UltraViolet Index & Ferguson Zones

Updated: Dec 28, 2021

The aim when designing an enclosure for any animal is to replicate their natural habitat as closely as possible, otherwise there are further ethical considerations as to whether these animals should be kept privately.


For reptiles, we can refer to the Ferguson Zones to understand the environment we need to recreate with regards to sunlight. Aptly named the Ferguson Zones, after Professor Gary Ferguson, his team from Texas Christian University pioneered one of the first comprehensive studies by measuring the daily voluntary basking habits of 15 different reptiles, and recording the UV range to the exposure time using the UV Index (UVI). UVI measurements are especially important as it measures UV irradiance in the wavelengths that enable vitamin D3 synthesis - crucial to keeping reptiles.


From this study, reptiles could now, with a greater deal of accuracy, be categorized into one of four zones based on voluntary UVB exposure observed in the wild.


They are as follows:

  • Zone 1 = crepuscular or shade dweller, thermal conformer. Average exposure: UVI 0-0.7; maximum recorded: UVI 0.6-1.4

  • Zone 2 = partial sun/occasional basker, thermoregulator. Average exposure: UVI 0.7-1.0; maximum recorded: UVI 1.1-3.0

  • Zone 3 = open or partial sun basker, thermoregulator. Average exposure: UVI 1.0-2.6; maximum recorded: UVI 2.9-7.4

  • Zone 4 = mid-day sun basker, thermoregulator. Average exposure: UVI 2.6-3.5; maximum recorded: UVI 4.5-9.5


Whilst this study was conducted in the southern US and Jamaica and only observed 15 species of lizards and snakes, follow-on research has assigned a further 304 species to zones using field studies and observing captivity behaviour.



Using our tool Find your Reptiles Ferguson Zone, you can find the zone for your specific animal and design your enclosure accordingly. The main takeaway is to know the UV index, and whether the reptile needs light via the shaded method, or the sunbeam method (note that some allow both).






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