Abstract
Age determination of animals by measuring the weight of their eye lenses is a widely used method in wildlife biology. In general, it is recommended to prepare lenses immediately after trapping to avoid errors in the age estimation due to decomposition of lens tissue. However, in many field studies, large numbers of animals need to be trapped over long periods of time in huge areas and by many different field workers. Therefore, the immediate preparation of eye lenses imposes a considerable logistic constraint that could be avoided by prior freezing of trapped animals. To assess the impact of freezing, weights of lens of frozen and unfrozen eyes of 114 Arvicola terrestris were compared pair wise. The frozen lenses weighed at average 3.3% (95% CI: 2.4–4.1%) more than the unfrozen ones from the same animals. Freezing time, weight of lenses and mean temperature of the trapping day as an indicator of decomposition speed did not affect the freezing-induced weight increase. Age estimates based on weights of unfrozen lenses varied between 24 and 445 days. Estimates based on frozen lenses were systematically higher. Applying a constant correction factor of 1.033−1 for the weight of frozen lenses corrects this overestimation of age. We conclude that age determination with frozen lenses of small rodents can yield valid age estimates if a correction factor for freezing is applied. Thus, age determination can be organised much more efficiently in field studies, which is highly advantageous for many ecological, agricultural and epidemiological research projects.
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Acknowledgements
We are grateful to Marianne Fritzsche from Grün Stadt Zürich and to Andreas Weidmann, Simon Franz and all other field workers that delivered the trapped rodents. We thank Alexander Mathis for valuable comments on the manuscript and acknowledge the support of our research activities by the Swiss Federal Veterinary Office (Nr. 1.07.04). This work represents part of the dissertation of Pierre Burlet, veterinarian.
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The authors declare that the study comply with the current laws of the country in which they were performed.
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The authors declare that they have no conflict of interest.
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Burlet, P., Deplazes, P. & Hegglin, D. Efficient age determination: how freezing affects eye lens weight of the small rodent species Arvicola terrestris . Eur J Wildl Res 56, 685–688 (2010). https://doi.org/10.1007/s10344-010-0390-0
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DOI: https://doi.org/10.1007/s10344-010-0390-0