This is a problem we often encounter - people read online and find conflicting opinions on a topic. In thee cases it makes sense to delve deeper, because frankly we have no idea WHO the people we are reading are, what their experience or expertise is, are they just pet keepers? Vets? Scientists?
So I am going to point out a few home truths about geckos and WHY UV is beneficial for them.
The problem with being an insectivorous gecko is that very few insects indeed contain anything more than trace amounts of D3; the ones that do tend to be diurnal; so there is nowhere the geckos can actually get this important prehormone other than sunlight.
D3 is used in calcium metabolism. Without it, geckos develop a disease known as Metabolic Bone Disorder (MBD). More on this later.
We ALSO know from various studies that supplementing D3 in the diet does not fully compensate a lizard for D3 gained from sunlight; it does not compare, to the point that in SEVERAL studies lizards only fed the supplement but denied UV were found to have serum D3 levels so low they were virtually suffering from hypovitaminosis; and on the flip side, that the serum D3 levels of the lizards exposed to UV were significantly higher than those that had been supplemented. This indicates that geckos kept without UV are probably suffering from mild hypovitaminosis as a baseline (since providing UV ALWAYS raised their serum D3 levels, even when they were also being supplemented).
It is also worth pointing out that MBD is still the SINGLE most common pathology presented among captive geckos despite D3 supplements being commonplace. It can even occur when the gecko is being supplemented. UV would have solved this issue easily.
This is DESPITE the continuous denial of people like the "big breeders" who try to convince the easily led that minimalistic is better for the animals... But however honest you think they are, none of us sees the percentage of animals that suffer MBD or indeed other maladies out of the thousands they breed. None of us sees what happens to the large surplus of "normal" animals that result from their morph breeding projects. However much these big breeders may make themselves out to be "the messiahs" of the hobby - they aren't. They are businessmen with overheads and profit margins, and they meet these profit margins by keeping as many animals as possible in as poor conditions as they can get away with.
Let's be frank. Breeding these animals is EASY. Anybody can do it. Doing it on a huge scale proves nothing.
We KNOW Leopard geckos bask; this does not mean they bask all the time. However pointing out they do not bask all the time is hardly relevant; they have the choice to bask as and when they want and we know that they DO bask as and when they need to; serum D3 can last some time in the body and it is probable that basking for short spells lasts the gecko a few weeks.
Anybody that has observed geckos for any period of time in the wild will assure you these are not isolated occurrences and, despite the fact some geckos probably do spend more time hidden than exposed, this is not the same as not being able to expose themselves for any time at all - the whole point about providing UV using the "light and shade" method is allowing the animal to regulate its own exposure.
Again, we know from scientific studies that the presence of UV affects nocturnal gecko behaviour to the extent that geckos living on east-facing walls remain active later (to bask in the first rays of sunlight at dawn) and geckos living on west-facing walls emerge earlier (to bask in the setting sun's rays).
Not only that, crepuscular geckos exhibit FAR more efficient D3 synthesis than diurnal lizards of the same size, indicating that - whilst exposing themselves to sunlight might be risky in terms of exposing themselves to possible predation to diurnal predators
Here are some studies showing the benefits of UV on Leopard geckos.
Gould, Amelia, et al. "Evaluating the Physiologic Effects of Short Duration Ultraviolet B Radiation Exposure in Leopard Geckos (Eublepharis macularius)." Journal of Herpetological Medicine and Surgery (2018).
Wangen K, Kirshenbaum J and Mitchell MA "Measuring 25-Hydroxy vitamin D levels in leopard geckos exposed to commercial ultraviolet B lights." Association of Reptilian and Amphibian Veterinarians, 42. (2013)
Wangen, Kim, Jill Kirshenbaum, and Mark A. Mitchell. "Measuring 25-hydroxy vitamin D levels in leopard geckos exposed to commercial ultraviolet B lights." Proceedings of the Association of Reptilian and Amphibian Veterinarians conference. Indianapolis (IN). 2013.
On the efficiency of nocturnal geckos at synthesising D3 from UV light:
Carman, Elliott N., et al. "Photobiosynthetic opportunity and ability for UV-B generated vitamin D synthesis in free-living house geckos (Hemidactylus turcicus) and Texas spiny lizards (Sceloporus olivaceous)." Copeia 2000.1 (2000): 245-250.
On the use of shade-basking at low UV indices.
Passos, Daniel Cunha, Djan Zanchi, and Carlos Frederico Duarte Rocha. "Basking in shadows and climbing in the darkness: microhabitat use, daily activity and thermal ecology of the gecko Phyllopezus periosus Rodrigues, 1986." (2013): 171-174.
There is of course also the point that many "nocturnal" geckos see well into the ultraviolet spectrum:
Loew, Ellis R. "A third, ultraviolet-sensitive, visual pigment in the Tokay gecko (Gekko gekko)." Vision research 34.11 (1994): 1427-1431.
Yokoyama, Shozo, and Nathan S. Blow. "Molecular evolution of the cone visual pigments in the pure rod-retina of the nocturnal gecko, Gekko gekko." Gene 276.1 (2001): 117-125.
