Abstract
The development of age determination methods in marine mammals is reviewed with particular reference to the use of teeth Growth Layer Groups (GLGs) formed in the dentine and cement of carnivores. Using this background, practices for sampling, tooth extraction and collection, storage and different methods of preparation of teeth as well as reading and counting GLGs are discussed and evaluated for the harbour seal (Phoca vitulina). The paper includes comments on best practices for counting GLGs with new examples from known-age seals, and also a detailed examination of confounding factors in interpreting GLGs such as mineralization anomalies and the phenomena of accessory lines, “false annuli” and “paired laminae” which have not been discussed previously. The paper concludes with recommendations for undertaking age estimation in harbour seals from sampling through final GLG interpretation with special emphasis on standardization of methods with other researchers.
Highlights
The ability to age individuals is critical in the investigation of the life history parameters and structure of any population
This paper provides as a background, a general review of the methods used to age mammals using annual growth layers in teeth, highlighting how different approaches have been adopted to age individuals of both the same and different species
The use of incisors and premolars has developed. This has enabled the removal of teeth from live animals (Arnbom et al 1992, Coy and Garshelis 1992), and has increased the ability for comparison of Growth Layer Groups (GLGs) between teeth taken from the same individual over a period of time (Myrick et al 1984)
Summary
The ability to age individuals is critical in the investigation of the life history parameters and structure of any population. The cementum which covers the dentine in the root of the tooth, has a similar composition to bone (Boyde, 1980) and is derived from the cementoblasts in the gum tissue adjacent to and surrounding the tooth. In general up to 20% of this tissue is organic material, including cementocytes, ground substance (containing proteoglycans) and both intrinsic and extrinsic collagen fibres (Maas, 2002). Both between and within species differences in the percent by mass of carbon in cementum has been examined (Mackey, 2004). But continuous deposition, in conjunction with the incremental growth layers, means that it provides an ideal long term recording structure (Klevezal 1980, Maas 2002, Raspanti et al 2000)
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