Does reproduction accelerate the growth of eye lens mass in female voles?

Although the eye lens mass method has long been used for determining age in small rodents from natural populations, the effects of breeding on growth rates of lenses are rarely considered. Under laboratory conditions, we examined eye lens mass in two groups of 100-day old females of the common vole (Microtus arvalis) : one comprising the females that have already bred, each delivering two litters, and another comprising controls that did not reproduce. The breeding females were heavier than the nonbreeding ones and also had heavier dried eye lenses. However, the effect of breeding on eye lens mass disappeared when the variation in body mass was accounted for in the statistical model. The total number and mass of offspring that the females produced did not affect the lens mass. We conclude that different reproductive histories did affect the growth of eye lens in female common voles through its influence on body size. These results suggest that besides age, the construction of calibration curves for aging voles in natural populations should also include individual body mass as an additional covariate to account for variation in body mass due to differences in reproductive condition.