Age-related variations in the hippocampus mass in the late postnatal ontogeny of bank voles (Clethrionomys glareolus) from the spring and autumn generations are considered in a comparative aspect. The mass of the hippocampus of the spring voles increases regularly over their short life cycle and approaches the values maximum for the species at two months of age. In the autumn voles, a two-peak growth curve is detected, the hippocampus increasing until September, after which an autumn-winter regression of the mass of this brain region was observed. The hippocampal growth of autumn voles resumed at the age of 7-9 months and coincided with the beginning of the period of reproduction. The hippocampus of autumn voles reached a definitive size at the age of 10-11 months. The results obtained characterize specific features of hippocampal growth in spring and autumn voles. The mechanisms that switch the alternative types of ontogeny of the seasonal generations are discussed.
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