Advantages of Monogamy over Polygamy and Vice Versa In Nature?
Polygamy spreads genetics further, but usually you trade that spread for care of the offspring. 

Monogamy means less genetic diversity, but the offspring usually get more attention and care, meaning they're more likely to survive. 

So it's more babies where more will die, or less babies that take more energy to grow and nurture but more likely to survive.