Abstract
Knowledge of age structure and longevity (maximum age) are essential for modelling marine mammal population dynamics. Estimation of age in common dolphins (Delphinus spp.) is primarily based on counting Growth Layer Groups (GLGs) in the dentine of thin, decalcified and stained sections of teeth. An annual incremental deposition rate was validated for Delphinus spp. 30-years ago through the use of tetracycline. However, it is not known if the pulp cavity becomes occluded in older individuals or GLGs continue to be deposited in dentine tissue. To investigate the deposition of GLGs in dentine tissue, teeth samples were obtained during the necropsies of two short-beaked common dolphins (Delphinus delphis) that were held in captivity for 31 and 33 years in New Zealand. Individuals were captured together in Hawkes Bay, North Island, New Zealand and classified as juveniles based on physical appearance. Teeth were processed in two ageing laboratories, using four different bone decalcifiers, two sectioning techniques incorporating the use of both a freezing microtome (-20°C) and paraffin microtome, and two different stains. An age was estimated for one of the dolphins, in line with that proposed based on estimated age at capture and period in captivity. However, a hypomineralised area was observed in the dentine tissue close to the pulp cavity of the second individual, preventing estimation of maximum age. The presence and structure of this anomaly is explored further within the study.
Highlights
Age provides important information on an individual’s life history including growth and development, both sexually and physically
Growth Layer Groups (GLGs) may be recognised by their cyclic repetition and must involve at least one change that can be defined as a countable unit representing, for most species, a 1-year period (Klevezal 1980, Perrin and Myrick 1980, Scheffer and Myrick 1980, Myrick 1998, Murphy et al 2012)
Teeth processing methodologies Methodologies varied between ageing laboratories based in the United Kingdom and New Zealand for varying reasons; (1) due to available equipment, (2) ongoing laboratory practices, and most importantly (3) for a comparison of techniques, including the use of decalcifiers, sectioning apparatuses, and histological stains
Summary
Age provides important information on an individual’s life history including growth and development, both sexually and physically. The neonatal line, a hypocalcified band in the dentine, is laid down at birth and is thought to be caused by hormonal stress due to abrupt environmental and nutritional changes (Avery 1976, Lockyer 1995, Danil 2004). It delineates the pre- and post-natal dentine tissue, and after which the first incremental layer or Growth Layer Group (GLG) begins to be deposited. As common dolphins have small teeth, dentine deposition may occlude the pulp cavity before an animal dies (Gurevich et al 1980), though this has not been investigated in individuals of known-age
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
