Abstract
An examination of litter size, interval between litters, the ‘litter index’ (a measure of the persistency of breeders), preweaning mortality and productivity 70, 120 and 170 days after mating was made in 9 inbred lines of mice. The data were collected over a period of at least a year from the breeding records of 360 monogamous pairs of mice. Litter size was not significantly affected by parity. There were significant strain differences in the pattern of intervals between litters. Typically, the interval between mating and the first litter was short, followed by a longer interval between the first and second litter, followed again by a shorter interval both between the second and third and between the third and fourth litters. This pattern was most pronounced in the BALB/c, but did not occur in the DBA/2, where the intervals between all litters were approximately equal.
There were statistically significant strain differences in all seven characters, and heritabilities in the broad sense ranged from 0.36 for the interval between litters to 0.89 for preweaning mortality. The heritabilities of the other three components of 170–day productivity (litter size, litter index and interval between litters) were less than the heritability of 170-day productivity itself. The shape of the productivity curve was found to be genetically determined, although in the typical case there was a linear decline in productivity from the first to the third 50–day period.
No significant linear relationship was found between litter size or interval between litters and 170–day productivity using simple linear regression. There was a significant linear relationship between 170–day productivity and preweaning mortality, and between 170-day productivity and the litter index.
Multiple linear regression showed that the four factors: litter size, interval between litters, preweaning mortality and litter index, accounted for about 32, 10, 32 and 23 per cent respectively of the variation in 170-day productivity.