Efficient age determination: how freezing affects eye lens weight of the small rodent species Arvicola terrestris

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.