Why do reptiles need UVB light? And do all reptiles need it, or just some?
The majority of husbandry across zoological collections, as well as the reptile industry, is rooted in traditional practices that are not necessarily founded in empirical evidence. The avoidance, or lack, of negative welfare indicators as a basis for justifying current practices in captive husbandry is widely and mistakenly perceived to equate to positive welfare. However, providing animals with “what they want” (determined via preference testing) may provide a wider range of positive welfare states, rather than providing only the basic necessities of life.
For example, the Animal Welfare Act (2006) states that keepers and owners are responsible for ensuring the welfare of their animals in accordance to the five animal needs. In particular, the need to exhibit normal behaviour patterns and the need for a suitable environment. This may be achieved through replication of natural environments, including parameters that encourage the expression of natural behaviours via choice-based self-exposure.
It is relatively well known that certain reptiles need UVB light in their captive environment in order to survive. Self-exposure to ultraviolet-B light (UVB) in reptiles is well documented in scientific literature, so provision of UVB should be considered an established husbandry norm. However, provision of artificial UVB is scarce among some species (possibly even a majority), even though vitamin D deficiency is documented in literature, especially when synthetic vitamin D3 supplementation is absent.
Vitamin D is a hormone essential for calcium metabolism in most vertebrates. Coupled with calcium and phosphorus, it plays an essential role in healthy growth, bone development, reproduction, cardiovascular health, and immune function. Low levels of vitamin D can cause metabolic bone disease (MBD), which is recognised as low bone density, the bending of long bones, and fractures.
Vitamin D precursors (7-dehydrocholesterol) are naturally produced within the dermis (skin), and are converted into previtamin D by UVB light (290-315nm). When exposed to heat, the previtamin D becomes vitamin D3, which is carried in the blood plasma to the liver where it is converted into the hormone calcediol (25-hydroxy vitamin D3). It is then carried to the kidneys where a part of it is converted into calcetriol (1,25-dihydroxyvitamin D), the active metabolite in the kidneys. 25-hydroxy vitamin D3 or 25(OH)D3 is the most stable metabolite and is also present in highest levels in the blood plasma and serum, making it an excellent indicator of vitamin D status.
Historically, rickets (MBD) has been reported when animals were housed indoors without access to natural sunlight, leading to the practice of synthetic vitamin D3 being orally supplemented through the diet. This is problematic as the exact doses required per species, sex, life stage, season, or body weight may be unknown. Supplementation of synthetic vitamin D3 given in excess can be harmful to the animal. Whereas, excess production of 25-hydroxy vitamin D3 under UVB cannot occur and is self-regulating due to a build-up of a reversible pool of inert metabolites in the dermis. For this reason, exposure to UVB light is a safer alternative to dietary supplementation, as the risk of hypervitaminosis is rendered redundant.
Even though evidence of vitamin D3 deficiency is described in literature as affecting many species, use of artificial fluorescent UVB lighting in reptiles such as leopard geckos, blue tongue skinks, and especially snakes is rare and still considered a new and controversial practice. Melfi (2009) states that offering preference-tested choices and monitoring of indicators of positive welfare such as good neonate growth, reproductive ability and the expression of pleasurable behaviours is the route to higher captive animal welfare. This source also argues that the five animal needs stated in the Animal Welfare Act (2006) only enforce satisfactory welfare at best, as it only focuses on the avoidance of negative welfare.
Therefore, providing UVB at levels similar to what each species experiences in its wild habitat could be argued as an essential part of replicating the species’ natural environment. Providing UVB light for reptiles complies with the need for a suitable environment, the need to be able to exhibit normal behaviour patterns, and the need to be protected from pain, suffering, injury, and disease.
