime sleeping locations or disturbance by potential prey or predators are legitimate causes of albeit infrequent exposure.

What does this mean for the reptile-keeping community?

Much of this research consolidates current widely-used and good practices in the reptile and amphibian keeping community.

In particular, it is well understood that monitors require high UV-B exposure because they bask in midday and amphibians have lower UV-B exposure due to the risks of desiccation and burns. Most of this is not counter-intuitive, and we can generalise these findings to further species not on the current list such as Ackies monitors (desert, midday baskers.) However, it adds much controversy which is highly debated in online forums – and it can be a very touchy subject if you go asking in certain groups.

Much of keepers’ opinions and ‘evidence’ is anecdotal – based on their experiences and perhaps people they know. This is not to say that their experiences and opinions are invalid, as it is true that these animals can live long lives and appear to be relatively healthy throughout. However, if one is to become a better animal keeper using science, we must accept that anecdotal evidence does not constitute good science. The outcome of one animal is not representative of all individuals. Furthermore, reptiles and even amphibians are inherently tolerant animals and it is well understood that illnesses and diseases often do not become visible until the animal is so unwell that they are too weak to hide it or the disease has gained a debilitating foothold. Therefore, animals kept in sub-optional conditions may very well tinker on and appear superficially healthy, but this does not mean at all that they are in optimum health and fitness.

Perhaps the most popular yet ill-treated reptile of all is the Royal python, python regius. Keepers generally assume this species is either crepuscular or nocturnal because they are mostly active in the evening or at night. During the daytime, they are largely inactive and hide away most of the time. Therefore, UV-B is widely omitted in their captive husbandry and there is a widespread claim that ‘royal pythons do not require UV-B’ because they ‘are not susceptible to metabolic bone disease like lizards.’ It is true that MBD in Royal pythons is extremely rare, but as discussed previously UV-B functions beyond calcium metabolism, having downstream effects in over 2000 gene functions and bodily processes including immunity, muscle contraction, heart function and neurological transmission. According to their behaviour and Ferguson zone allocation, they are in zone 2 and therefore are partial sun or occasional baskers, and should be provided with a shade-dwelling 5.0%-6.0% T8 fluorescent tube, with plenty of hide coverage.


Special Considerations

There may be instances where it is reasonable for the welfare of your animal to provide lower level UV-B intensities or for reduced durations. Amelanistic animals (commonly referred to as albinos) have reduced or completely lack melanin (brown) pigmentation and appear yellow. These and similarly hypopigmented (low pigment) morphs may be at increased risk of UV-B damage. Melanin in the skin strongly absorbs UV-B and confers protection to the body. In its absence, UV-B can penetrate deeper into the sub-cutaneous layers causing burns and mutations that could cause cancers.

Different species may undergo ontogenetic changes; changes throughout an organism’s development or have cryptic lifestyles; with young animals being less bold and living more sheltered lives compared to adults so have very different UV-B exposures throughout life. For example, iguanas are arboreal but larger dominant individuals occupy higher levels of the tree canopy, becoming exposed to more UV-B. Savannah monitors transition from a terrestrial-fossorial lifestyle in the wet season to a terrestrial-arboreal lifestyle in the dry season, and animals such as Tegus and Giant African Bullfrogs undergo extensive periods of underground brumation and aestivation, respectively.



Providing UV-B for captive reptiles and amphibians remains a highly debated topic in the reptile keeping community. For some species, such as monitors, it is crystal clear that UV-B is essential to prevent MBD. For others, such as Royal pythons who are often kept in rack-systems, UV-B is largely omitted even though they are allocated to Zone 2. What is true, is that UV-B exposure is species-specific and the intensity and duration varies based on the thermoregulatory behaviours, microhabitats and daytime activity pattern of that species, that recent scientific studies have elucidated upon to help us better our captive husbandry techniques. Whilst many animals that in the wild have minimal UV-B exposure can survive in captivity without UV-B and appear healthy, it does not mean that they are thriving. Moreover, UV-B is important in a variety of processes beyond just calcium absorption. Here at the Reptile Academy, we are certainly advocating an environmentally-enriching husbandry approach where the appropriate UV-B exposure is important, based on the science we currently have available.


Baines, Frances & Chattell, Joe & Dale, James & Garrick, Dan & Gill, Iri & Goetz, Matt & Skelton, Tim & Swatman, Matt. (2016). How much UV-B does my reptile need? The UV-Tool, a guide to the selection of UV lighting for reptiles and amphibians in captivity. Journal of Zoo and Aquarium Research 4(1): 42 - 63. 2. Ferguson G.W., Brinker A.M., Gehrmann W.H., Bucklin S.E., Baines F.M., 52 Journal of Zoo and Aquarium Research 4(1) 2016 Baines et al. Mackin S.J. (2010) Voluntary exposure of some westernhemisphere snake and lizard species to ultraviolet-B radiation in the field: how much ultraviolet-B should a lizard or snake receive in captivity? Zoo Biology 29: 317–334


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