FACTORS IN PREGNANCY BLOCKING: AGE AND REPRODUCTIVE BACKGROUND OF FEMALES: NUMBERS OF STRANGE MALES

Abstract

Summary. Pregnancy blocking experiments involving 731 female laboratory mice (control and experimental) indicate: (1) an increased resistance among older females; (2) an equal susceptibility among parous and non-parous females; and (3) a lower pregnancy rate among females exposed to six males than those exposed to one or three males. An exteroceptive block to pregnancy in the laboratory mouse has been de¬ scribed by Bruce (1959, 1960) wherein recently inseminated mice returned to oestrus when placed in a cage with a strange male or males. In these studies parous females were thought to be less readily blocked than non-parous females when placed individually in contact with a strange male, although this differ¬ ence was not apparent when the females were caged inside the male stock box (and thus exposed to more than one male). The latter situation was considered to supply a stronger stimulus. In a subsequent study (Bruce, 1963), however, increasing the number of males to which a female was simultaneously exposed did not increase the incidence of pregnancy blocking, although the question of parous versus non-parous mice was not investigated. Most investigations on pregnancy blocking have involved young virgin mice. Parous mice may be assumed to be older than such females. Thus the apparent resistance of parous females could be due to age. The initial purpose of the present study was to investigate the effects of both age and parity on pregnancy blocking in labora¬ tory mice. During the course of this investigation certain observations suggested that the incidence of pregnancy blocking could be enhanced by increasing the number of males to which individual females were exposed without contact. This effect was investigated further. Swiss strain females from an outbred colony maintained in this laboratory were used in all experiments. Wild males previously shown to be capable of inducing pregnancy blocking served as strange males. Standard laboratory maintenance procedures and cages were employed throughout except for the special test cage (described below) used in Experiment 2. Females were housed