Reproductive biology

Seasonal regimes of resource availability favor high fecundity during the flush of resources occurring during summer. Therefore, the reproduction of soricine shrews is strongly seasonal, occurring when food is highly available and weather conditions are optimal. Soricines produce significantly larger litters than crocidurines, on average 5.1 young per litter (5.9 in Sorex), but more than 10 young can also be born in a litter. One female can produce one to four litters per year. Increased reproductive output may be a major advantage derived from the high rate of metabolism in many soricines.

Evidence from the field and laboratory studies suggests that female shrews of a variety of species show a tendency to mate with many different males. Multiple paternity in the litter of wild-caught females has been demonstrated in the promiscuous common shrew, with up to six different fathers per litter. Most soricines have a short gestation period (on

An ornate shrew (Sorex ornatus) eating a lizard. (Photo by Alan Blank. Bruce Coleman, Inc. Reproduced by permission.)
A southern short-tailed shrew (Blarina carolinensis) eating an insect. (Photo by R. Wayne Van Devender. Reproduced by permission.)

average 21 days) and a longer weaning period. Young are born in a very early stage of development and are among the most altricial placental mammals. The pups develop quickly, and usually they are weaned within three to four weeks. After the final break of mother-offspring bond, the young leave the nest and their dispersal begins. Juvenile shrews usually do not reproduce during their first summer because generally they mature in the next spring. High social intolerance and aggressiveness with increased population density are important factors inhibiting the maturation and reproduction of juveniles. After reproduction, and as the autumn approaches, the parental population quickly dies. The average life-span of a reproducing soricine shrew is 14.7 months.

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