Aspartic acid racemization rate in narwhal (Monodon monoceros) eye lens nuclei estimated by counting of growth layers in tusks

Ages of marine mammals have traditionally been estimated by counting dentinal growth layers in teeth. However, this method is difficult to use on narwhals (Monodon monoceros) because of their special tooth structures. Alternative methods are therefore needed. The aspartic acid racemization (AAR) technique has been used in age estimation studies of cetaceans, including narwhals. The purpose of this study was to estimate a species-specific racemization rate for narwhals by regressing aspartic acid D/L ratios in eye lens nuclei against growth layer groups in tusks (n = 9). Two racemization rates were estimated: one by linear regression (r(2) = 0.98) based on the assumption that age was known without error, and one based on a bootstrap study, taking into account the uncertainty in the age estimation (r(2) between 0.88 and 0.98). The two estimated 2k(Asp) values were identical up to two significant figures. The 2k(Asp) value from the bootstrap study was found to be 0.00229 +/- 0.000089 SE, which corresponds to a racemization rate of 0.00114(-yr) +/- 0.000044 SE. The intercept of 0.0580 +/- 0.00185 SE corresponds to twice the (D/L)(0) value, which is then 0.0290 +/- 0.00093 SE. We propose that this species-specific racemization rate and (D/L)(0) value be used in future AAR age estimation studies of narwhals, but also recommend the collection of tusks and eyes of narwhals for further improving the (D/L)(0) and 2k(Asp) estimates obtained in this study.