Determination of the age of over wintered water voles inhabiting garden plots

The role of long-lived individuals in the functioning of rodent populations is not clear. It is associated with the difficulty in determining the age of animals. To study the age composition of the overwintered individuals in natural populations, a method to determine the age by measuring the bones of the pelvic limbs was developed. The method using discriminant analysis has an accuracy of up to a year. The discriminant function was found on a control sample of captive-bred animals (41 males and 42 females of known age). Then, the discriminant function was used to determine the age of the animals in the control sample; 100% of the individuals were classified correctly. The accuracy of age determination of individuals in the test sample, which was not part of the control sample, was 89.7% for males (n = 29) and 81.5% for females (n = 27). The reliability of the method was also confirmed by the results received in the third group (7 males and 7 females). They were caught in nature with a known relative age and died in the vivarium, the accuracy their age determination was 85.7%. Using the developed method, we compared the age composition of overwintered water voles from two populations, which inhabited the garden plots in the vicinity of Oyosha and Akademgorodok (Novosibirsk region). The number of the first population was declining for several years, while that of the second one was increasing. It was found that in the diminishing population the proportion of individuals that lived more than two winters was slightly higher (33.3%) than that in the rising population (18.8%). In overwintered males statistically significant age differences in craniometric and reproductive traits were found. It was concluded that long-lived individuals could play a significant role in maintaining the long-term existence of populations in pessimal environmental conditions.