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Litter size declines with age and parity in females of many, but not all, species of polytocous domestic and wild mammals (Sadleir, 1969; Nalbandov, 1976). Some researchers reported that most female shrews of the genus Sorex produced one litter, rarely two, during the breeding season (Clothier, 1955; Rudd, 1955; Johnston and Rudd, 1957), whereas others reported postpartum pregnancies in 12-50% of females examined and speculated that some individuals may have produced three litters (Jameson, 1955; Gashwiler, 1976). By examining repeatedly live-trapped vagrant shrews (Sorex vagrans) Hawes (1975) found 5 (33.3%) of 15 females produced two litters and six (40.0%) produced three litters; litter size could not be determined because of the nature of the study. Despite short periods of gestation and lactation, the short life-span of most Sorex (Churchfield, 1990) probably precludes production of more than three litters by the most fecund females.
Among pregnant western shrews (Sorex), …