Abstract
In order to estimate the age of the Norway rat, the lens weight was examined using two strains. The data on the mean weight of paired lenses of individual rats were obtained from the wild strain (males 72,females 65) and the Wistar albino strain (males 120,females 88) which were 1 to 24 months old. The relation between the age and the mean weight of paired lenses was given in Figs. 1 and 3. On the basis of the data, regression lines of the mean lens weight (y) to the age (x, log scale) were obtained as follows : Wild strain : male, y=17.68x+8.39 (correlation coefficient r=0.838,t value t_0=32.54,p<0.01), female, y=17.18x+8.36 (r=0.923,t_0=27.40,p<0.01). Wistar albino strain : male, y=18.01x+8.40 (r=0.97,t_0=62.73,p<0.01), female, y=17.29x+8.40 (r=0.975,t_0=57.41,p<0.01). From correlation coefficients and highly significant t value in both strains, it was concluded that the lens weight was closely related to age in the Norway rat and that the age could be estimated by lens weight with high reliability. However, when examining the relationship between the body weight or body length and lens weight at each age by correlation coefficient (r) and its significance, no correlation was observed, with few exceptions (Table 1). No difference was found in lens weight between offsprings born in winter and spring (Table 2). However, between the lens weights of litters kept at low and high densities, there was a significant difference (Table 3). From the results mentioned above, the lens weight method is recognized useful to estimate the age of Norway rats.
