[5] The Bruce effect is also observed in deer-mice,[6] meadow voles,[7] collared lemmings,[8] and it has also been proposed, but not confirmed, in other non-rodent species such as lions[9] and geladas.
To test this hypothesis, Bruce and her colleague Alan Parkes recruited perfumers to smell pieces of cloth from the mouse cages.
[13] Male mouse urine contains MHC class I peptides that bind to receptors in the female's vomeronasal organ,[3][14] a mucus-filled structure in the nasal septum.
When the vasopressin 1b receptor gene is knocked out in females, the presence of an unfamiliar male does not trigger pregnancy disruption.
Females treated with an oxytocin antagonist are unable to recognize the urinary scent of their mate, and will terminate pregnancy when exposed to any male, known or unknown.
[3] As the corpus luteum can no longer release progesterone, the uterus remains unprimed for embryo implantation, and the pregnancy fails.
Small steroid molecules such as E2 can enter the bloodstream directly via nasal ingestion[13] and travel to the uterus, which has a high density of suitable receptors.
[3] Pregnancy is only terminated if exposure to novel male scent coincides with two prolactin surges, one of these occurring in a daylight period.
Females can control their likelihood of terminating pregnancy by pursuing or avoiding novel male contact during their most susceptible periods.