Pregnancy block effect

Peele, P., Salazar, I., Mimmack, M., Keveme, E. B. and Brennan, P. A. (2003) Low molecular weight constituents of male mouse urine mediate the pregnancy block effect and convey information about the identity of the mating male. Eur. J. Neurosci. 18, 622-8. 

Li C., Kaba H. and Seto K. (1994). Effective induction of pregnancy block by electrical-stimulation of the mouse accessory olfactory-bulb coincident with prolactin surges. Neurosci Lett 176, 5-8. 

Despite extensive investigation of pregnancy block since its discovery, a convincing explanation for its functional significance and evolutionary development has remained elusive. The postponement of reproduction inevitably impairs reproductive success, but in order to evolve the Bruce effect must offer an overall benefit. The costs and benefits may be very different for males and females. 

Thus, rather than providing a reproductive benefit to males as traditionally assumed, the Bruce effect may have evolved solely to female advantage. Notably, this response also increases selective pressure on stud males to increase their investment in the territorial defence of nest sites that are preferred by females (Ims 1987). Females may improve their own reproductive success through threat of pregnancy block, compelling stud males to invest more heavily in nest defence. 

Male scent is typically used as the pregnancy-blocking stimulus during investigation of the Bruce effect. However experiments addressing the androgen-dependency... 

The list of pheromone controls on reproductive behavior can be extended female pheromones inhibit estrus in adult females and delay puberty in juvenile females, Lee-Boot effect male pheromones can block pregnancy, Bruce effect pheromones from dominant females can suppress reproduction in subordinate females and so on. Have a look at Wyatt s book for an abundance of examples. 

Effect of cy proterone acetate on the pregnancy-blocking ability of male mice and the possible chemical nature of the pheromone. Jottrna/ ofReproduction and Eertility 84, 387-392. 

Four main experimental paradigms have been used in the study of MHC-associated odours (Eggert et al., 1999). These are mate choice pregnancy block (the Bruce effect) trained discrimination and habituation-dishabituation. 

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