One scute ring per year in <em>Testudo graeca</em>? A novel method to identify ring deposition patterns in tortoises

Abstract

A reliable estimation of individuals’ age is helpful to conduct demographic studies on wildlife populations. In tortoises, many studies have estimated individuals’ age by counting growth rings on their scutes, assuming one ring per year (1:1 ratio). However, the accuracy of this method is controversial. The ring deposition pattern can vary depending on species, or even populations, and should be studied comprehensively. We studied the ring deposition pattern of Testudo graeca in southeastern Iberian Peninsula, using recaptures of 156 individuals between 2004 and 2010. We used a novel approach to explore the ring deposition pattern and to test possible differences between localities and individuals. Our results revealed that most analysed individuals (57.4%) showed a 1:1 ratio, in which rings were deposited during months of activity (spring to autumn). However, we found a trend to count less rings than years, which underestimated 1 year every 3 or 4 years. No differences in the deposition patterns were found among sites, sizes or sexes because the halt in growth during hibernation equally affects all tortoises in all sites. Our results support that the assumed 1:1 ratio in the assignment of individuals’ age is too simplistic. Since ring deposition patterns are complex, the use of statistical approaches capable of handling deviations from the assumed deposition ratios can help to better depict population age structures.