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The ancient reptiles need ultraviolet (UV) light to get vitamin D, which helps the bones absorb calcium and reduces the risk of broken bones and other painful conditions.
A new study from Massey University researchers found four of 18 tuatara facilities in New Zealand had ineffective UV light sources, while others were not providing enough light, The Press reported.
There was "a clear relationship" between light availability and the risk of developing bone problems, the study said.
The study did not name individual facilities, but all had been told to ensure tuatara had UV light equivalent "to what you see on a cloudy day in New Zealand".
The researchers did not note whether any deaths had resulted from inadequate lighting.
Tara Atkinson, head of native species at Christchurch's Orana Wildlife Park, said the research could save lives, but came too late for two baby tuatara that died at the park several years ago.
"Their bones were not up to it. They were too soft to support growth."