From the forests to teeth: Visual crossdating to refine age estimates in marine mammals

Estimation of the age of individuals is a fundamental parameter for understanding the biology of individuals and ecology of populations. Incremental growth deposits in tooth dentine or cementum, known as growth layer groups (GLGs), have been used to estimate the age of toothed marine mammals (odontocetes, pinnipeds) since the 1950s (Scheffer and Myrick 1980). In most species, it is assumed that GLGs are deposited annually (Hohn 2009). Validation of this assumption has only been possible in a small number of marine mammal species (e.g., Bowen et al. 1983, Myrick et al. 1984, Hohn et al. 1989, Lockyer 1993, Oosthuizen 1997), due to impracticalities in obtaining validated ages (Evans et al. 2002, Hohn 2009). For species in which validation of age is not possible, absolute age or the accuracy of age estimates cannot be confirmed (Campana 2001). Further, the precision of estimates (i.e., the closeness of repeat counts from an individual tooth) may not necessarily reflect the best estimate of age and, at present, an objective method to assist with identification of GLGs for age estimation purposes has not been established (Evans et al. 2002, Campana and Stewart 2014).