Context The Testudine shell has contributed to the success of this clade for 200 million years by conferring protection against predators. Despite this, the slow life history of Testudine species increases their vulnerability in this rapidly changing world, and many are threatened with extinction. Ex situ conservation programs can rescue species from extinction, but maintaining optimal environmental conditions in captivity can be challenging. One species for which captive breeding is instrumental for recovery is the critically endangered western swamp tortoise (Pseudemydura umbrina). Anecdotal evidence suggests that the shells of captive tortoises are softer than those of individuals in the wild, which has the potential to have significant survival implications for zoo-bred individuals that are released into the wild. Aims The aims of our investigation were to (i) compare variation in environmental parameters, including ultraviolet B (UVB) levels that is important for Testudine shell growth, between a captive and wild population of the western swamp tortoise, and (ii) test empirically whether zoo-bred western swamp tortoises have softer shells than wild individuals. Methods We used remote sensing equipment to measure environmental variables (UVB light, temperature and relative humidity) within the captive and wild environments. Additionally, we used a tension-calibrated digital micrometer to evaluate differences in shell hardness among zoo-bred and wild individuals. Results We found that shell hardness in juvenile zoo-bred tortoises was significantly lower than that in wild juveniles, but did not detect differences among adults from the two populations. Our environmental measurements showed that UVB levels in the captive environment were significantly lower than levels recorded in the wild, and temperature and relative humidity were significantly higher. Conclusions Taken together, our results indicated that differences between wild and captive conditions are evident and may influence shell hardness of zoo-bred juveniles, an important insight for the captive breeding of a critically endangered species. Further research is needed to investigate the implications of softer shells for released tortoises, for example, regarding the likelihood of predator attack success. Implications Our research highlighted the need to consider the environmental conditions of the captive environment in ex situ conservation programs of endangered Testudines.
Firman et al. (Mon,) studied this question.