Lifetime parental productivity in twenty-seven crosses of mice. I. Birth traits

Twenty-seven crosses derived from three random-mating, unselected strains of mice (SWO, J, PGH) with 16 matings/pureline or cross were cohabited continuously and monogamously for 365 d to evaluate long-term reproductive performance of the parents. Data from 3,249 litters born were analyzed to study the effects of parity (number of litters) and type of cross (pureline, two-way, backcross, three-way) on the following traits: 1) continuity of reproduction (COR), 2) litter size at birth (LSB), 3) age at first litter (AFL), 4) fraction alive at birth-the ratio of number born alive to LSB (FAB), 5) birth to birth interval (BBI) and 6) reproductive rate-the ratio of number born alive to birth to birth interval (RRT). Reproductive rate was also studied in a cumulative manner rather than on a per parity basis by regressing cumulative number born alive on cumulative birth to birth interval. Parity effects were important (P less than .01) for all traits measured. Differences among cross-types were highly significant for COR, LSB and RRT. Three-way crosses were more productive than backcrosses, and both were superior to two-way and pureline matings. Uniformity was greater among three-way crosses for all traits except COR. Cross-types differed importantly in the pattern of change in performance over parities for COR, LSB, FAB, RRT and cumulative number of live young (P less than .01). Number of parities were most important for variation in longevity of reproduction (COR) and somewhat for litter size (LSB). Mean and cumulative reproductive rates were influenced more by longevity and litter size than by interval between births.