The Effect of the Manipulation of Social Structure on Reproduction in House Mice

The influence of social and demographic parameters on reproduction was examined by removing either adult ♂ ♂, breeding ♀♀, or subadults of both sexes from pairs of populations for 15 mo; the 4th pair of 8 populations was not manipulated. Removal of ♂ ♂ was associated with a decreased proportion of adult ♀♀ which became parous, low body growth rates for maturing ♀ ♀, low association of perforate and nonperforate ♀♀ with adult ♂ ♂, and a high association of adult ♂ ♂ with nonperforate ♀ ♀. Removal of parous ♀♀led to earlier maturation and breeding of ♂ ♂, negative association between perforate ♀♀ and adult ♂ ♂, and higher levels of aggression in the populations. Multiple regression techniques indicated that estrous cycling and lactation were best explained by the activity of nonperforate ♀♀ and their association with adult ♂ ♂, the number of young weaned the previous month, the percentage of adult ♂ ♂ in the populations, and the aggressive tendency of these ♂♂. Aggressive tendency was measured in ♂ ♂ removed from the populations and paired with nonaggressive control mice. Nulliparous ♀ ♀ were more sensitive than parous ♀ ♀ to changes in social] behavior or demography of a population; litters born to previously nulliparous ♀ ♀ were clustered on certain months, whereas litters of parous ♀ ♀ were born randomly. Although most ♀ ♀ failed to reproduce during the later stages of population growth, this inhibition of reproduction was not always permanent; when nonbreeding ♀ ♀ were removed from the populations, transferred to small cages, and paired with adult ♂♂ from the populations, one half reproduced within 1 mo. Nestling survival was influenced negatively by the number of aggressive acts, the proportion of adults of either sex in the population, and the proportion of perforate and lactating ♀ ♀. Only ♂ dominance had a positive correlation with litter survival. The sex ratio of recently weaned mice varied greatly and was similar to that found among adults, even when the removal of adults of 1 sex caused the adult sex ratio to be markedly unequal.