MATERNAL NEST-BUILDING PERFORMANCE AND FETAL NUMBER IN ROCKLAND-SWISS ALBINO MICE

Summary. A significant correlation was shown in Rockland-Swiss albino mice between the number of fetuses and maternal nest-building performance as assessed by the amount of nesting material gathered into the cage. The greater the number of fetuses, the more nesting material was gathered. A significant correlation first appeared on Day 4 and was maintained until Day 18 of gestation. Mice construct one of two types of nests depending upon their reproductive state. Non-pregnant female and male mice build small saucer-shaped nests that have been referred to as 'sleeping nests'; pregnant mice construct large enclosed nests called 'maternal ' or 'brood nests ' because it is within these nests that the young subsequently are maintained (Koller, 1952, 1956). Progesterone is apparently responsible for eliciting the building of brood nests. Plasma progesterone levels in mice normally are high throughout most of the gestation period and decline just before parturition (Murr et al., 1974). Maternal nest-building follows a similar time course. The administration o