Abstract
We attempted to settle the potential problems of bias caused by too soft earplugs and poor formation of the growth layers in age readings of common minke whales. Thus, we examined the feasibility of a new technique of incorporating gelatin in order to collect earplugs for age assessment. Frozen sectioning and histology of the earplug core were also used as methods to improve age estimation. Earplugs were collected by filling the space in the external auditory meatus with gelatin, hardening the gelatin, earplug and its fragments, by spraying with cooling gas, and removing the earplug embedded in gelatin. In 174 trials with common minke whales in the Western North Pacific of coastal waters of Japan in 2007–2009, it was revealed that embedding earplugs with gelatin minimized breakage and protected the neonatal line (NL). This method was particularly effective in younger animals. As a result, the readability was improved. We also examined the histological sections, which were sliced using the Kawamoto specialized frozen sectioning technique, and stained them separately with toluidine blue, haematoxylin and eosin, Sudan III, Sudan VII, and alizarin red S to display a clearer core surface image of the growth layers. The histological sections stained with alizarin red S provided the clearest images, in which we could easily identify both dark and pale laminations. This suggested a close relationship with the seasonal changes in calcium intake from feeding. Earlier age estimation methods focused on fat content in the growth layers; however, we found potential for an improvement in the readability of unclear growth layers when focusing on calcium.
Highlights
Animal age is important basic information for population studies
An earplug from the right external auditory meatus collected by the standard procedure had lost the neonatal line (NL); whereas, the gelatinized earplug from the other side held the NL
Since the NL, which is important for age estimation, is formed at the apical portion of the core or the border between the outer covering and the core, an earplug with an immature outer covering could break and the NL could be lost
Summary
Earplugs were examined as an age character for the first time in baleen whales by Purves (1955). The outer covering is secreted by epithelial cells in the external auditory meatus, whereas, the core, comprising concentric light and dark laminae, is secreted by papillae on the surface of the glove finger (Purves 1955). Since many baleen whales migrate between a breeding place in low-latitude waters (winter) and a feeding place in highlatitude waters (summer), which is approximated by an annual cycle, such a living cycle is reflected as a growth layer; one growth layer represents 1 year (Best 1982, Ohsumi 1964, Gabriele et al 2010, Lockyer 1972; 1984b, Roe 1967). The neonatal line (NL) which is created at birth (Ichihara 1964), is formed at the apical portion of the core, and germinal layers which are newly created, are formed at the basal portion (Fig. 2)
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