Reproductive biology

In the tropics, shrews form monogamous pairs and breed throughout the year, while those living in northern temperate zones usually breed from March to November. The known gestation periods are 17-28 days. There are one or several litters a year with 2-10 young in each litter. The young are born naked and blind in a nest of dried grass or leaves placed under a shelter or in a ground cavity. Weaning appears to occur at 2-4 weeks in most forms. Some zoologists estimate the life span in the wild to be 12-18 months, possibly longer. The oldest known Crocidurinae lived approximately four years.

Among Crocidurinae, the young are hairless for the first week and fully haired at 16 days. The auditory canals of Cro-cidurinae species open between the fifth and ninth days of life and the eyes open between the thirteenth and fifteenth days. Crocidurinae species begin to wean their young after 20-22 days. The young are practically adults and are sexually mature by 2-3 months.

Within the Crocidura, gestation is 27-31 days; litter size varies from one to 10. Larger litter sizes are associated with higher energy demands on the mother. The young weigh about 0.04 oz (1 g) at birth and are weaned at around 20 days. In Crocidura russula, young females can conceive at approximately 30 days of age.

Some Crocidurinae species and at least one Sorex species, display an unusual caravan behavior. The mother initally carries the young infants in her mouth. Starting on the sixth to tenth day, depending on the species, the mother and her young move in a caravan. At the slightest suspicious sound, one of the baby shrews grasps its mother's fur near the base of the tail. A second shrew grabs onto the first, a third onto the second and so on until the whole litter is lined up behind the female who then pulls her train of offspring.

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