Hatch size, somatic growth rate and size-dependent survival in the endangered ploughshare tortoise Original Research Article

Successful conservation requires a good understanding of both the direct and indirect causes of any decline in population size. Harvests of wild populations often target the largest, oldest individuals that have the greatest economic value. If these individuals contribute disproportionately more to recruitment than conspecifics, the harvest will cause a greater reduction in population viability than initially anticipated. The ploughshare tortoise, endemic to Madagascar, has been reduced to <600 individuals due to exploitation and habitat loss. A trial release of captive-bred juveniles has been carried out but the relative suitability of individuals for release, in terms of body size, was not considered. Using data from a long-term mark-recapture study of an unharvested wild population of ploughshare tortoises, we found that larger juveniles were significantly more likely to survive their first year of life than smaller tortoises. Juveniles that survived beyond their first year of life generally grew at the same rate, but had a significantly larger hatch size (mean = 41.7 mm), compared with juveniles that did not survive (mean = 39.3 mm). The conservation implications of these results are that release of captive-bred ploughshare tortoises is likely to be more successful if larger individuals are released. Previous studies have reported larger female ploughshare tortoises laying larger eggs which hatch larger juveniles. This study shows individuals with a larger hatch size retain their size advantage over smaller conspecifics and are more likely to survive their first year of life. This suggests a harvest that targets the largest females in the ploughshare tortoise population could be highly detrimental to population viability.