The southwest of Madagascar is a dry, hot region flooded by the subtropical sun. At the coast the sunlight is additionally reflected by the sea, so that the local flora and fauna are exposed to a relatively high and direct solar radiation all year round. As a typical inhabitant of this semi-arid zone, the entire organism of Astrochelys radiata is also optimally adapted to intense sunlight. The entire activity cycle of Astrochelys radiata is influenced by the wandering of the sun and the associated brightness and temperature. Light is primarily associated by tortoises with warmth, which is needed to become active and to reach an optimal metabolic temperature. However, the amount of light, light spectrum and light intensity prevailing in the habitat are also essential for the health and well-being of Astrochelys radiata. The characteristics and composition of natural sunlight have a direct influence on hormonal balance, metabolism, vitamin D synthesis and the immune system of the animals.

Radiated Tortoises are particularly heliophilic reptiles. Therefore, there is only one optimal light source and that is the natural sunlight. This can neither be permanently replaced by HQI nor other professional UV lamps. Whenever possible, animals in human care should have access to natural sunlight, the more the better. In the European summer, daily exposure to a sunny outdoor enclosure is an absolute must, and in the cold season, daylight indoor housing is best accomplished in conservatory or greenhouse-like accommodations. Housing concepts that incorporate natural daylight have a clear advantage and also help to save material and electricity costs (see Fig. 1). Indoor housing without access to natural daylight must be generously illuminated in bad weather and during the cold season with professional lighting similar to daylight. A common mistake in this regard is insufficient basic illumination of the indoor enclosure. At least 2/3 of the total indoor area should be brightly illuminated with daylight-like lamps for 8-10 hours! Lighting that consists only of isolated and limited light cones below the spots is insufficient and irritates the turtles in spatial light perception (see Fig. 3). Optimal illumination of an indoor facility always consists of higher hanging and flat HQI base lighting and lower hanging HQI spots that provide both heat and sufficient UV light (see Fig. 2).

Commercial terrarium lighting is usually perceived by us humans as more intense and brighter than by reptiles, yet these artificial illuminants do not come close to the intensity and quality of natural sunlight. If Radiated Tortoises are kept under suboptimal light conditions, they usually wander around the enclosure for days restlessly looking for more sunlight, which is often mistakenly interpreted by the keeper as regular daytime activity. If the lack of light persists, the animals become increasingly inactive and crawl into hiding places to compensate for the unfavorable light situation by shutting down their metabolism. After days, however, the animals become completely inappetent and lethargic and hardly move from the spot. An acute light or UV deficiency always has both mental and physical consequences for Radiated Tortoises. The relationships are complex and interact with each other. An unbalanced and fluctuating hormone balance due to light deficiency makes the animals depressed and apathetic. In this state, neither further energy can be absorbed through feed nor important vitamin D can be synthesized. These deficiency symptoms result in a disturbed metabolism, which in turn leads to an unfavorable and lasting weakening of the immune system. In addition to irreversible bone, shell and organ damage, sudden flare-ups of infections can lead to the rapid and agonizing death of the animals.

Our tortoise house has a sloping and south-facing roof with two large, recessed and temperature-controlled hinged windows, through which diffuse daylight always falls even on cloudy or rainy days. In summer, the animals can move independently from the indoor to the outdoor enclosure and move with the sun. Despite great efforts to integrate as much European sun as possible into the daily routine of our Radiated Tortoises, the local sunshine hours in central Switzerland are far from reaching subtropical conditions. Therefore, on cold and low-light days, it is absolutely necessary to simulate more daylight in the indoor facility with HQI spotlights. However, if our Radiated Tortoises have the choice to stay under a spotlight or in a light cone below the skylight, they always prefer the intense light color of natural daylight.

Seasonally we use several 400 Watt HQI spotlights as basic lighting in the indoor enclosure and some Lucky Reptile Bright Sun Desert spotlights 150 Watt with ballast in exposed areas. The 400 Watt HQI lamps illuminate an area of 0.5×0.6 m (Lux: 60000/30cm) at a distance of about 50 cm from the ground. In addition to a large-area basic illumination, one lower-hanging, large HQI spotlight per adult pair must be reckoned with for heat and UV uptake. Despite a comfortable mixed light situation with natural daylight coming through the skylights, additional UV, light and heat sources are needed at exposed locations in the indoor enclosure, such as on the laying mound. We always make sure that our Radiated Tortoises are never exposed to artificial lights in the indoor enclosure for more than 4-5 months at a time (approximately November through March) during the cold season. In order to allow our Radiated Tortoises to benefit from natural sunlight as much and as long as possible, we also let them out into the outdoor enclosure during the transitional periods (November/March), even if only for a few hours, if the weather is mild and dry. The outside temperature (from 15°C) is less important than the experience of a sunny, bright day. Direct access from the indoor to the outdoor enclosure allows the animals to warm up again independently and quickly in the heated indoor enclosure at any time.

Due to the different dimensions and conditions of indoor facilities, a general recommendation of illuminants for Radiated Tortoises is not possible. The choice and number of suitable lamps depends on the size and height of the indoor facility as well as the area to be illuminated and the number of animals kept. All illuminants and mounting distances must always be adapted to the respective conditions for safety reasons. Please also observe the operating, mounting and safety instructions of the respective manufacturer! Illumination and heat should always be checked several times and over a longer period of time before the animals move in. With all UV lamps the UV radiation decreases continuously after a certain burning time. Even if the lamps still shine brightly, they will not emit sufficient UV radiation after approximately one year. Although it is economically and ecologically questionable, we always recommend to replace all lamps after one year as a preventive measure to ensure a continuous and optimal UV radiation. If you want to use HQI lamps longer than one year, an additional use of strong UV lamps such as the Osram Vitalux over a period of 3-4 hours over the midday period for a sufficient UV supply is recommended.

The following light sources have been very successful in indoor keeping as light, heat and UV illumination: