Age Estimation

Abstract

This chapter focuses on the age estimation, a tool for obtaining a numerical value of age for animals for which the actual age is not known. Currently, age is estimated primarily from counts of growth layers deposited in several persistent tissues, primarily teeth, less often bone, and in some cases from other layered structures or from chemical signals. Growth layers in the persistent structures are similar in concept to growth rings in trees. Until the use of growth layers became a feasible means of age estimation, relative measures of age, such as tooth wear, pelage or skin color, or fusion of cranial sutures allowed individuals to be placed in age groups; these techniques largely have been replaced with methods that allow for estimation of absolute age by counting growth layers. Marine mammalogists pioneered age estimation from counting growth layers in teeth, initially in pinnipeds; this discovery was followed by widespread use for terrestrial mammals as well. Much of the development of this field has focused on how to ensure that age estimates are accurate and precise. That focus has been directed toward verifying the amount of time represented by a growth layer (i.e., calibration or validation), developing increasingly better ways to prepare samples for optimizing counts, and standardizing methods to ensure that growth layer counts are consistent among studies.