Interpretation of growth layers in the periosteal zone of tympanic bulla from minke whales Balaenoptera acutorostrata

Abstract Growth layers in the periosteal zone of tympanic bullae from minke whales, Balaenoptera acutorstrata , have been analysed. Different methods for preparation of bulla sections were used to show the growth layers. A comparison of the number of growth layers identified in thick etched sections and in thin translucent sections using light microscopy and scanning electron microscopy show similar readability in the different preparation techniques. Acid-etched surfaces of thick sections from 297 bullae and thin translucent section from 399 bullae were then examined by reflected and transmitted light in a binocular light microscope. All the etched and 390 of the thin sections were examined by two readers, and 157 of the thin sections were also read by a third reader. Comparisons of the readability of growth layers in etched and thin sections were tested for each individual reader and between readers. There is some problem in counting growth layers, but there is substantial evidence suggesting that about 80% of the minke whales were correctly aged within ±1 year. The deposition of periosteal growth layers is discussed in relation to body length of the animals, accumulation of corpora in the ovaries of females and other biological data.