Effectiveness of phalanx skeletochronology to estimate age in living reptiles

Demographic studies are fundamental in population ecology, as well as in conservation biology and wildlife management. However, few methods are available to determine the exact age of animals. Mark-recapture is usually the main method to study demography, but this approach is highly time-consuming and needs long-term monitoring. For species for which recapture is not feasible, this method is not valid. However, in vertebrates with indeterminate growth, such as fish, amphibians, and reptiles, skeletochronology is a method that allows age to be estimated from a bone. Nevertheless, studies of skeletochronology frequently involve the death of the animal to obtain the bone. In the present study, we test the reliability of phalanx skeletochronology, comparing the readings from the most commonly used bones in reptile skeletochronology (femur and humerus) with the age estimated from phalanges. Our results show phalanx skeletochronology to be a reliable method for estimating age in lizards without killing them. Cross-section readings from all bones studied presented a high correlation and repeatability, regardless of the phalanx chosen. These findings imply that, to apply skeletochronology, phalanges must be used instead of other bones that mean the death of the animal, and the killing of lizards for skeletochronology studies is no longer justified. This alternative is especially relevant for endangered species, considering that obtaining a representative sample usually requires a considerable number of individuals.