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When a pregnant pine vole is placed in a cage with a strange (unfamiliar) male, one that did not cause her pregnancy, this female goes into "heat", indicating she has rejected the embryos she was carrying. This pine vole can later be impregnated by the strange, or second male. This phenomenon of pregnancy termination, called the "Bruce effect", was first noticed by Hilda Bruce, who saw it occur in labratory mice. Many investigators have noted it in a variety of labratory mice, deer mice, and other species of voles. We set out to see if the Bruce effect was reproducible in pine voles.