If this is to be considered baseline satisfactory welfare, then the provision of artificial UVB must become a necessary component of captive husbandry, not just an option. It must become acknowledged that all reptiles need UVB light, not just some. This becomes even more relevant for species that are housed outdoors, yet do not have access to a high enough UV index (a measurement of ultraviolet light intensity on an index scale) due to the geographical location in which they are kept. In addition, the Zoo Licensing Act (1981) requires zoos to provide each animal with an environment well adapted to meet its physical, psychological, and social needs while providing a husbandry program which includes preventative and curative veterinary care and nutrition. Again, it could be argued that not supplying adequate levels of UVB in any capacity fails this.
A study by Drake et al. (2017) tested the provision of artificial UVB lights on indoor housed tropical birds to prevent suspected occurrences of vitamin D3 deficiency. (Editor’s note: Although birds are not reptiles, they are very similar. So similar, in fact, that technically speaking, birds can arguably be called reptiles due to the fact that they are evolutionarily descended from them. Exotic veterinarians frequently reference avian data to make educated decisions pertaining to reptile health and treatment.) Artificial UVB provision is not common practice for captive birds, as it is considered unnecessary. But within this study, the birds were observed to choose to bask when given the opportunity. Annual occurrences of MBD dropped from 14.3% to 3.2% over a course of three years, and there was a significant increase of 25(OH)D3 (65.5%) after 12 months.
There are also several reptile-specific studies which yielded similar findings and results.
For example, the age-old of argument of, “It’s nocturnal—it doesn’t need UVB,” is rendered redundant when studies testing for a rise in 25-hydroxyvitamin D in nocturnal and crepuscular species such as Burmese Pythons (Python bivittatus), Leopard Geckos (Eublepharis macularius), and Corn Snakes (Pantherophis guttatus) all show a positive correlation between UVB and high 25-hydroxyvitamin D levels. And even while many species are yet to be scientifically tested through blood withdrawals, there is a multitude of anecdotal evidence of species of all kinds basking under UVB either partially or fully exposed.
One such anecdotal case is the author’s own Mexican Black Kingsnake (Lampropeltis getula nigrita), which basks fully exposed daily at a UVI of 3-4, even though it is considered a nocturnal or crepuscular species. This species is yet to be described as using UVB in a formal study, but intentional UVB exposure is very much part of this animals’ daily routine.
While the potential benefits of UVB exposure seems to be strong, further research is required to determine suitable UV provision recommendations on a species by species basis. A 2016 paper by Frances Baines et al. is one such attempt to do so. In the meantime, estimations can be made using UVI measurement data taken from the habitat or microhabitat of the chosen species and provide similar a UVI at the basking site of the enclosure, leaving ample room left for the rest of the enclosure to cascade into lower light levels. We are all aware of the importance of thermal gradients, but a gradient of ultraviolet light and visible light is also just as important. These should coincide with the thermal gradient to encourage natural behaviours. The association between high light levels, heat and UVB is a basic part of most species’ biology, whether it be via the pineal gland or by visually seeing into the ultraviolet spectrum.
There is significant evidence in scientific literature to suggest that all reptiles need UVB light, and UVB provision should be more commonplace in both zoological collections and the reptile industry than it is now. It could also be argued that not doing so fails conditions of animal welfare legislation in the UK. The provision of UVB should not be considered a supplementary measure, but a necessary part of herpetological husbandry regardless of the species in question. Giving the option to use UVB should be just as commonplace as giving the option of multiple hides. The question shouldn’t be: “Should I provide UVB?” It should be: “How much?”
Learn more about UVB provision for reptiles:
- Introduction to UVB, part 1: What is UVB, and Why is it Important to Reptile Husbandry?
- Introduction to UVB, part 2: How to Use UVB in Your Reptile Enclosure
- Does My Snake Need UVB Light and Does My Snake Need Good Welfare
- Testing UVB Bulbs with The Bio Dude
About the author:
Liam Sinclair is a keen reptile owner that has worked as a zookeeper as well as in a specialist reptile shop in the UK. He is currently pursuing a degree in Animal Management, and he currently runs the YouTube channel, Reptiles and Research. His goal is to help people achieve higher welfare for their animals.
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