Tuatara probably have the slowest growth rates of any reptile, continuing to grow larger for the first 35 years of their lives. The average lifespan is about 60 years, but they can live to be well over 100 years old, barring tortoises, tuatara is the reptile with the longest lifespan. One such male even reproduced successfully for the first time at 111 years of age with an 80-year-old female. Some experts believe that captive tuatara could live as long as 200 years. This may be related to genes that offer protection against reactive oxygen species. The tuatara genome has 26 genes that encode selenoproteins and 4 selenocysteine-specific tRNA genes. In humans, selenoproteins have a function of antioxidation, redox regulation and synthesis of thyroid hormones. It is not fully demonstrated, but these genes may be related to the longevity of this animal or may have emerged as a result to poor levels of selenium and other trace elements in the New Zealand terrestrial systems.