The Evolutionary Significance of the Olfactory Block to Pregnancy

Abstract

Mammalian pheromones influence a wide variety of behaviours and physiological states, few of which have attracted interest and stimulated the imagination more than the olfactory block to pregnancy, or the “Bruce effect” as it is more colloquially termed, after its discovery by Hilda Bruce in the early 60’s. This pheromonal effect, classed as “primer” to distinguish it from the “releasing” action on behaviour of signalling pheromones, is but one of a number of physiological influences which urinary pheromones have on rodent reproduction. Indeed, murine rodents are unique among mammals for the extent to which this aspect of their chemical environment influences reproduction (Bronson, 1979). Over the past twenty years several different effects from primer pheromones have been demonstrated, particularly in laboratory female mice, following exposure to male urine. These include the acceleration of puberty (Vandenbergh, 1969), induction of oestrus in grouped females (Whitten, 1956) and the blocking of pregnancy in newly mated females (Bruce, 1959). Exposure of female mice to female urine has the converse effect of delaying puberty (Cowley and Wise, 1973), inducing anoestrus (Whitten, 1959) and protecting against the pregnancy block (Bruce, 1963). Some of these effects have also been demonstrated in wild mice, prairie deer mice, the laboratory rat, and several microtine rodents, although the latter appear to require the presence of the male in addition to urinary pheromones, these being ineffective in themselves (Milligan, 1976).