We tested the hypotheses that fecundity in four taxa of Sorex declines with age and that older individuals enter a postreproductive state. As the teeth erode in shrews the dentary shortens and masticatory muscles increase in mass, thereby maintaining bite force. To be adaptive these changes must be subject to natural selection which, in turn, requires that older shrews be reproductively active. Number of embryos was not related significantly to age based on tooth wear in any taxon. Females with the most heavily worn teeth were pregnant or lactating and were among those producing the largest litters. The former suggests that they were capable of producing offspring essentially to the end of their natural lifespans. Thus, continued fecundity in older females provides a mechanism by which the bite-force trait is adaptive.
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Vol. 142 • No. 2