In the shrews Sorex araneus Linnaeus, 1758 and S. minutus Linnaeus, 1766 a short period of fast post-natal development in the nest is followed by a longer summer period of developmental stasis and then by autumn regression. The aim of this paper has been to test whether this period of developmental stability also is accompanied by inhibited development of the eye lens - the most continuously growing organ of the mammalian body. If it is not, then can this be used as an age indicator in studies on shrews, and can it serve in distinguishing seasonal cohorts of young born in consecutive summer months? This aspect was examined on material comprising dead shrews collected from live traps and pitfalls set for rodent research in the marshy habitat of a river valley in Białowieża Forest, Poland, in 1997-1999. In fact, the eye lens in shrews was shown to increase in size continuously; therefore it may be used as a reliable age indicator. Body mass of weaned young shrews also continued to increase slowly with age until autumn. On the basis of the distribution of individuals in successive lens-mass classes, three age groups (cohorts) of young could be distinguished each season in the S. araneus population, and two in S. minutus. In the former species, consistent patterns to the relative abundance of the three seasonal cohorts were observed. The consequences of time of birth on individual fitness in Sorex shrews are discussed.
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