Age determination of mysticete whales using 210 Pb/ 226 Ra disequilibria

• Published in: Canadian Journal of Zoology
• Main Authors: Kastelle, Craig R, Shelden, Kim EW, Kimura, Daniel K
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2003
• Subjects: Animal Science and Zoology; Ecology, Evolution, Behavior and Systematics; Balaena mysticetus
• Online Access: http://dx.doi.org/10.1139/z02-214; http://www.nrcresearchpress.com/doi/pdf/10.1139/z02-214

Accurate age determination is fundamental to the study of population structure and individual growth rates of mysticete whales. Here the disequilibrium between 210 Pb and 226 Ra in the tympanic bullae of two mysticete whale species was investigated for use as a chronometer. Radiometric ageing depends upon accumulation of the naturally occurring radionuclide 226 Ra (exclusive of other 238 U decay-chain members) in the bullae and subsequent retention of its progeny 210 Pb. Ages are determined from the 210 Pb/ 226 Ra activity ratio. Samples were obtained from five gray whales (Eschrichtius robustus) with lengths of 4.5 (a neonate), 7.8, 8.7, 10, and 11.5 m, and two bowhead whales (Balaena mysticetus) with lengths of 12.9 and 17.4 m. In gray whales, radiometric ages were estimated in the three largest whales. In the neonate, the 210 Pb/ 226 Ra ratio was above one and was not usable. The 7.8-m gray whale was used to determine the initial 210 Pb/ 226 Ra ratio required for age determination. We propose a theory of gray whale bullae growth starting at the fetal stage with an open system (with a 210 Pb/ 226 Ra > 1 and a fast growth rate), which transitions by 1 year old to a closed system (with a 210 Pb/ 226 Ra << 1 and a slow growth rate). In both bowhead whales, radiometric age could not be estimated because the 210 Pb/ 226 Ra ratio was above one. The excess 210 Pb in these bullae samples was likely accumulated from the whales' environment via prey, or in the case of the neonate gray whale, across the placental boundary. Our results indicate that the underlying assumptions of the 210 Pb/ 226 Ra radiometric ageing method may not hold true in bowhead whales. Successful application of this method to bowhead whales is therefore doubtful.