How to escape male infanticide: mechanisms for avoiding or terminating pregnancy in mammals
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The phenomenon whereby pregnancy may be inhibited or terminated when a female is exposed to non-sire males after mating is often, and rather generally, referred to as the ‚Bruce effect‘. Widespread and indiscriminate use of the term for any case of pregnancy failure following exposure to an unfamiliar male, however, masks distinct physiological and social causes of the blocking or termination of pregnancy. Within the available literature, we identify four basic processes by which mammalian females can terminate pregnancy, and thus minimise risks of wasted reproductive costs which might result from male infanticide by a subsequent consort of any progeny carried to full-term where he was not the father. Physical contact with a non-sire male may induce pregnancy failure either before implantation (pregnancy block) or after implantation (pregnancy disruption). By direct contrast, in other species, physical presence of a familiar non-sire male may act to prevent the blocking or disruption of pregnancy, while separation from this non-sire male may act to trigger termination. We propose that use of the term ‚Bruce effect‘ should be restricted to situations in which pregnancy failure is induced primarily by physical contact and/or odour stimuli from a non-sire male as in its initial formulation. Blanket use of this single term for all situations of pregnancy block or disruption, implying by default that pregnancy failure is the consequence of pre-implantation pregnancy block of an inseminated female as a result of physical contact with an unfamiliar male or his olfactory cues, masks the fact that, in many circumstances or species, very different mechanisms may operate in the prevention or disruption of pregnancy. The implicit presumption that pregnancy failure is a single and uniform phenomenon also discourages further research into the range of rather different circumstances and mechanisms by which pregnancy disruption may be triggered.
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