Before talking about temperatures lets have a look at Varanus melinus natural habitat weather statistics. That is going to dismiss all the speculations on our end and make everything easier.
Weather conditions in Indonesia are uniform throughout the year and that is going to make our job a bit easier, as there is no real need to mimic seasonal fluctuations. The most important thing for us is to create proper temperature GRADIENT. Monitor lizards need to have a temperature range to thermoregulate. They do require hot basking areas and hot corners, but also absolutely need to have areas with lower temperatures. In the wild they have very hot sun-lit spots, cooler shaded areas, cooler ocean breezes and all other climatic events that provide necessary temperature range. This is why uniformly hot or uniformly cool enclosure is not going to work for an animal. And this is why custom enclosure of a large size is very important - it is impossible to create proper temperature gradient in a small enclosure.
V. melinus basking and observing winter in northern hemisphere
Basking area. Having basking area at 125-130 °F (52-55 °C)
is essential for monitor lizards to properly digest and metabolize their food and get
their body temperature up to perform daily activities.
The best way to provide adequate basking area is to position dark colored stone slate under the chain of halogen lights of proper wattage. It has been observed, that it is much more beneficial to use a chain of smaller wattage halogen flood lights versus one of a higher wattage. Smaller halogens have lower profile, cooler to the touch, spread heat evenly across animal body, don’t affect humidity as much, don’t fail all at once, can be automated to simulate daylight changes. Stone or similar slate will help in heating animal up from below, so the monitor will be positioned in a “heat sandwich” and will acquire necessary temps faster and more evenly. Basking time for monitors should be brief, after that they proceed with their business. If monitor basks for long periods of time that may indicate that basking temp is too low or may also indicate some health issues.
Failure to provide necessarily high-temp basking area and proper ambient temperatures range will lead to low activity, lethargy, stunted growth, low muscle tone, metabolic issues, digestive issues up to an impaction, weak immune system, respiratory infections and other connected health problems.
Lower row: temps and humidity range inside the enclosure
Day ambient temperatures. We need to aim for 78-90 °F (25-32 °C) gradient. To provide necessary temperature gradient in the enclosure most of the time all we need is just properly large and tall enclosure with basking area in one of its corners. In case of arboreal Quince monitor basking area can be raised up to the upper half of the enclosure - that will concentrate heat up top with slight gradient across the top of the enclosure. It will leave bottom area with a substrate at necessarily low temperatures. And it will also provide all the necessary gradient from cooler bottom to warmer mid-level to hot upper level. Now we just need to make sure all those areas are fully accessible to the animal and provide multiple climbing features, levels and shelves for arboreal animal to move freely between different temperature zones.
Night ambient temperatures. Can safely be set around 76-80 °F (24-27 °C)
range. Monitors are diurnal creatures and don’t need the enclosure to be lit during night hours.
Night time temperatures can be provided by using ceramic heat emitters connected to thermostats.
Just need to make sure animal can’t access them as they run really hot. There is no need to
provide reptile-specific red or moonlight incandescent bulbs as they serve no useful purpose
and fail frequently.
Actually it is best to avoid thin-walled incandescent bulbs inside the enclosure - they tend to explode on a contact with water and may harm the animal.
UVB. Debatable topic. Many keepers claim monitors can successfully be kept without source of UVB as they get all the necessary vitamin D through whole prey diet. They do, however, receive decent amounts of UVB in the wild so it could be beneficial to provide it in their enclosures. Just avoid compact coil-type UVB bulbs - there have been problems with them in the past. Two good solutions are: mercury vapor bulbs to combine heat and UVB (can be used for basking) or T5 HO 10.0 fluorescent lamps, that give good daylight spectrum in addition to UVB. All UVB bulbs have to be replaced per manufacturer specs every 6 months to a year.
V. melinus basking outside the enclosure
Heat retention and ventilation. Monitor enclosures don’t have to be sealed air tight. But it is good to design them with heat(and humidity) retention in mind. That includes thicker walls and ceiling, thicker glass or plexiglas, no vents or mesh on top. Thick substrate layer on the bottom works as a good insulation, good heat retention, good humidity support and good burrowing support for activity enrichment. If you think you do need a ventilation system it is good to position it closer to the bottom of enclosure, make smaller openings and equip them with fans to force better ventilation at given time. Fans should be positioned outside with fine mesh protecting the animal from reaching them with any part of their body including tongues.
Additional light sources. After temperatures are dialed in but you still need additional source of light you can use fluorescent tubes and/or LED lights. They can give you necessary light spectrum without significantly increasing enclosure temperature. Large transparent doors and windows in the enclosure to provide additional lightning. We just need to make sure they are positioned at the front, so animal can retreat to more secluded areas deep inside the enclosure.
Additional enrichment and daylight change simulation. It is beneficial to simulate daylight changes with multiple heating-light sources available. For example, if basking area is equipped with 3 sources, they can be automated to turn on in the morning one after another at 30 minute time intervals to gradually increase temperature and light inside the enclosure. After that any additional lights and UVBs can be turned on at calculated intervals. Then in the evening they can be automated to turn off in reverse order.
Example of light and heating layout
Tools and Equipment. Bringing down to essentials we need: