Bruce effect

When a newly impregnated female mouse is confined with a strange male, that is a male which has an odor different from that of her mate, implantation failure and a rapid return to estrus is observed. This pregnancy-blocking is not observed if the female is exposed to a castrated male, but treatment of the male with testosterone restores his production of pregnancy-blocking pheromone (342). 

Gangrade and Dominic (341) observed that this effect is high if a female is placed and housed below correlated males, but is reduced when the female is housed above the males and concluded that the pheromone involved in the Bruce effect is apparently nonvolatile and acts on females through contact. 

Marchlewsbca-Koj (330) fractionated male mouse urine and found that the pregnancy-blocking pheromone is a peptide or a substance bound with the peptide fraction. 

The rate of pregnancy-blocking is also reduced when the strange male belongs to an inbreeding strain or on administration of alpha-methyl dopa suggesting that dopaminergic nervous system are involved in the Bruce effect (338). 

A sodobiological approach (144) suggests that the Bruce effect has evolved to promote heterogeneity in the population since the female is available for reinsemination. However, Keverne and de la Riva 

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Rajendren G. and Dominic J. (1986). Effect of bilateral transection of the lateral olfactory tract on the male-induced implantation failure (Bruce effect) in mice. Physiol Behav 36, 587-590. 

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... 

Mahady, S.J. and Wolff, J.O. (2002) A field test of the Bruce effect in the monogamous prairie vole (Microtus ochrogaster). Behav. Ecol. Sociobiol. 52,31-37. 

Schwagmeyer, RL. (1979) Bruce effect - evaluation of male-female advantages. Am. Nat. 114, 932-938. 

Thomas, K.J. and Dominic, C.J. (1987) Evaluation of the role of the stud male in preventing male-induced implantation failure (the Bruce effect) in laboratory mice. Anim. Behav. 35, 1257-1259. 

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. 

Bruce effect blockage of pregnancy by male pheromones. 

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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