The Breeding Season and Age Structure of Feral House Mouse Populations near San Francisco Bay, California

Abstract

Five feral populations of Mus musculus in the salt marshes and adjacent areas of San Francisco Bay, California, were studied. Age of specimens was determined by comparing toothwear with that found in feral Mus stock reared in the laboratory under simulated field conditions. Young mice first appeared in the trapped samples commencing with the 16 May–15 June sample and during subsequent samples increased in proportion to over-wintering mice, resulting in polygraph figures resembling those of expanding populations. The polygraph figures continued to resemble those of expansion up to the 16 November–15 December sample period at which time a figure resembling a stationary condition became apparent. The population figures appeared to contract from 16 November through 15 April. Males contained sperm in all sample periods. During fall and winter, the proportion of males containing sperm was low. During the latter portion of the breeding season and the early portion of the non-breeding season, many males did not contain sperm even though old enough chronologically for breeding. Females showed no pregnancies from 16 December through 15 April. Females with uterine scars were apparent in the population at all times with a slightly lower proportion during the non-breeding season.