And this study determining that the background colour changes of a Gecko are not for thermoregulation but a response to light, and that they reflect less UV by day (Crested Geckos sleep among the leaves of growing plants; those leaves are in daylight. Shade, yes... but still daylight. And interestingly, they go pale when asleep. This allows increased penetration of the skin by daylight including UVB that is always present in daylight (and of course in higher amounts in direct sunlight, which they avoid). These little guys get virtually all their vitamin D3 from UVB in the wild; wild fruits and insects contain none:
Vroonen, Jessica, et al. "Physiological colour change in the Moorish gecko, Tarentola mauritanica (Squamata: Gekkonidae): effects of background, light, and temperature." Biological Journal of the Linnean Society 107.1 (2012): 182-191.
Just a scant few of the studies we have available in the literature describing the relationship between geckos and UV.
When it comes to studies actually done prescribing correct amounts of UV to offer this specific species of gecko, you do of course have the following study, performed by veterinarians and zoos with specific requirements of over 100 reptiles species:
Baines, Frances, et al. "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 (2016): 42.
Within it states that Leopard Geckos inhabit Ferguson Zone 1 (Shade Dwellers) and therefore require a UV gradient of 0.7 to 1.4. This was calculated by average irradiance at basking sites of randomly encountered individuals in the field.
SO. We have seen hat UV certainly DOES benefit Leopard geckos. What of the people that swear blind they keep them without it? Well, of course it is possible to keep these animals without UV. Nobody is denying that. However if you are not using UV, then you MUST supplement the diet with D3 to compensate (the fact you need to do this should tell you something about how the animal has evolved and what it is adapted to do).
The problem with this approach is that there has been NO study testing exactly how much D3 they actually need in the first place, and even if there had been it would need to factor in the differing D3 requirements of growing juveniles, gravid females and so on (i.e. the D3 requirement is not static throughout a gecko's life).
So when supplementing your gecko's diet, how much D3 is in "a pinch". Are your fingers big or small? Is your powder fresh or stale? Have the crickets cleaned it all off by the time they are eaten? Too little D3 and you risk hypovitaminosis. Too much, and you possibly risk hypervitaminosis or vitamin D toxicity. That's not easy-care, is it?
Easy-care is installing a simple ordinary T8 6% or 5% UVB tube or compact lamp over the top mesh of your terrarium on a timer for 12hrs/day and forgetting about it, knowing your gecko will choose to sleep just as far away or near to it as his vitamin D3 needs dictate.
Just because it is possible to keep Geckos with or without UV does NOT mean the two methods are equal. One method is better, that has been demonstrated time and again. Because UV is good for more than just stimulating D3 synthesis. It also is important for the melatonin/ serotonin cycle, which is linked to circadian rhythms, wakefulness and activity levels. The only actual scientific studies that have compared geckos being kept under both treatments (some of which I have linked above) have shown that they are NOT kept with "the same success" - the animals kept with UV have had significantly different levels of serum D3 (implying that without UV they are not getting enough D3), different behaviours and different physiology to the point that we have been shown x-rays of geckos kept with and without UV with very obvious bone density differences even to the naked eye.
One final thought, now I think of it:
It is telling that perhaps the largest commercial breeder of geckos in the world has gone on record more than once stating that the common reptile-branded D3 supplements are flat out not good enough for his purposes and he uses D3 supplements designed for cats and dogs - much larger animals - for his geckos... This alone should say something.
It is already known that D3 supplemented solely by diet is not as effective as simply providing UV; several studies have shown that serum D3 levels are just not as high after using just supplements as opposed to providing UV. Also, how is one to know exactly how much supplement to provide? Vitamin D is toxic in large amounts and too much can cause hypervitaminosis, how does one calculate exactly how much to provide to each gecko?
Ignoring this most common facet of providing UV - that of D3 synthesis and calcium metabolism, there are plenty of other known benefits of providing UV. Geckos are VERY well known to see well into the UV spectrum and in fact UV reflectance is known to be a major stimulus to some of them.
Whatsmore, as has been pointed out above, UV is a completely separate visible colour to them. We know for example that even "nocturnal" geckos like Tokays have visual pigments in their eyes that enable them to perceive UV (364 nm λmax). That this UV-absorbing pigment was truly a visual pigment was confirmed by its dichroism, behaviour following exposure to UV radiation and “nomogram” fit. (Loew, 1994).
From experiments on various lizards and their behavioural reactions to light, we know it stimulates everything from basking behaviour to reproductive activity (Clausen et al. 1937; melatonin production and synthesis of methoxyindoles (Firth & Kennaway, 1987) and therefore Circadian rhythm (Janik et al. 1990). At night, it effects serotonin production and thus the presence or absence of light affects production of different hormones (Enrbretson & Lent 1976).
The final "proof of the pudding" though is the behavioural response typical to these animals when provided with UV they spend plenty of time actively basking under it. I have several geckos siting right out in the open in front of me as I type this. Some of them are over twenty years old.
Is it possible to keep these animals without UV? Certainly. But by doing so you are forced to account for the deficiency with the supplements you provide, and denying them several important functions of UV not related to D3 metabolism. So it certainly benefits them more to be provided with UV, and that is absolutely not in doubt.
Bottom line: USE UV.
