Prenatal Mortality in Wild-Caught House Mice

Abstract

The litter size of a mammal is compounded of genetical, maternal and environmental factors. In the laboratory mouse many of these factors have been identified and quantified, and the paper is an attempt to apply this knowledge to wild-living animals. Counts of the number of corpora lutea, dead and living embryos in 99 pregnant house mouse females caught on Pembrokeshire farms and on the Isle of May showed an excess of corpora lutea over implantations of 20% in the farm mice, 34% in the island mice. Post-implantation death was 2.5% of the number of eggs shed in island mice, 10.4% in rick mice, and 17.1% in mice trapped in a barn. Litter size in commensal and rick mice is constant throughout the year, but in mice on the island of Skokholm it varies in the same way as breeding intensity, rising from an average of 5.0 implantation sites in March (11% of females pregnant) through 10.7 in July (37.5% of females pregnant) to 6.0 in Oct. (7% of females pregnant). Post-implantation mortality throughout the season is of the same order as that in the Isle of May mice. Deleterious genetical factors appear to cause considerable post-implantation mortality in laboratory mice. The low rate of post-implantation mortality in wild-living mice suggests that the "genetic load" of these populations is small. Litter size is positively correlated with maternal size in mice caught on farms (correlation coefficient c. 0.6), but this correlation is virtually absent in island mice, i.e. some other factor than maternal size is limiting litter size under island conditions. This result supports Lack''s arguments about the action of natural selection on litter sizes.