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Are there any mammals in which polyspermy produces viable zygotes? In the wikipedia page it is mentioned that there is a delicate equilibrium between female defenses against many sperms, which ocassionally results in low fertility rates (which in not all cases might be an evolutionary disadvantage, but i disgress)

My question is if there are cases where a ovum can be fertilized by several sperms, and the zygote somehow manages to select fairly between a subset of all chromosomes, avoiding polyploidy and producing a viable embryo.

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The short answer appears to be "no".

The longer answer seems to be dependent on what you mean by "viable". Triploid humans do exist that apparently arise from dispermy, but most such fetuses die shortly after birth. Those that do not die soon after birth have a variety of health issues such as intellectual disability; "The few [triploid] infants that do survive to term have multiple severe birth defects." They're almost certainly sterile, though that source doesn't say anything about it.

The existence of elaborate physiological mechanisms for blocking polyspermy in sexual organisms generally, and further mechanical means for limiting sperm number in mammals, argue that polyspermy is something that we spend a lot of effort because it's bad. Here is a recent review of polyspermy in mammals; it has a lot more information about both mechanisms and statistics on the outcomes of dispermy.

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